How to 'tag' a location in a C source file for a later breakpoint definition?

user_3609976 0
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Problem: I want to be able to put different potentially unique or repeated "tags" across my C code, such that I can use them in gdb to create breakpoints.

Similar Work:

  • Breakpoints to line-numbers: The main difference with breakpoints on source lines, is that if the code previous to the tag is modified in such a way that it results in more or less lines, a reference to the tag would still be semantically correct, a reference to the source line would not.

  • Labels: I am coming from my previous question, How to tell gcc to keep my unused labels?, in which I preconceived the idea that the answer was to insert labels. Upon discussion with knowledgeable members of the platform, I was taught that label's names are not preserved after compilation. Labels not used within C are removed by the compiler.

  • Injecting asm labels: Related to the previous approach, if I inject asm code in the C source, certain problems arise, due to inline functions, compiler optimizations, and lack of scoping. This makes this approach not robust.

  • Define a dummy function: On this other question, Set GDB breakpoint in C file, there is an interesting approach, in which a "dummy" function can be placed in the code, and then add a breakpoint to the function call. The problem with this approach is that the definition of such function must be replicated for each different tag.

Is there a better solution to accomplish this? Or a different angle to attack the presented problem?

assembly gdb gcc breakpoints c
4 Answers

Using SDT (Statically Defined Tracing) probe points appears to satisfy all the requirements.

GDB documentation links to examples of how to define the probes.

Example use: (gdb) break -probe-stap my_probe (this should be documented in the GDB breakpoints section, but currently isn't).

Employed_Russian_506170

You could create a dummy variable and set it to different values. Then you can use conditional watchpoints. Example:

#include <stdio.h>
static volatile int loc;
int main()
{
    loc = 1;
    puts("hello world");
    loc = 2;
    return 0;
}

(gdb) watch loc if loc == 2
Hardware watchpoint 1: loc
(gdb) r
Starting program: /tmp/a.out 
hello world

Hardware watchpoint 1: loc

Old value = 1
New value = 2
main () at test.c:8
8       return 0;

You can of course wrap the assignment in a macro so you only get it in debug builds. Usual caveats apply: optimizations and inlining may be affected.

user_5479810

Use python to search a source file for some predefined labels, and set breakpoints there:

def break_on_labels(source, label):
    """add breakpoint on each SOURCE line containing LABEL"""
    with open(source) as file:
        l = 0
        for line in file:
            l = l + 1
            if label in line:
                gdb.Breakpoint(source=source, line=l)

main_file = gdb.lookup_global_symbol("main").symtab.fullname()
break_on_labels(main_file, "BREAK-HERE")

Example:

int main(void)
{
  int a = 15;
  a = a + 23; // BREAK-HERE
  return a;
}
user_19833980

You could insert a #warning at each line where you want a breakpoint, then have a script to parse the file and line numbers from the compiler messages and write a .gdbinit file placing breakpoints at those locations.

user_6349190
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