mprotect always returns invalid arguments

I'm trying to modify a value in the .text segment using protect to give me writing access:

 int pageSize = sysconf(_SC_PAGE_SIZE);

 int *toModify = (int *)(foo+5);
 if (mprotect(toModify, pageSize, PROT_WRITE) < 0 ) {
      perror("mprotect failed with error:");
      return -1;
  }
  *toModify = 5;
  printf("Modify :%i",foo());

mprotect does never work. It always returns an mprotect failed with error:: Invalid argument error.

foo is a method that returns an int that is stored 5bytes after the function(thats the reason for foo+5)

I have executed the following code on OS X 10.9, and it appears to have the desired behavior. The output is “foo returns 23.”

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

#include <unistd.h>
#include <sys/mman.h>


extern int foo(void);


int main(void)
{
    //  New value to write into foo+5.
    int NewValue = 23;

    //  Find page size for this system.
    size_t pagesize = sysconf(_SC_PAGESIZE);

    //  Calculate start and end addresses for the write.
    uintptr_t start = (uintptr_t) &foo + 5;
    uintptr_t end = start + sizeof NewValue;

    //  Calculate start of page for mprotect.
    uintptr_t pagestart = start & -pagesize;

    //  Change memory protection.
    if (mprotect((void *) pagestart, end - pagestart,
            PROT_READ | PROT_WRITE | PROT_EXEC))
    {
        perror("mprotect");
        exit(EXIT_FAILURE);
    }

    //  Write new bytes to desired location.
    memcpy((void *) start, &NewValue, sizeof NewValue);

    //  Some systems could require an invalidate of instruction cache here.

    //  Try modified function.
    printf("foo returns %d.\n", foo());

    return 0;
}

For foo, I used this assembly code. Both sources were built with cc -arch i386.

    .globl  _foo
_foo:
    nop
    nop
    nop
    nop
    mov $42, %eax
    ret

You should modify code this way only as a learning exercise and not use it in any deployed application.