CODEWITHSUNDEEP
c
Nikhil
Nikhil

Reputation: 2318

C: Writing data to memory mapped file shows junk

I'm trying to write data to memory mapped file and it showing junk. Not sure what I'm doing wrong. I'm creating a memory mapped file and writing int to it. I'm seeing junk output.

The file I'm opening as read/write and I ideally want other process to read the data written to it

Code below

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

#define FILEPATH "test"
#define NUMINTS  (1000)
#define FILESIZE (NUMINTS * sizeof(int))

int main(int argc, char *argv[])
{
    int i;
    int fd;
    int result;
    int *map;  /* mmapped array of int's */

    /* Open a file for writing.
     *  - Creating the file if it doesn't exist.
     *  - Truncating it to 0 size if it already exists. (not really needed)
     *
     * Note: "O_WRONLY" mode is not sufficient when mmaping.
     */
    fd = open(FILEPATH, O_RDWR | O_CREAT | O_TRUNC, (mode_t)0600);
    if (fd == -1) {
    perror("Error opening file for writing");
    exit(EXIT_FAILURE);
    }

    /* Stretch the file size to the size of the (mmapped) array of ints
     */
    result = lseek(fd, FILESIZE-1, SEEK_SET);
    if (result == -1) {
    close(fd);
    perror("Error calling lseek() to 'stretch' the file");
    exit(EXIT_FAILURE);
    }

    /* Something needs to be written at the end of the file to
     * have the file actually have the new size.
     * Just writing an empty string at the current file position will do.
     *
     * Note:
     *  - The current position in the file is at the end of the stretched
     *    file due to the call to lseek().
     *  - An empty string is actually a single '\0' character, so a zero-byte
     *    will be written at the last byte of the file.
     */
    result = write(fd, "", 1);
    if (result != 1) {
    close(fd);
    perror("Error writing last byte of the file");
    exit(EXIT_FAILURE);
    }

    /* Now the file is ready to be mmapped.
     */
    map = mmap(0, FILESIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
    if (map == MAP_FAILED) {
    close(fd);
    perror("Error mmapping the file");
    exit(EXIT_FAILURE);
    }

    /* Now write int's to the file as if it were memory (an array of ints).
     */
    for (i = 1; i <=10; ++i) {
        map[i] = i;
    }

    /* Don't forget to free the mmapped memory
     */
    if (munmap(map, FILESIZE) == -1) {
    perror("Error un-mmapping the file");
    /* Decide here whether to close(fd) and exit() or not. Depends... */
    }

    /* Un-mmaping doesn't close the file, so we still need to do that.
     */
    close(fd);
    return 0;
}

Upvotes: 1

Views: 1207

Answers (1)

DevSolar
DevSolar

Reputation: 70391

The contents of test look just as I would have expected:

$ hexdump -C test
00000000  00 00 00 00 01 00 00 00  02 00 00 00 03 00 00 00  |................|
00000010  04 00 00 00 05 00 00 00  06 00 00 00 07 00 00 00  |................|
00000020  08 00 00 00 09 00 00 00  0a 00 00 00 00 00 00 00  |................|
00000030  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00000fa0

There are your 10 ints.

Of course, in a text editor that looks like garbage, because it's the binary value of the ints, not the ASCII "1", "2"...

Upvotes: 4

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