Question

Why does this code segment give segmentation fault?

#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <string.h>

int main()
{
    void *ptr;

    ptr=mmap(NULL, 10, PROT_READ|PROT_WRITE, MAP_ANONYMOUS, -1, 0);
    strcpy(ptr, "Hello");

}

Or better, i would like to have: char *ptr=malloc(10); then pass this argument to mmap. Both gives SIGSEGV.

Answer 1

Check the return values of your system calls!

The flags argument to mmap must have exactly one of these two options:

MAP_SHARED
  Share  this mapping.  Updates to the mapping are visible to other processes
  that map this file, and are carried through to the underlying file. The file
  may not actually  be updated until msync(2) or munmap() is called.

MAP_PRIVATE
  Create  a private copy-on-write mapping.  Updates to the mapping are not
  visible to other processes mapping the same file, and are not carried through
  to the underlying file.   It is  unspecified whether changes made to the file
  after the mmap() call are visible in the mapped region.

You’re not providing that, so mmap is most likely failing (returning (void*)-1) with errno set to EINVAL.