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clinuxsegmentation-faultclone

9th call to clone produces seg fault

I am having a problem with the clone function in that it give me a segmentation fault after the 9th call to it. Program works fine until I get up to using 9+ threads.

here is my call to clone:

void **child_stack = (void **) malloc(SIGSTKSZ);
clone (func,
       (child_stack + SIGSTKSZ),
       CLONE_VM | CLONE_FILES | CLONE_PARENT_SETTID,
       (void *) argsForFunc,
       &pid);

I am using clone and not any higher level threads library like pthreads.

If it helps, this is the error I get when using GDB:

Program received signal SIGSEGV, Segmentation fault.
clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:66
66              movq    %rcx,8(%rsi)
Current language:  auto; currently asm
Solution

  • TJD's comment above made me see the problem right away: your pointer arithmetic to get the end of the stack is incorrect. If you allocate the stack as:

    void **child_stack = (void **) malloc(SIGSTKSZ);

    And then you calculate the top of the stack as:

    child_stack + SIGSTKSZ

    The actual address passed to clone will be

    child_stack + sizeof(void*)*SIGSTKSZ

    Maybe you meant for child_stack to have type char*? sizeof(char) is 1 by definition, so that would give you the right result.