By Patrick McDonald


2009-03-03 10:13:02 8 Comments

In this question, someone suggested in a comment that I should not cast the result of malloc, i.e.

int *sieve = malloc(sizeof(int) * length);

rather than:

int *sieve = (int *) malloc(sizeof(int) * length);

Why would this be the case?

26 comments

@ashiquzzaman33 2015-10-09 21:23:21

From the Wikipedia:

Advantages to casting

  • Including the cast may allow a C program or function to compile as C++.

  • The cast allows for pre-1989 versions of malloc that originally returned a char *.

  • Casting can help the developer identify inconsistencies in type sizing should the destination pointer type change, particularly if the pointer is declared far from the malloc() call (although modern compilers and static analyzers can warn on such behaviour without requiring the cast).

Disadvantages to casting

  • Under the ANSI C standard, the cast is redundant.

  • Adding the cast may mask failure to include the header stdlib.h, in which the prototype for malloc is found. In the absence of a prototype for malloc, the standard requires that the C compiler assume malloc returns an int. If there is no cast, a warning is issued when this integer is assigned to the pointer; however, with the cast, this warning is not produced, hiding a bug. On certain architectures and data models (such as LP64 on 64-bit systems, where long and pointers are 64-bit and int is 32-bit), this error can actually result in undefined behaviour, as the implicitly declared malloc returns a 32-bit value whereas the actually defined function returns a 64-bit value. Depending on calling conventions and memory layout, this may result in stack smashing. This issue is less likely to go unnoticed in modern compilers, as they uniformly produce warnings that an undeclared function has been used, so a warning will still appear. For example, GCC's default behaviour is to show a warning that reads "incompatible implicit declaration of built-in function" regardless of whether the cast is present or not.

  • If the type of the pointer is changed at its declaration, one may also, need to change all lines where malloc is called and cast.

Although malloc without casting is preferred method and most experienced programmers choose it, you should use whichever you like having aware of the issues.

i.e: If you need to compile C program as C++(Although those are separate language) you should use malloc with casting.

@Spikatrix 2016-05-14 06:26:46

What does "Casting can help the developer identify inconsistencies in type sizing should the destination pointer type change, particularly if the pointer is declared far from the malloc() call" mean? Could you give an example?

@Peter Cordes 2016-10-24 15:02:51

@CoolGuy: See an earlier comment on another answer. But note that the p = malloc(sizeof(*p) * count) idiom picks up changes in the type automatically, so you don't have to get warnings and go change anything. So this isn't a real advantage vs. the best alternative for not-casting.

@Peter A. Schneider 2016-11-15 17:00:22

This is the proper answer: There are pros and cons, and it boils down to a matter of taste (unless the code must compile as C++ -- then the cast is mandatory).

@Michaël Roy 2017-06-18 01:46:57

Point 3 is moot, since If the type of the pointer is changed at its declaration, one should check every instance of malloc, realloc and free inolving that type. Casting will force you to do just that.

@Aashish 2017-10-03 13:13:38

The casting of malloc is unnecessary in C but mandatory in C++.

Casting is unnecessary in C because of:

  • void * is automatically and safely promoted to any other pointer type in the case of C.
  • It can hide an error if you forgot to include <stdlib.h>. This can cause crashes.
  • If pointers and integers are differently sized, then you're hiding a warning by casting and might lose bits of your returned address.
  • If the type of the pointer is changed at its declaration, one may also need to change all lines where malloc is called and cast.

On the other hand, casting may increase the portability of your program. i.e, it allows a C program or function to compile as C++.

@user10133158 2018-09-16 05:58:00

@Aashish, in C++ you generally do not do any memory allocations. Not even using new. ps: I do not downvote too.

@Slothworks 2015-10-09 17:47:38

This is what The GNU C Library Reference manual says:

You can store the result of malloc into any pointer variable without a cast, because ISO C automatically converts the type void * to another type of pointer when necessary. But the cast is necessary in contexts other than assignment operators or if you might want your code to run in traditional C.

And indeed the ISO C11 standard (p347) says so:

The pointer returned if the allocation succeeds is suitably aligned so that it may be assigned to a pointer to any type of object with a fundamental alignment requirement and then used to access such an object or an array of such objects in the space allocated (until the space is explicitly deallocated)

@Lundin 2014-03-20 15:53:14

You don't cast the result of malloc, because doing so adds pointless clutter to your code.

The most common reason why people cast the result of malloc is because they are unsure about how the C language works. That's a warning sign: if you don't know how a particular language mechanism works, then don't take a guess. Look it up or ask on Stack Overflow.

Some comments:

  • A void pointer can be converted to/from any other pointer type without an explicit cast (C11 6.3.2.3 and 6.5.16.1).

  • C++ will however not allow an implicit cast between void* and another pointer type. So in C++, the cast would have been correct. But if you program in C++, you should use new and not malloc(). And you should never compile C code using a C++ compiler.

    If you need to support both C and C++ with the same source code, use compiler switches to mark the differences. Do not attempt to sate both language standards with the same code, because they are not compatible.

  • If a C compiler cannot find a function because you forgot to include the header, you will get a compiler/linker error about that. So if you forgot to include <stdlib.h> that's no biggie, you won't be able to build your program.

  • On ancient compilers that follow a version of the standard which is more than 25 years old, forgetting to include <stdlib.h> would result in dangerous behavior. Because in that ancient standard, functions without a visible prototype implicitly converted the return type to int. Casting the result from malloc explicitly would then hide away this bug.

    But that is really a non-issue. You aren't using a 25 years old computer, so why would you use a 25 years old compiler?

@Don Hatch 2016-11-02 08:43:50

"pointless clutter" is dismissive hyperbole that tends to derail any possibility of convincing anyone who doesn't already agree with you. A cast certainly isn't pointless; Ron Burk's and Kaz's answers make arguments in favor of casting that I very much agree with. Whether those concerns weigh more than the concerns you mention is a reasonable question to ask. To me, your concerns look relatively minor compared to theirs.

@chux 2017-07-27 21:07:44

"A void pointer can be converted to/from any other pointer type without an explicit cast" is not supported by 6.3.2.3. Perhaps you are thinking of "pointer to any object type"? "void pointer" and "pointer to a function" are not so readily convertible.

