Trace Function Calls Using GDB Revisited

In an earlier post I discussed how to trace calls using GDB so that function calls and their arguments can easily be viewed. What I neglected to mention was rbreak , a feature of GDB to be able to set breakpoints using a regular expression.

Using rbreak you can get the same functionality but with much less effort. For example, to get the same behavior as before (setting a breakpoint on each function call and printing a trace of the bottom most level) all you need to provide to GDB are the following commands:

set args 4 10

rbreak file.c:.
command
silent
backtrace 1
continue 
end

run

I have placed these commands in a file called gdb_commands. The rbreak command above tells GDB to set a breakpoint on any function call within file.c. With a quick call to GDB we find almost the exact same functionality as we had before.

dustymabe@laptop: gdbpost>gdb -quiet -command=gdb_commands ./a.out
Reading symbols from
/content/gdbpost/a.out...done.
Breakpoint 1 at 0x400578: file file.c, line 12.
int main(int, char **);
Breakpoint 2 at 0x400546: file file.c, line 7.
int modulo(int, int);
Breakpoint 3 at 0x40055f: file file.c, line 8.
int plus(int, int);
Breakpoint 4 at 0x400533: file file.c, line 6.
int squared(int);
#0  main (argc=3, argv=0x7fffffffe698) at file.c:12
#0  squared (x=4) at file.c:6
#0  squared (x=10) at file.c:6
#0  modulo (x=100, y=16) at file.c:7
#0  plus (x=100, y=100) at file.c:8
#0  modulo (x=200, y=16) at file.c:7
#0  plus (x=200, y=100) at file.c:8
#0  modulo (x=300, y=16) at file.c:7
#0  plus (x=300, y=100) at file.c:8
#0  modulo (x=400, y=16) at file.c:7
LCM is 400
[Inferior 1 (process 1579) exited with code 013]
Missing separate debuginfos, use: debuginfo-install
glibc-2.15-56.fc17.x86_64
(gdb) quit

In my previous example, if you had 50 functions that you wanted to trace, you needed 50 break commands as well as 50 command blocks.

Considering this fact, it is apparent that rbreak is much more efficient and friendly for when you are setting breakpoints from the GDB cli. Another extremely useful feature is that it also works for c++ classes, so you can break on any function in a class regardless of what file the function is defined in.

To show an example of this I converted all the math function calls from the program in file.c to a c++ class. I put the result in a file called file.cc (shown below).

#include <stdio.h>
#include <stdlib.h>
// A program that will square two integers and then find the LCM
// of the resulting two integers. 
class Math {
    public:
        int squared(int x)       { return x*x; }
        int modulo(int x, int y) { return x%y; }
        int plus(int x, int y)   { return x+y; }
};

int main(int argc, char *argv[]) {
    int x, y, x2, y2, tmp;
    if (argc != 3) return 0; 

    Math* m = new Math;

    x = atoi(argv[1]);  
    y = atoi(argv[2]);
    x2       = m->squared(x);
    y2 = tmp = m->squared(y);

    while (1) {
        if (m->modulo(tmp,x2) == 0) break;
        tmp = m->plus(tmp,y2);
    }
    printf("LCM is %d\n", tmp);
}

I then used the rbreak Math:: command in the following GDB commands file (I named this one gdb_commands2) to run GDB and set up the break points.

set args 4 10

rbreak Math::
command
silent
backtrace 1
continue 
end

run

And.. Here we are with the final result:

dustymabe@laptop: gdbpost>gdb -quiet -command=gdb_commands2 ./a.out
Reading symbols from
/content/gdbpost/a.out...done.
Breakpoint 1 at 0x400758: file file.cc, line 9.
int Math::modulo(int, int);
Breakpoint 2 at 0x400776: file file.cc, line 10.
int Math::plus(int, int);
Breakpoint 3 at 0x400741: file file.cc, line 8.
int Math::squared(int);
#0  Math::squared (this=0x601010, x=4) at file.cc:8
#0  Math::squared (this=0x601010, x=10) at file.cc:8
#0  Math::modulo (this=0x601010, x=100, y=16) at file.cc:9
#0  Math::plus (this=0x601010, x=100, y=100) at file.cc:10
#0  Math::modulo (this=0x601010, x=200, y=16) at file.cc:9
#0  Math::plus (this=0x601010, x=200, y=100) at file.cc:10
#0  Math::modulo (this=0x601010, x=300, y=16) at file.cc:9
#0  Math::plus (this=0x601010, x=300, y=100) at file.cc:10
#0  Math::modulo (this=0x601010, x=400, y=16) at file.cc:9
LCM is 400
[Inferior 1 (process 2304) exited normally]
Missing separate debuginfos, use: debuginfo-install
glibc-2.15-56.fc17.x86_64 libgcc-4.7.0-5.fc17.x86_64
libstdc++-4.7.0-5.fc17.x86_64
(gdb) quit

Enjoy!

Dusty

PS - If you are new to GDB I have found a great reference for beginners here.