Reading a file to string with mmap

c, file, file-io, mmap, segmentation-fault

Solution

`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);

Problem

Original source