@Lundin 2018-04-12 08:55:08

Indeed the reference was incomplete. The relevant part for the "implicitness" is the rule of simple assignment 6.5.16.1. "one operand is a pointer to an object type, and the other is a pointer to a qualified or unqualified version of void". I've added this reference to the answer for completeness.

@proski 2016-06-29 07:30:41

I prefer to do the cast, but not manually. My favorite is using g_new and g_new0 macros from glib. If glib is not used, I would add similar macros. Those macros reduce code duplication without compromising type safety. If you get the type wrong, you would get an implicit cast between non-void pointers, which would cause a warning (error in C++). If you forget to include the header that defines g_new and g_new0, you would get an error. g_new and g_new0 both take the same arguments, unlike malloc that takes fewer arguments than calloc. Just add 0 to get zero-initialized memory. The code can be compiled with a C++ compiler without changes.

@user2371524 2017-08-16 12:25:30

No, you don't cast the result of malloc().

In general, you don't cast to or from void *.

A typical reason given for not doing so is that failure to #include <stdlib.h> could go unnoticed. This isn't an issue anymore for a long time now as C99 made implicit function declarations illegal, so if your compiler conforms to at least C99, you will get a diagnostic message.

But there's a much stronger reason not to introduce unnecessary pointer casts:

In C, a pointer cast is almost always an error. This is because of the following rule (§6.5 p7 in N1570, the latest draft for C11):

An object shall have its stored value accessed only by an lvalue expression that has one of the following types:
— a type compatible with the effective type of the object,
— a qualified version of a type compatible with the effective type of the object,
— a type that is the signed or unsigned type corresponding to the effective type of the object,
— a type that is the signed or unsigned type corresponding to a qualified version of the effective type of the object,
— an aggregate or union type that includes one of the aforementioned types among its members (including, recursively, a member of a subaggregate or contained union), or
— a character type.

This is also known as the strict aliasing rule. So the following code is undefined behavior:

long x = 5;
double *p = (double *)&x;
double y = *p;

And, sometimes surprisingly, the following is as well:

struct foo { int x; };
struct bar { int x; int y; };
struct bar b = { 1, 2};
struct foo *p = (struct foo *)&b;
int z = p->x;

Sometimes, you do need to cast pointers, but given the strict aliasing rule, you have to be very careful with it. So, any occurrence of a pointer cast in your code is a place you have to double-check for its validity. Therefore, you never write an unnecessary pointer cast.

tl;dr

In a nutshell: Because in C, any occurrence of a pointer cast should raise a red flag for code requiring special attention, you should never write unnecessary pointer casts.


Side notes:

  • There are cases where you actually need a cast to void *, e.g. if you want to print a pointer:

    int x = 5;
    printf("%p\n", (void *)&x);
    

    The cast is necessary here, because printf() is a variadic function, so implicit conversions don't work.

  • In C++, the situation is different. Casting pointer types is somewhat common (and correct) when dealing with objects of derived classes. Therefore, it makes sense that in C++, the conversion to and from void * is not implicit. C++ has a whole set of different flavors of casting.

@P__J__ 2017-08-22 16:38:37

In your examples you avoid void *. there is a difference between cast from double * to int * and vice versa. malloc returns pointel aligned to the largest standard type so there are no aliasing rules broken even if someone casts form this aligned pointer to other type.

@user2371524 2017-08-22 16:43:08

Aliasing has nothing at all to do with alignment and for the rest of your comment -- you obviously didn't get the point.

@user2371524 2017-08-22 16:48:26

@PeterJ: just in case, the point is to avoid an unnecessary pointer cast, so it doesn't look like a piece of code you have to pay special attention to.

@Lundin 2017-08-23 06:33:07

The strict aliasing issue doesn't really have anything to do with void pointers. In order to get bugs caused by strict aliasing violations, you must de-reference the pointed-at data. And since you can't de-reference a void pointer, such bugs are per definition not related to the void pointer but to something else.

@Lundin 2017-08-23 06:36:40

Rather, you would have to make a rule to ban all pointer casts. But then how would you write things like serialization routines and hardware-related programming? Things that are C's strength. Such casts are fine if you know what you are doing.

@user2371524 2017-08-23 07:10:51

@Lundin that's exactly the point. You sometimes need these casts, but you know you have to be extra careful with them, therefore you don't write unnecessary pointer casts.

@unwind 2009-03-03 10:17:53

No; you don't cast the result, since:

  • It is unnecessary, as void * is automatically and safely promoted to any other pointer type in this case.
  • It adds clutter to the code, casts are not very easy to read (especially if the pointer type is long).
  • It makes you repeat yourself, which is generally bad.
  • It can hide an error if you forgot to include <stdlib.h>. This can cause crashes (or, worse, not cause a crash until way later in some totally different part of the code). Consider what happens if pointers and integers are differently sized; then you're hiding a warning by casting and might lose bits of your returned address. Note: as of C11 implicit functions are gone from C, and this point is no longer relevant since there's no automatic assumption that undeclared functions return int.

As a clarification, note that I said "you don't cast", not "you don't need to cast". In my opinion, it's a failure to include the cast, even if you got it right. There are simply no benefits to doing it, but a bunch of potential risks, and including the cast indicates that you don't know about the risks.

Also note, as commentators point out, that the above talks about straight C, not C++. I very firmly believe in C and C++ as separate languages.

To add further, your code needlessly repeats the type information (int) which can cause errors. It's better to dereference the pointer being used to store the return value, to "lock" the two together:

int *sieve = malloc(length * sizeof *sieve);

This also moves the length to the front for increased visibility, and drops the redundant parentheses with sizeof; they are only needed when the argument is a type name. Many people seem to not know (or ignore) this, which makes their code more verbose. Remember: sizeof is not a function! :)


While moving length to the front may increase visibility in some rare cases, one should also pay attention that in the general case, it should be better to write the expression as:

int *sieve = malloc(sizeof *sieve * length);

Since keeping the sizeof first, in this case, ensures multiplication is done with at least size_t math.

Compare: malloc(sizeof *sieve * length * width) vs. malloc(length * width * sizeof *sieve) the second may overflow the length * width when width and length are smaller types than size_t.

@n.m. 2016-12-08 12:30:35

Please consider updating the answer. The cast is no longer dangerous, and repeating oneself is not necessarily a bad thing (redundancy can help catch errors).

