By John Carter


2008-10-31 10:33:14 8 Comments

I want to examine the contents of a std::vector in GDB, how do I do it? Let's say it's a std::vector<int> for the sake of simplicity.

4 comments

@Nikhil 2009-03-01 14:53:31

'Watching' STL containers while debugging is somewhat of a problem. Here are 3 different solutions I have used in the past, none of them is perfect.

1) Use GDB scripts from http://clith.com/gdb_stl_utils/ These scripts allow you to print the contents of almost all STL containers. The problem is that this does not work for nested containers like a stack of sets.

2) Visual Studio 2005 has fantastic support for watching STL containers. This works for nested containers but this is for their implementation for STL only and does not work if you are putting a STL container in a Boost container.

3) Write your own 'print' function (or method) for the specific item you want to print while debugging and use 'call' while in GDB to print the item. Note that if your print function is not being called anywhere in the code g++ will do dead code elimination and the 'print' function will not be found by GDB (you will get a message saying that the function is inlined). So compile with -fkeep-inline-functions

@kristopolous 2012-01-03 05:02:54

That link is a 404.

@badeip 2014-08-26 06:57:03

put the following in ~/.gdbinit

define print_vector
    if $argc == 2
        set $elem = $arg0.size()
        if $arg1 >= $arg0.size()
            printf "Error, %s.size() = %d, printing last element:\n", "$arg0", $arg0.size()
            set $elem = $arg1 -1
        end
        print *($arg0._M_impl._M_start + $elem)@1
    else
        print *($arg0._M_impl._M_start)@$arg0.size()
    end
end

document print_vector
Display vector contents
Usage: print_vector VECTOR_NAME INDEX
VECTOR_NAME is the name of the vector
INDEX is an optional argument specifying the element to display
end

After restarting gdb (or sourcing ~/.gdbinit), show the associated help like this

gdb) help print_vector
Display vector contents
Usage: print_vector VECTOR_NAME INDEX
VECTOR_NAME is the name of the vector
INDEX is an optional argument specifying the element to display

Example usage:

(gdb) print_vector videoconfig_.entries 0
$32 = {{subChannelId = 177 '\261', sourceId = 0 '\000', hasH264PayloadInfo = false, bitrate = 0,     payloadType = 68 'D', maxFs = 0, maxMbps = 0, maxFps = 134, encoder = 0 '\000', temporalLayers = 0 '\000'}}

@user1541776 2017-04-17 08:46:31

thank you for the code! I guess there is a typo and "print *($arg0._M_impl._M_start + $elem)@1" should be "print *($arg0._M_impl._M_start + $arg1)@1"? I use the following modification: define print_vector if $argc == 2 if $arg1 >= $arg0.size()-1 printf "Error, %s.size() = %d, printing last element:\n", "$arg0", $arg0.size()-1 end print *($arg0._M_impl._M_start + $arg1)@1 else print *($arg0._M_impl._M_start)@$arg0.size() end end

@typelogic 2019-10-20 05:44:48

el magnifico! mochas gracias

@John Carter 2008-10-31 10:33:23

With GCC 4.1.2, to print the whole of a std::vector<int> called myVector, do the following:

print *(myVector._M_impl._M_start)@myVector.size()

To print only the first N elements, do:

print *(myVector._M_impl._M_start)@N

Explanation

This is probably heavily dependent on your compiler version, but for GCC 4.1.2, the pointer to the internal array is:

myVector._M_impl._M_start 

And the GDB command to print N elements of an array starting at pointer P is:

print [email protected]

Or, in a short form (for a standard .gdbinit):

p [email protected]

@John Carter 2008-10-31 11:10:42

Hehe, it's something that's bugged me before, so I just looked it up this morning and added it as a memo to myself (as Jeff himself recommended).

@mariner 2014-01-07 01:26:43

Also if you want just a particular vector element, myVector._M_impl._M_start + n (for the nth element)

@wallyk 2015-04-30 17:29:49

Not working for me. Cannot evaluate function -- may be inlined

@jfritz42 2015-08-31 22:08:06

To print a single element, e.g. the 2nd element: print (myVector._M_impl._M_start)[2]

@Eponymous 2015-11-30 17:53:27

To find the special names (_M_impl etc) for your compiler under GDB 7.0+, use print /r myVector

@bcumming 2015-12-01 10:34:00

with GCC 5.2.0 (though I am sure it works for earlier versions), you can use print *(&myVector[0])@myVector.size(), which requires no specific knowledge of GCC stdlib internals.

@agam 2019-05-23 19:22:56

Thanks @Eponymous !! I was wondering how to arrive at the right inner member names.

@typelogic 2019-10-20 05:23:46

Not sure how much it helps in debugability, but in addition to this tip I also compiled with g++ -ggdb -O0

@MichaƂ Oniszczuk 2010-01-23 13:23:51

To view vector std::vector myVector contents, just type in GDB:

(gdb) print myVector

This will produce an output similar to:

$1 = std::vector of length 3, capacity 4 = {10, 20, 30}

To achieve above, you need to have gdb 7 (I tested it on gdb 7.01) and some python pretty-printer. Installation process of these is described on gdb wiki.

What is more, after installing above, this works well with Eclipse C++ debugger GUI (and any other IDE using GDB, as I think).

@wallyk 2015-04-30 17:34:44

This works fine as long as the vector elements are directly interpretable. But it doesn't help if the vector contains pointers to the items of interest.

@Enrico Maria De Angelis 2020-01-05 13:38:43

I frankly don't find the gdb wiki page particularly readable, maybe because it's "slightly" outdated now? For instance, I had the impression that the suggested content of the $HOME/.gdbinit was necessary. At the moment I end up with no such file at all and gdb correctly showing the content of std::vector. However, since during my "rambling" attempts I just installed and then unistalled cgdb, and I already had libstdc++5 installed, I have no idea why the pretty printing did not work while now it works.

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