CODEWITHSUNDEEP
clinuxmmap
RajSanpui
RajSanpui

Reputation: 12064

mmap: map_anonymous why does it give SIGSEGV?

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.

Upvotes: 1

Views: 2498

Answers (2)

Basile Starynkevitch
Basile Starynkevitch

Reputation: 1

You probably get MAP_FAILED (that is, (void*)-1) as the result of your mmap with EINVAL in errno. The man page of mmap(2) says that it fails with

   EINVAL We don't like addr, length, or offset (e.g., they are too large,
          or not aligned on a page boundary).

Your second argument to mmap (called length in the man page) cannot be 10. It should be a multiple of the page length (at least 4K).

Upvotes: 0

Mat
Mat

Reputation: 206679

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.

Upvotes: 10

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