@n.m. 2016-12-08 12:40:04

Compilers have changed. An up-to-date compiler will warn you about a missing declaration of malloc.

@unwind 2016-12-08 12:48:47

@n.m. Ok. I think it's bad to assume that anyone reading here has a particular compiler. Also, since C11 the entire "implicit function" concept is gone, I didn't know that. Still, I don't see the point in adding a pointless cast. Do you also do int x = (int) 12; just to make things clear?

@n.m. 2016-12-08 13:10:15

I've encountered a case where redundancy of the malloc cast helped to find a bug. This doesn't happen often but still.

@Braden Best 2016-12-17 18:02:52

When using sizeof with a specific type, I like to separate sizeof and (type) with a single space (I.e. sizeof (int)) just to reinforce that the argument to sizeof is (int), not int

@Braden Best 2016-12-17 18:49:29

@n.m. if explicitly casting a void pointer "helped" solve a bug, you more likely encountered undefined behavior, which would mean the program in question likely has a far worse, undiscovered bug that you haven't run into yet. And one day, on a cold winter evening, you'll come home from work to find your GitHub page flooded with issue reports complaining about demons flying out of the users' noses

@n.m. 2016-12-17 20:30:04

@BradenBest No, this is not the case, even remotely so. You can look at the program in question here.

@Amin Negm-Awad 2017-01-08 04:30:50

@unwind Even I agree with you, (int)12 is not comparable. 12 is an int, the cast does simply nothing. The retval of malloc() is void *, not the pointer type casted to. (If it is not void *. So the analogy to (int)12 would be (void*)malloc(…) what nobody is discussing.)

@P.P. 2017-01-18 12:50:35

"I think it's bad to assume that anyone reading here has a particular compiler. Also, since C11 the entire "implicit function" concept is gone, I didn't know that" - It's gone since C99 (not just C11). Any C99 compiler is required to issue a diagnostic if #include <stdlib.h> isn't included.Out of the 4 bullets, the first 3 are subjective (there's sufficient proof that some tend to favor it in this page) and the 4th has been "fixed" in C language about 10 years before this answer. While I am not advocating in favor of the cast, it's really a case of making a mountain out of a mole here.

@Ryan Haining 2017-03-03 19:08:47

@unwind comparing with double d = (double)12; or unsigned int u = (unsigned int)12; might make more sense.

@Nguai al 2017-05-11 06:08:26

"the second may overflow the length * width when width and length are smaller types that size_t." - I don't get this. I thought the variables get promoted to larger type. How is the overflow possible?

@Gewure 2017-05-29 11:24:25

how comes the compiler is throwing warnings unless i cast the result of malloc?

@unwind 2017-05-30 08:51:19

@Nguaial No, promotion has nothing to do with the type of the left-hand side of an assignment. This is a common misconception. double a = 1 / 2; is 0 since it's an integer division.

@unwind 2017-05-30 08:52:31

@Gewure Obviously quite hard to answer without your code, but probably because you're failing to #include <stdlib.h> for malloc()'s prototype, thus making the compiler assume it returns int, which you can't assign to a pointer without a cast. The cast is not enough, in that case.

@Michaël Roy 2017-06-17 21:57:36

Not reviewing ALL allocations in the code when changing a data type size/switching to a different data type is asking for troubles. you can use casting to your an advantage to quiclky find all malloc calls for a certain type.

@tilz0R 2017-06-29 06:27:10

Let's say we have C++ project and C library file where header has extern "C"... setup. When compiler compiles .c file, will it work if there is no casting of malloc, because we are in C++ project?

@unwind 2017-06-29 08:04:01

@tilz0R Yes, of course. The extern "C" is for the C++ compiler, it just tells it not to name-mangle when looking for the C function. The C function is built by a C compiler and is written in C.

@tilz0R 2017-06-29 08:06:45

So then I don't know why we always fight over you cannot write C and C++ code for the same project. I'm using extern "C" approach in every my C library so then it is allowed to not cast malloc and will still compile. Case closed for me then. I'm not casting it anymore then if this is really hidden error if you cast result. In general to me, your 4th point why not cast has no sense, or at least I don't get it. If non NULL is returned, memory is allocated, you can cast it how you want.

@unwind 2017-06-29 08:13:25

@tilz0R Pre-C11, functions without prototype were assumed to return int. But int is often smaller than void * (32 vs 64 bits), so doing foo *x = (foo *) malloc(sizeof *x); might have hidden a warning caused by making the (undeclared, assumed int) return value from malloc() be treated as a pointer, when it in fact is not large enough and thus won't work.

@user2371524 2017-08-16 12:30:31

@unwind "Pre-C11, functions without prototype were assumed to return int" <- this was already gone in C99.

@autistic 2017-09-07 16:46:43

@unwind, actually, I think that point which you say is no longer relevant is still relevant because if you try to compile code without specifying --std=c11 (as many do), you'll get but a mere warning... In fact, even when specifying --std=c11 I get "warning: implicit declaration of function ‘malloc’", when I've not included the standard library header for it. This isn't an uncommon implementation, either; gcc version 5.4.0 20160609...

@autistic 2017-09-08 07:53:17

@unwind There's room for misinterpretation there; some users might think this means "this point is no longer relevant, since it just emits a warning and (erroneously) we can ignore the warning".

@savram 2017-09-10 02:28:14

"Compare: malloc(sizeof *sieve * length * width) vs. malloc(length * width * sizeof *sieve) the second may overflow the length * width when width and length are smaller types than size_t" I don't understand what overflow can happen here.

@Daniel H 2017-09-19 22:10:15

@savram If length and width are ints, where int has fewer bits than size_t, then length*width might be bigger than the largest int. This is less likely to be an issue if you use size_t math, which you can do by putting the sizeof part first (since the compiler “reads” multiplications from left to right)

@savram 2017-09-20 01:45:44

@DanielH Thank you, I understand about the int now. But why changing the order solves anything? What if the multiplication of sizeof * length exceeds the largest sizeof?

