C 'mmap' causing segmentation fault. Ideas? - c

Question

I am trying to write a program that reads a file using 'mmap' for school. I am having some difficulty creating the map. Specifically, I am getting a segmentation fault. I am not really sure what I am doing wrong here so some concrete help would be appreciated. Thank you.
#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
int main(int argc, char* argv[])
{
printf("Hello world!\n");
FILE* fp;// File pointer
int fd;// File descriptor
size_t size;// Length of the file
char* map;// File mmap
/* Open the file */
fp = fopen("data.txt", "r+");
/* Get the file descriptor */
fd = fileno(fp);
printf("FD: %d\n", fd);
/* Get the size of the file */
fseek(fp, 0, SEEK_END);
size = ftell(fp);
fseek(fp, 0, SEEK_SET);
printf("SIZE: %d\n", size);
/* Map the file with mmap */
map = mmap(NULL, size, PROT_READ, 0, fd, 0);
if (map == MAP_FAILED)
{
printf("MMAP FAILED\n");
} else {
printf("MMAP SUCEEDED\n");
}
/* Do something with the map */
int i;
for (i = 0; i < size; i++)
{
char c;
c = map[i];
putchar(c);
}
fclose(fp);
return(0);
}

Answer 1

You are not specifying anything as the flag argument, you must either specify MAP_PRIVATE or MAP_SHARED as specified here:
The flags argument determines whether updates to the mapping are
visible to other processes mapping the same region, and whether
updates are carried through to the underlying file. This behavior is determined by including exactly one of the following values in flags:
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. (To precisely control when updates are
carried through to the underlying file requires the use of
msync(2).)
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.
In your case, since you are just reading the file, MAP_PRIVATE should be enough.
Try with:
map = mmap(NULL, size, PROT_READ, MAP_PRIVATE, fd, 0);