gdb nostop SIGSEGV on a specific thread

Question

I have a program that purposely segfaults on one threads, but I have a problem that the other thread is segfaulting, I'd like to catch it with GDB, I saw that I can:

handle SIGSEGV nostop noprint

but I'd like to do that only on the thread that purposely does that.. is it possible?

I'll explain: I have 2 threads, one thread is segfaulting(and recovers(mprotect read only and then releasing memory)), that works fine, the other thread does something else, but sadly, there is a bug and it is segfaulting, I want to catch that segfault, and not the other ones that occur in the other thread.

Answer 1

As I know, depending on the OS, and I assume linux for my answer and the answer is 'NO'!

Posix exceptions can have a sigmask per thread but only one handler per task. So it is not possible to set different handling for each thread. sigaction will handle it for the complete process. So I see no way for gdb to change this.

Answer 2

I'll explain: I have 2 threads, one thread is segfaulting(and recovers(mprotect read only and then releasing memory)), that works fine, the other thread does something else, but sadly, there is a bug and it is segfaulting, I want to catch that segfault, and not the other ones that occur in the other thread

You have to tell gdb to ignore the first SIGSEGV signal. So after the first sagfault use the signal 0 command in this thread. Your program will resume execution under gdb and that is that you want. Then it will stop at the second segfault in your second thread and this is what you want to inspect.

(gdb) help signal
Continue program with the specified signal.
Usage: signal SIGNAL
The SIGNAL argument is processed the same as the handle command.

An argument of "0" means continue the program without sending it a signal.
This is useful in cases where the program stopped because of a signal,
and you want to resume the program while discarding the signal.

So

1. Do not use handle SIGSEGV nostop noprint. Run your program under gdb.
2. When it segfaults in the first threead do signal 0. Your program resumes execution.
3. Then it segfaults in another thread. Now use backtrace to see the problem.

Or if your two thread are not dependent on each other you can wait in the thread that first segfaulted while another segfault happen. Just do call sleep(60) in the first thread as soon as it causes a segfault and wait for another segfault in another thread. Your first thread will wait:

Program received signal SIGFPE, Arithmetic exception.
[Switching to Thread 0x7ffff7fde700 (LWP 25744)]
0x000000000040075d in my_thread_func1 (arg=0x0) at my_test_2.cpp:17
17        ptr1 = ptr1 / 0;
(gdb) call sleep(60)
Thread 140737343510272:

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7ffff75dd700 (LWP 25745)]
0x00000000004007a3 in my_thread_func2 (arg=0x0) at my_test_2.cpp:27
27        *ptr2 = *ptr2 + 2;
The program received a signal in another thread while
making a function call from GDB.
Evaluation of the expression containing the function
(sleep) will be abandoned.
When the function is done executing, GDB will silently stop.
(gdb)