@Daniel H 2017-09-20 14:57:40

@savram The malloc function takes a size_t as input. If the whole multiplication exceeds that, then you just can't do the allocation you want. This is unlikely in a bug-free program, because on most 64-bit systems that would be asking for 18 exabytes of memory, more than I think has actually been manufactured ever. On the other hand, the maximum int on such systems is often 2^31-1, or just over two gigabytes; a lot of programs would want to allocate that much.

@autistic 2018-02-25 04:11:56

@DanielH I'd suggest that the bigger problem there is... why was int chosen as the type for length and width? We should take care in choosing types for our variables, and unless it absolutely makes sense for a length and/or width to be negative, all signed options should be discarded from the choices. After all, you wouldn't want a two-dimensional array where each dimension can have negative indices, right? That's just begging for undefined behaviour. Wouldn't you rather suggest that length and width be refactored as size_t, to avoid further complications?

@Daniel H 2018-02-25 13:31:01

@Sebivor It depends on the context. Sometimes there are reasons for using signed types anyway, and more often length might be unsigned but smaller than size_t for some reason. I agree that, all else being equal, it's better if length is a size_t. I was also answering savram's question about how an overflow could happen with length * width * sizeof *sieve, as mentioned in the question; just saying to always make length and width be of type size_t would not be an answer.

@DimeCadmium 2018-06-27 18:15:31

Regarding sizeof() vs sizeof: yes, it's not a function, but it behaves somewhat like a function (or more accurately a macro I suppose). And also () makes it clear to anyone reading EXACTLY what size you're taking. Personally I feel strongly that the ability to leave off the paren's is a bug, not a feature. @Kevin: this is why you use sizeof(*x), as mentioned in the answer.

@HostileFork 2018-07-30 14:04:32

I very firmly believe in C and C++ as separate languages. => You shouldn't be firm about things that fly in the face of both history and present practice. C++ didn't inherit C's syntax because of its intrinsic beauty; it was so that C codebases can incrementally adopt C++ features. Plus, there's a lot of value in being able to compile a C codebase with a C++ compiler whether that's how you ultimately ship your binary or not, see the C++ core guidelines. One can write a lot of neat type_traits-based static checks for C.

@Ryan 2019-01-10 05:07:50

@HostileFork That may have been true back when C++ was just a plain preprocessor on top of C, with a solid philosophy guiding the language. But in the past couple of decades, syntactic creep has left it nearly unrecognizable as a dialect of C. A lot of compliant C code refuses to compile with a C++ compiler (try creating a struct named 'template' with a field named 'class'). Meanwhile the ISO C++ committee has neglected to keep up with modern developments to the ISO C standards, leaving the languages on two separate tracks: one promising portability, and the other boasting flexibility.

@Cacahuete Frito 2019-01-30 18:41:48

@HostileFork @Ryan You don't even have to use C++ keywords to get C/C++ incompatibility: I recently had a problem when I first used C++ because the library I used (OpenCV) was being deprecated in C (WTF!), and the only stable option was to use C++. When I tried to use my C library which includes functions like this one void foo(int n, int arr[n]), I realized C++ doesn't allow that, which I think is really stupid (tell me otherwise), and then I decided to do separate headers for C (.h) and for C++ (.hpp) (both referring to C code), even if that means repeating most of the code in the headers.

@Mohit 2016-11-08 10:09:54

  1. As other stated, it is not needed for C, but for C++.

  2. Including the cast may allow a C program or function to compile as C++.

  3. In C it is unnecessary, as void * is automatically and safely promoted to any other pointer type.

  4. But if you cast then, it can hide an error if you forgot to include stdlib.h. This can cause crashes (or, worse, not cause a crash until way later in some totally different part of the code).

    Because stdlib.h contains the prototype for malloc is found. In the absence of a prototype for malloc, the standard requires that the C compiler assumes malloc returns an int. If there is no cast, a warning is issued when this integer is assigned to the pointer; however, with the cast, this warning is not produced, hiding a bug.

@Ron Burk 2013-02-14 16:15:37

You do cast, because:

  • It makes your code more portable between C and C++, and as SO experience shows, a great many programmers claim they are writing in C when they are really writing in C++ (or C plus local compiler extensions).
  • Failing to do so can hide an error: note all the SO examples of confusing when to write type * versus type **.
  • The idea that it keeps you from noticing you failed to #include an appropriate header file misses the forest for the trees. It's the same as saying "don't worry about the fact you failed to ask the compiler to complain about not seeing prototypes -- that pesky stdlib.h is the REAL important thing to remember!"
  • It forces an extra cognitive cross-check. It puts the (alleged) desired type right next to the arithmetic you're doing for the raw size of that variable. I bet you could do an SO study that shows that malloc() bugs are caught much faster when there's a cast. As with assertions, annotations that reveal intent decrease bugs.
  • Repeating yourself in a way that the machine can check is often a great idea. In fact, that's what an assertion is, and this use of cast is an assertion. Assertions are still the most general technique we have for getting code correct, since Turing came up with the idea so many years ago.

@hyde 2013-03-26 11:09:25

@ulidtko In case you did not know, it's possible to write code which compiles both as C and as C++. In fact most header files are like this, and they often contain code (macros and inline functions). Having a .c/.cpp file to compile as both is not useful very often, but one case is adding C++ throw support when compiled with C++ compiler (but return -1; when compiled with C compiler, or whatever).

@paulm 2013-05-06 17:55:52

If someone had malloc calls inline in a header I wouldn't be impressed, #ifdef __cplusplus and extern "C" {} are for this job, not adding in extra casts.

@Jonathan Baldwin 2013-07-20 00:28:01

Also nets you compatibility with compilers which compile C as C++ by default. (<cough>MSVC</cough>)

@Elias Van Ootegem 2013-12-23 15:37:42

Well, point 1 is irrelevant, since C != C++, the other points are also trivial, if you use the variable in your malloc call: char **foo = malloc(3*sizeof(*foo)); if quite full-proof: 3 pointers to char pointers. then loop, and do foo[i] = calloc(101, sizeof(*(foo[i])));. Allocate array of 101 chars, neatly initialized to zeroes. No cast needed. change the declaration to unsigned char or any other type, for that matter, and you're still good

@Norwæ 2014-04-16 11:10:00

Casting will not hide an error - in fact it introduces the possibility for further errors (casting an int to pointer, most obviously)

@Dr Beco 2014-09-26 03:22:34

When I tought I got it, there it comes! Fantastic answer. Its the first time here in StackOverflow that I +1 two opposite answers! +1 No, you dont cast, and +1 Yes, you do cast! LOL. You guys are terrific. And for me and my students, I made my mind: I do cast. The kind of errors students make are more easily spotted when casting.

