c - How to debug the entry-point of fork-exec process in GDB?

Question

I have a C linux application (A) that spawns another process (P) when it is started. When I want to debug P I start A as usual and I connect with ddd/gdb to P.

Problems appear when I want to debug the entry-point (start of main) of P. If I follow the usual approach when I connect the debugger to P is already to late. The solution I've found was to insert a sleep at the begining of the main of P so I have time to connect with gdb but this is not a very elegant solution.

I've also tried using asm("int $3") but it doesn't seems to work.

Do you have any idea how I could solve this problem? (preferably without altering the code of A or P)

Answer 1

You should use this option:

set follow-fork-mode mode

Where mode is one of parent, child or ask.

To follow the parent (this is the default) use:

set follow-fork-mode parent

To follow the child:

set follow-fork-mode child

To have the debugger ask you each time:

set follow-fork-mode ask

So basically you'd start out connecting gdb to A, then set gdb to follow the child, and then when A spawns P, gdb will connect to P and detach from A.