linker - Set a breakpoint into LibC with gdb -

why cannot set breakpoint (using gdb) in exported function within libc? being libc dynamically linked, must contains symbols of functions exports. shouldn't able set breakpoint of these functions?

i merely tried do:

(gdb) b _io_vfprintf@@glibc_2.2.5 function "_io_vfprintf@@glibc_2.2.5" not defined. 

but looking @ dynamyc-symbols table in elf file symbol exist:

 127: 0000000000049cf0 20904 func    global default   12 _io_vfprintf@@glibc_2.2.5 

i don't know how came symbol name using, here's see on system (ubuntu 14.04.1):

$ objdump --dynamic-syms /lib/x86_64-linux-gnu/libc.so.6 |grep vfprintf 0000000000049cf0 g    df .text  00000000000051a8  glibc_2.2.5 _io_vfprintf 00000000001097e0 g    df .text  0000000000000111  glibc_2.3.4 __vfprintf_chk 0000000000049cf0 g    df .text  00000000000051a8  glibc_2.2.5 vfprintf 

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here's demonstration program:

   #include <stdio.h>    #include <stdarg.h>  int myprintf( const char *format, ... )    {    va_list ap;    va_start( ap, format );    int result = _io_vfprintf( stderr, format, ap );    va_end(ap);    return result;    }  int main()    {    myprintf( "hello world! %s %s %s\n", "abc", "def", "ghi" );    myprintf( "goodbye world! %d %d\n", 123, 456 );    return 0;    } 

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i found complains less if first run main(), then set breakpoint b _io_vfprintf.

$ make cflags="-wall -werror -g" test && ./test  $ objdump --disassemble test |grep vfprintf ## verify call isn't inlined 0000000000400480 <_io_vfprintf@plt>:   40061e:   e8 5d fe ff ff          callq  400480 <_io_vfprintf@plt>  $ gdb --quiet ./test reading symbols ./test...done.  (gdb) b main breakpoint 1 @ 0x400635: file test.c, line 16.  (gdb) run starting program: .../test   breakpoint 1, main () @ test.c:16 16     myprintf( "hello world! %s %s %s\n", "abc", "def", "ghi" );  (gdb) b _io_vfprintf breakpoint 2 @ 0x7ffff7a5ecf4  (gdb) cont continuing.  breakpoint 2, 0x00007ffff7a5ecf4 in vfprintf () /lib/x86_64-linux-gnu/libc.so.6 

so yes, works...

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taking next level -- can step through libc source applying following commands...

$ sudo apt-get install libc6-dbg ## debug symbols $ apt-get source libc-dev-bin ## download source (on ubuntu or similar) $ gdb --quiet --directory ./eglibc-2.19/stdio-common ./test 

related notes here.