@Ray Toal 2014-10-18 23:13:37

Interesting that @Leushenko was the only commenter here to state the very important point that the casting is not DRY. IMHO that is a showstopper and why I do not cast in C. I suck it up in C++ because I have to. That said, "repeating yourself" does have its use: in unit tests. But certainly not in production code. Repeating oneself is just asking for trouble. I'm glad we can all agree to disagree but I can't see myself ever being swayed by the casters' arguments. ;-)

@Jean-Baptiste Yunès 2015-01-27 10:55:48

Maybe donotcasters could read this document: securecoding.cert.org/confluence/display/seccode/…. I'm convinced that do not cast is more dangerous than casting and that casting the result of malloc is not "repeating yourself".

@Grzegorz Szpetkowski 2015-02-25 22:22:42

@Jean-BaptisteYunès: It's worth looking into comments below of this document. The rationale of p = malloc(sizeof(gadget)) has little sense, as more idiomatic form would be just p = malloc(sizeof *p). They didn't mention, that cast may be dangerous, producing nasty bugs, when <stdlib.h> header is not present (and C89 compilers are not obligated for any diagnostic in this case), as malloc is assumed to return an int.

@Mohit Jain 2015-05-12 05:27:56

So, do you also cast the argument of free() to void *?

@PC Luddite 2015-09-02 22:43:56

@MohitJain That argument makes no sense. In Java, would you cast the argument of a method that takes Object?

@supercat 2015-09-18 17:46:23

@Leushenko: Repeating yourself in a way that cannot be validated by machine nor by local inspection is bad. Repeating yourself in ways that can be validated by such means is less bad. Given struct Zebra *p; ... p=malloc(sizeof struct Zebra);, the malloc can't avoid duplciating information about p's type, but neither the compiler nor local code inspection would detect any problem if one type changed but the other didn't. Change the code to p=(struct Zebra*)malloc(sizeof struct Zebra); and the compiler will squawk if the cast type doesn't match p, and local inspection will reveal...

@supercat 2015-09-18 17:49:09

...if the cast type doesn't match the sizeof type. Someone examining the version of code with the typecast can know that (if the code compiles cleanly) p is either void* or struct Zebra* without having to look at the actual declaration of p. To my mind that's a win.

@Albert van der Horst 2016-04-01 17:29:52

"Repeating yourself in a way that the machine can check". No! If you leave out a cast (in general) you will get a warning if you do something unreasonable, or reasonable and not portable. If you use a cast, you say to the compiler: "Shut up! I've looked into it, and I know better.". Novices should not be taught the habit of putting up casts. malloc() is the one example where adding the cast does comparatively little harm.

@Don Hatch 2016-11-02 08:31:30

Great answer, thank you for it. Implicit conversions cause so many problems, and you do a good job of articulating why. Maybe reword the "stupendously stupid" part though? That would probably increase the chance of reaching people who don't already agree with you, rather than putting them on the defensive and making them unable hear the sensible things you're saying.

@Tom Lint 2016-11-14 15:18:19

@AlbertvanderHorst Novices should enable the compiler flag that turns implicit conversions into errors.

@AnT 2017-06-14 15:45:11

Lots of fallacies in this answer. Basically, the only valid argument is the first one: cross-compilable C/C++ code. But it is only applicable to cross-compilable C/C++ code, not a reason to use the cast everywhere. The remaing arguments are fallacies based on the failure to undestand a simple fact: avoiding the cast is not a purpose in itself, but a part of wider idiomatic practice - writing type-independent/type-agnostic code.

@AnT 2017-06-14 15:48:26

Tha above reasoning tries to solve problems present in T *p = malloc(N * sizeof(T)). But you shouldn't do it this way in the first place. The proper idiomatic type-agnostic way to allocate memory calls for T *p = malloc(N * sizeof *p), i.e. the type names are not mentioned in size expression at all. This version is completely free of any issues the above answer tries to "solve". In other words, explicit cast on the result of memory allocation function is a classc "solution looking for a problem". A problem you yourself created. Don't create the problem, and you won't need the solution.

@Luis Masuelli 2018-04-26 16:33:16

Why do most of you say casting is bad because if you cast an pointer as an integer you may lose bits depending on the platform if the whole point here is regarding casting void* to an arbitrary but needed T*?

@Kemin Zhou 2018-11-10 17:00:32

Look the example, do not use a case: en.cppreference.com/w/c/memory/malloc

@Jonathan Leffler 2018-11-19 18:44:43

@Jean-BaptisteYunès: The SEI reference for MEM02-C: Immediately cast the result of a memory allocation function call into a pointer to the allocated type is at a new location (link valid today; who knows about tomorrow).

@user877329 2015-06-12 15:23:56

The best thing to do when programming in C whenever it is possible:

  1. Make your program compile through a C compiler with all warnings turned on -Wall and fix all errors and warnings
  2. Make sure there are no variables declared as auto
  3. Then compile it using a C++ compiler with -Wall and -std=c++11. Fix all errors and warnings.
  4. Now compile using the C compiler again. Your program should now compile without any warning and contain fewer bugs.

This procedure lets you take advantage of C++ strict type checking, thus reducing the number of bugs. In particular, this procedure forces you to include stdlib.hor you will get

malloc was not declared within this scope

and also forces you to cast the result of malloc or you will get

invalid conversion from void* to T*

or what ever your target type is.

The only benefits from writing in C instead of C++ I can find are

  1. C has a well specified ABI
  2. C++ may generate more code [exceptions, RTTI, templates, runtime polymorphism]

Notice that the second cons should in the ideal case disappear when using the subset common to C together with the static polymorphic feature.

For those that finds C++ strict rules inconvenient, we can use the C++11 feature with inferred type

auto memblock=static_cast<T*>(malloc(n*sizeof(T))); //Mult may overflow...

@Toby Speight 2015-06-25 21:06:29

Use a C compiler for C code. Use a C++ compiler for C++ code. No ifs, no buts. Rewriting your C code in C++ is another thing entirely, and may - or may not be - worth the time and the risks.

