How to use alloca to allocate C function pointers?

Question

C is a mystery all the time!

I am implementing a work-crew thread execution model in which I am trying to use alloca as a faster memory allocation option. I have a strange segmentation fault while trying to execute code via function pointers stored on the stack using alloca.

Here's a tooth-pick code which results in a similar segmentation fault:

#include <stdlib.h>
#include <stdio.h>

typedef void* (*foo)(void*);

typedef struct task
{
    foo f;
} task;

void *blah(void* v)
{
    printf("addr:%p\n", &v);
    return v;
}

int main()
{
    void *queue[10]; 

    task *t = (task*) alloca (sizeof(task));
    // No null check, excuse me!
    t->f = blah;

    queue[0] = (void*)t;
    char string[10] = "Bingo!";
    char *c = &string[0];

    task *tnew = (task*)&queue[0];
    tnew->f((void*)c);

    return 0; 
}

When I execute the above code I get a segmentation fault at the tnew->f() line. GDB backtrace did not help me much.

Kindly explain the error in the above code.. I am using alloca for the first time.

Thank you very much!

Answer 1

Change this line:

task* tnew = (task*)&queue[0];

to

task* tnew = (task*)queue[0];

Because queue[0] is already a pointer; you don't need to take the address of it. You have the same issue inside blah . Your printf won't crash, but it will print out the address of the pointer, not the value of the pointer, which probably isn't what you want.

Answer 2

Maybe you might also want to pass the parameter "v"?

t->f = blah; // BAD

t->f = blah (SOMETHING); // Better...