Reading a file to string with mmap

Question

I'm trying to read a file to a string using mmap.

I was following this example: http://www.lemoda.net/c/mmap-example/index.html

My code looks like this

unsigned char *f;
int size;
int main(int argc, char const *argv[])
{
    struct stat s;
    const char * file_name = argv[1];
    int fd = open (argv[1], O_RDONLY);

    /* Get the size of the file. */
    int status = fstat (fd, & s);
    size = s.st_size;

    f = (char *) mmap (0, size, PROT_READ, 0, fd, 0);
    for (i = 0; i < size; i++) {
        char c;

        c = f[i];
        putchar(c);
    }

    return 0;
}

But I always receive a segemation fault when accessing f[i]. What am I doing wrong?

Answer 1

strace is your friend here:

$ strace ./mmap-example mmap-example.c

...
... (lots of output)
...
open("mmap-example.c", O_RDONLY)        = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=582, ...}) = 0
mmap(NULL, 582, PROT_READ, MAP_FILE, 3, 0) = -1 EINVAL (Invalid argument)
--- SIGSEGV (Segmentation fault) @ 0 (0) ---
+++ killed by SIGSEGV +++

The mmap man page tells you all you need to know ;)

• EINVAL We don't like addr, length, or offset (e.g., they are too large, or not aligned on a page boundary).
• EINVAL (since Linux 2.6.12) length was 0.
• EINVAL flags contained neither MAP_PRIVATE or MAP_SHARED, or
  contained both of these values.

The -EINVAL error is caused by flags that cannot be 0. Either MAP_PRIVATE or MAP_SHARED has to be picked. I have been able to make it work by using MAP_PRIVATE on Linux, x86-64.

So, you have just to add MAP_PRIVATE to mmap():

#include <stdio.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <sys/io.h>
#include <sys/mman.h>

int main(int argc, char const *argv[])
{
    unsigned char *f;
    int size;
    struct stat s;
    const char * file_name = argv[1];
    int fd = open (argv[1], O_RDONLY);

    /* Get the size of the file. */
    int status = fstat (fd, & s);
    size = s.st_size;

    f = (char *) mmap (0, size, PROT_READ, MAP_PRIVATE, fd, 0);
    for (int i = 0; i < size; i++) {
        char c;

        c = f[i];
        putchar(c);
    }

    return 0;
}

---

NOTE: My first answer did include another possible cause for the EINVAL:

size must be an integral multiple of the page size of the system. To obtain the page size use the function getpagesize().

This is not actually required, but you must take into account that either way, mapping will be always performed in multiples of the system page size, so if you'd like to calculate how much memory is actually been available through the returned pointer, update size as this:

int pagesize = getpagesize();
size = s.st_size;
size += pagesize-(size%pagesize);