@autistic 2015-12-07 16:06:56

I'd like to add to @TobySpeight advice: If you need to use C code in a C++ project, you can usually compile the C code as C (e.g. gcc -c c_code.c), the C++ code as C++ (e.g. g++ -c cpp_code.cpp), and then link them together (e.g. gcc c_code.o cpp_code.o or vice-versa depending upon the project dependencies). Now there should be no reason to deprive yourself of any nice features of either language...

@autistic 2015-12-08 00:54:29

@user877329 It's a more sensible alternative to painstakingly adding casts to code that reduce the code's legibility, only for the sake of being "C++ compatible".

@Peter Cordes 2016-10-24 15:11:33

Other advantages to C over C++: C99 variable-length arrays for very efficient allocation/deallocation of scratch space. You can't accidentally write non-constant initializers when you didn't mean to (e.g. if you thought you were being clever by putting static const __m128 ones = _mm_set1_ps(1.0f); at the global scope so multiple functions could share a constant, the fact that constructors aren't a thing in C stops you from generating worse code. (This is really finding a silver lining to a C limitation...))

@Peter Cordes 2016-10-24 15:13:48

Probably the main advantage in this context is that C lets you write p = malloc(sizeof(*p));, which doesn't need changing in the first place if p changes to a different type name. The proposed "advantage" of casting is that you get a compile error if p is the wrong type, but it's even better if it Just Works.

@dhana govindarajan 2016-07-18 10:36:57

A void pointer is a generic pointer and C supports implicit conversion from a void pointer type to other types, so there is no need of explicitly typecasting it.

However, if you want the same code work perfectly compatible on a C++ platform, which does not support implicit conversion, you need to do the typecasting, so it all depends on usability.

@Kaz 2016-03-30 00:23:45

I put in the cast simply to show disapproval of the ugly hole in the type system, which allows code such as the following snippet to compile without diagnostics, even though no casts are used to bring about the bad conversion:

double d;
void *p = &d;
int *q = p;

I wish that didn't exist (and it doesn't in C++) and so I cast. It represents my taste, and my programming politics. I'm not only casting a pointer, but effectively, casting a ballot, and casting out demons of stupidity. If I can't actually cast out stupidity, then at least let me express the wish to do so with a gesture of protest.

In fact, a good practice is to wrap malloc (and friends) with functions that return unsigned char *, and basically never to use void * in your code. If you need a generic pointer-to-any-object, use a char * or unsigned char *, and have casts in both directions. The one relaxation that can be indulged, perhaps, is using functions like memset and memcpy without casts.

On the topic of casting and C++ compatibility, if you write your code so that it compiles as both C and C++ (in which case you have to cast the return value of malloc when assigning it to something other than void *), you can do a very helpful thing for yourself: you can use macros for casting which translate to C++ style casts when compiling as C++, but reduce to a C cast when compiling as C:

/* In a header somewhere */
#ifdef __cplusplus
#define strip_qual(TYPE, EXPR) (const_cast<TYPE>(EXPR))
#define convert(TYPE, EXPR) (static_cast<TYPE>(EXPR))
#define coerce(TYPE, EXPR) (reinterpret_cast<TYPE>(EXPR))
#else
#define strip_qual(TYPE, EXPR) ((TYPE) (EXPR))
#define convert(TYPE, EXPR) ((TYPE) (EXPR))
#define coerce(TYPE, EXPR) ((TYPE) (EXPR))
#endif

If you adhere to these macros, then a simple grep search of your code base for these identifiers will show you where all your casts are, so you can review whether any of them are incorrect.

Then, going forward, if you regularly compile the code with C++, it will enforce the use of an appropriate cast. For instance, if you use strip_qual just to remove a const or volatile, but the program changes in such a way that a type conversion is now involved, you will get a diagnostic, and you will have to use a combination of casts to get the desired conversion.

To help you adhere to these macros, the the GNU C++ (not C!) compiler has a beautiful feature: an optional diagnostic which is produced for all occurrences of C style casts.

     -Wold-style-cast (C++ and Objective-C++ only)
         Warn if an old-style (C-style) cast to a non-void type is used
         within a C++ program.  The new-style casts (dynamic_cast,
         static_cast, reinterpret_cast, and const_cast) are less vulnerable
         to unintended effects and much easier to search for.

If your C code compiles as C++, you can use this -Wold-style-cast option to find out all occurrences of the (type) casting syntax that may creep into the code, and follow up on these diagnostics by replacing it with an appropriate choice from among the above macros (or a combination, if necessary).

This treatment of conversions is the single largest standalone technical justification for working in a "Clean C": the combined C and C++ dialect, which in turn technically justifies casting the return value of malloc.

@Phil1970 2016-06-10 15:53:04

As other pointed out, I would usually recommend to not mix C and C++ code. However, if you have good reason to do it, then macros might be useful.

@Kaz 2016-06-10 16:45:03

@Phil1970 It's all written in one cohesive dialect, which happens to be portable to C and C++ compilers, and takes advantage of some capabilities of C++. It must be all compiled as C++, or else all compiled as C.

@Kaz 2016-06-14 17:35:46

I.e. what I was trying to say in the previous comment is that there is no mixing of C and C++. The intent is that the code is all compiled as C or all compiled as C++.

@dirkgently 2009-03-03 10:17:21

In C, you don't need to cast the return value of malloc. The pointer to void returned by malloc is automagically converted to the correct type. However, if you want your code to compile with a C++ compiler, a cast is needed. A preferred alternative among the community is to use the following:

int *sieve = malloc(sizeof *sieve * length);

which additionally frees you from having to worry about changing the right-hand side of the expression if ever you change the type of sieve.

Casts are bad, as people have pointed out. Specially pointer casts.

@Michael Anderson 2015-04-30 07:02:33

@MAKZ I'd argue that malloc(length * sizeof *sieve) makes it look like sizeof is a variable - so I think malloc(length * sizeof(*sieve)) is more readable.

@Toby Speight 2015-08-20 13:01:17

And malloc(length * (sizeof *sieve)) more readable still. IMHO.

@chux 2015-12-10 16:40:42

@Michael Anderson () issue aside, note that your suggested style switched the order., Consider when element count is computed like length*width, keeping the sizeof first in this case insures multiplication is done with at least size_t math. Compare malloc(sizeof( *ptr) * length * width) vs. malloc(length * width * sizeof (*ptr)) - the 2nd may overflow the length*width when width,length are smaller types that size_t.

@Michael Anderson 2015-12-11 00:18:32

@chux it's not obvious, but the answer has been edited so that my comment is less pertinent - the original suggestion was malloc(sizeof *sieve * length)

@David C. 2016-02-12 23:12:57

C is not C++. Pretending that they are will ultimately lead to confusion and sadness. If you're using C++, then a C-style cast is also bad (unless you're using a very old C++ compiler). And static_cast>() (or reinterpret_cast<>() )is not compatible with any dialect of C.

@Cubic 2016-08-27 10:29:18

@TobySpeight And I'd argue that calloc(length, sizeof(*sieve)) is more readable than any of the alternatives I've seen proposed on this question.

@Daniel H 2017-09-19 22:11:43

@Cubic Sure, but if you don’t need the memory zeroed it can be slower on some systems.

@David 天宇 Wong 2018-11-26 18:22:49

This is ugly, and not initialized. You need to use calloc and sizeof()

@StephenG 2015-12-04 17:27:54

People used to GCC and Clang are spoiled. It's not all that good out there.

I have been pretty horrified over the years by the staggeringly aged compilers I've been required to use. Often companies and managers adopt an ultra-conservative approach to changing compilers and will not even test if a new compiler ( with better standards compliance and code optimization ) will work in their system. The practical reality for working developers is that when you're coding you need to cover your bases and, unfortunately, casting mallocs is a good habit if you cannot control what compiler may be applied to your code.

I would also suggest that many organizations apply a coding standard of their own and that that should be the method people follow if it is defined. In the absence of explicit guidance I tend to go for most likely to compile everywhere, rather than slavish adherence to a standard.

The argument that it's not necessary under current standards is quite valid. But that argument omits the practicalities of the real world. We do not code in a world ruled exclusively by the standard of the day, but by the practicalities of what I like to call "local management's reality field". And that's bent and twisted more than space time ever was. :-)

YMMV.

I tend to think of casting malloc as a defensive operation. Not pretty, not perfect, but generally safe. ( Honestly, if you've not included stdlib.h then you've way more problems than casting malloc ! ).

@ryyker 2017-04-25 15:13:02

Well stated alternative view, but what does YMMV mean? (your mileage may vary is the most common definition I could find, but do not see how it applies here :)

@user3949394 2015-09-10 15:58:49

Casting is only for C++ not C.In case you are using a C++ compiler you better change it to C compiler.

@August Karlstrom 2015-09-04 11:52:04

In the C language, a void pointer can be assigned to any pointer, which is why you should not use a type cast. If you want "type safe" allocation, I can recommend the following macro functions, which I always use in my C projects:

#include <stdlib.h>
#define NEW_ARRAY(ptr, n) (ptr) = malloc((n) * sizeof *(ptr))
#define NEW(ptr) NEW_ARRAY((ptr), 1)

With these in place you can simply say

NEW_ARRAY(sieve, length);

For non-dynamic arrays, the third must-have function macro is

#define LEN(arr) (sizeof (arr) / sizeof (arr)[0])

which makes array loops safer and more convenient:

int i, a[100];

for (i = 0; i < LEN(a); i++) {
   ...
}

@chux 2017-07-27 21:39:36

"a void pointer can be assigned to any object pointer" Function pointers are another issue, albeit not a malloc() one.

@chux 2017-12-26 19:09:04

Assigning a void* to/from a function pointer may lose information so "a void pointer can be assigned to any pointer," is a problem in those cases. Assigning a void*, from malloc() to any object pointer is not an issue though.

@chux 2017-12-27 15:44:20

The do loop comment related to macros involving a loop yet that is wondering away from the title question. Removing that comment. Will take this one down later too.

@iec2011007 2015-07-29 05:53:42

The concept behind void pointer is that it can be casted to any data type that is why malloc returns void. Also you must be aware of automatic typecasting. So it is not mandatory to cast the pointer though you must do it. It helps in keeping the code clean and helps debugging

@Toby Speight 2015-08-20 12:54:52

"It is not mandatory -- though you must do it" - I think there's a contradiction there!

@Bill Woodger 2015-09-08 12:28:48

I think you should read this post to someone, and see if they understand what you are trying to say. Then rewrite it, making it clear what you want to say. I really can't understand what your answer is.

@user968000 2013-02-07 22:22:34

It is not mandatory to cast the results of malloc, since it returns void* , and a void* can be pointed to any datatype.

@user3079666 2014-03-28 18:21:31

Just adding my experience, studying computer engineering I see that the two or three professors that I have seen writing in C always cast malloc, however the one I asked (with an immense CV and understanding of C) told me that it is absolutely unnecessary but only used to be absolutely specific, and to get the students into the mentality of being absolutely specific. Essentially casting will not change anything in how it works, it does exactly what it says, allocates memory, and casting does not effect it, you get the same memory, and even if you cast it to something else by mistake (and somehow evade compiler errors) C will access it the same way.

Edit: Casting has a certain point. When you use array notation, the code generated has to know how many memory places it has to advance to reach the beginning of the next element, this is achieved through casting. This way you know that for a double you go 8 bytes ahead while for an int you go 4, and so on. Thus it has no effect if you use pointer notation, in array notation it becomes necessary.

@Lundin 2014-05-27 07:56:53

Except as already mentioned, the cast might hide bugs and make the code harder to analyse for the compiler or static analyser.

@chux 2014-08-23 19:29:50

"Essentially casting will not change anything in how it works". Casting to the matching type should not change anything, but should the var's type change and the cast no longer match, could problems come up? IWOs, the cast and var type should be kept in sync - twice the maintenance work.

@chux 2017-07-27 21:38:01

I can see why Profs prefer casting. Casting may be useful from an educational standpoint where it conveys to the instructor information and the student code does not need to be maintained - its throw-away code. Yet from a coding, peer-review and maintenance perspective, p = malloc(sizeof *p * n); is so simple and better.

@Endeavour 2014-07-13 06:42:25

A void pointer is a generic pointer and C supports implicit conversion from a void pointer type to other types, so there is no need of explicitly typecasting it.

However, if you want the same code work perfectly compatible on a C++ platform, which does not support implicit conversion, you need to do the typecasting, so it all depends on usability.

@Toby Speight 2015-08-20 12:57:54

It's not a normal use case to compile a single source as both C and C++ (as opposed, say, to using a header file containing declarations to link C and C++ code together). Using malloc and friends in C++ is a good warning sign that it deserves special attention (or re-writing in C).

@chux 2018-08-21 14:05:48

"A void pointer is a generic pointer" --> "A void pointer is a generic object pointer". Function pointers sizes can exceed void * , thus void * is insufficient to store a function pointer well.

@swaps 2013-04-26 16:43:31

The returned type is void*, which can be cast to the desired type of data pointer in order to be dereferenceable.

@Toby Speight 2015-08-20 12:59:37

void* can be cast to the desired type, but there is no need to do so as it will be automatically converted. So the cast is not necessary, and in fact undesirable for the reasons mentioned in the high-scoring answers.

@Jeyamaran 2014-03-17 13:16:41

It depends on the programming language and compiler. If you use malloc in C there is no need to type cast it, as it will automatically type cast, However if your using C++ then you should type cast because malloc will return a void* type.

@August Karlstrom 2015-09-05 09:19:39

The function malloc returns a void pointer in C as well but the rules of the language are different from C++.

@Yu Hao 2013-06-09 17:31:35

Casting the value returned by malloc() is not necessary now, but I'd like to add one point that seems no one has pointed out:

In the ancient days, that is, before ANSI C provides the void * as the generic type of pointers, char * is the type for such usage. In that case, the cast can shut down the compiler warnings.

Reference: C FAQ

@Albert van der Horst 2016-04-01 17:33:56

Shutting up compiler warnings is a bad idea.

@Dan Bechard 2016-04-11 17:17:37

@AlbertvanderHorst Not if you're doing so by solving the exact problem the warning is there to warn you of.

@Albert van der Horst 2016-11-02 16:51:18

@Dan . If by solving the exact problem is meant a rewrite of a subroutine to return modern ANSI C types instead of char *, I agree. I wouldn't call that shutting up the compiler. Do not give in to managers who insists that there are no compiler warnings , instead of using them by each recompilation to find possible problems. Groetjes Albert

@PaulJWilliams 2009-03-03 10:18:12

In C you can implicitly convert a void pointer to any other kind of pointer, so a cast is not necessary. Using one may suggest to the casual observer that there is some reason why one is needed, which may be misleading.

@EFraim 2009-03-03 10:16:21

In C you get an implicit conversion from void* to any other (data) pointer.

@EFraim 2012-04-29 07:17:41

@Jens: OK, maybe the more proper wording is "implicit conversion". Like use of integral variable in floating point expression.

@Mad Physicist 2015-11-03 19:13:22

@EFraim That would actually result in a cast, and an implicit one at that.

@quinmars 2009-03-03 11:17:46

As other stated, it is not needed for C, but for C++. If you think you are going to compile your C code with a C++ compiler, for which reasons ever, you can use a macro instead, like:

#ifdef __cplusplus
# define NEW(type, count) ((type *)calloc(count, sizeof(type)))
#else
# define NEW(type, count) (calloc(count, sizeof(type)))
#endif

That way you can still write it in a very compact way:

int *sieve = NEW(int, 1);

and it will compile for C and C++.

@Hosam Aly 2009-03-04 06:13:08

Since you're using a macro anyway, why don't you use new in the definition of C++?

@quinmars 2009-03-04 08:51:30

Because there is no reason to do so. It is mainly for C programs that are compiled with a C++ compiler. If you are going to use 'new', the only thing you get are problems. You need then also a macro for free. And you need a macro to free an array, a differentiation that doesn't exists in C.

@quinmars 2009-03-04 08:53:28

Not to mention if it's not you who frees the memory but maybe a C library you are using, etc. Many possible problems without any gain.

@Hosam Aly 2009-03-04 12:38:52

Hmmm... I didn't think of that. Is it an error to use free() to free memory allocated with new?

@Graeme Perrow 2011-07-16 17:10:33

@Hosam: Yes, it definitely is. If you use new you must use delete and if you use malloc() you must you free(). Never mix them.

@chux 2013-06-07 15:24:18

Functions, variables, keywords and macros that only differ in case, though allowed, provide small differentiation. Maybe CNEW(t, n)

@quinmars 2013-10-25 20:28:55

@chux, well, new is not a keyword in C and in a real C++ program I wouldn't use this macro at all, so no confusion.

@mah 2014-04-16 15:23:27

If one is going to take this approach, calling the macro NEW is probably a bad idea since the resource is never returned using delete (or DELETE) so you're mixing your vocabulary. Instead, naming it MALLOC, or rather CALLOC in this case, would make more sense.

@Dave Branton 2014-09-29 06:44:18

I'd be pretty unhappy seeing a #define used in this way. I guess if there's really no other option, it might be ok. But one day you're going to wish you hadn't done it. At least give the damn thing a better name than NEW.

@quinmars 2014-09-29 08:05:59

@DaveBranton, in what way do you see the use of #define critically? The macro does not alter any variable, the variable is only used once, it's a simple expression and not a sequence of statements. Plus, it cannot be done with a inline function. So I wonder what makes you worry.

@Niccolo M. 2014-12-31 03:16:31

GLib ineed defines this macro. It calls it g_new, and g_new0.

@chqrlie 2015-02-26 19:57:01

Actually it is much safer to add the cast in C if you use this macro. It will allow the compiler to catch this mistake: long array = NEW_INT(int, n);

@Luis Colorado 2015-04-14 05:22:41

Why to #define a new NEW if there is an old new operator in C++ ??? malloc(3) and free(3) are severely discouraged use functions from C in C++ (they are supported only for the claim of portability of C into C++). C++ encourages the use of new and delete operators instead.

@milevyo 2015-10-12 16:51:17

is NEW a keyword in c++?. i guess not, and if i am right, the macro by @quinmars is quite a good solution. and there will be no troubles in using it.

@Dragon Energy 2015-12-13 06:26:16

@LuisColorado This solution is for interop-style code between C and C++. It's not about doing things in a C++-idiomatic way, it's about making C code compile and port easily to a C++ compiler.

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