ipc/mmap.c

   1 /*
   2  * Compile with:
   3  * gcc mmap.c -o mmap -Wall -W -Wextra -ansi -pedantic
   4  */
   5
   6 #include <fcntl.h>
   7 #include <stdio.h>
   8 #include <stdlib.h>
   9 #include <string.h>
  10 #include <unistd.h>
  11 #include <sys/mman.h>
  12
  13 /* Function prototypes */
  14 void diep(const char *s);
  15
  16 int main(int argc, char *argv[])
  17 {
  18     const char *message = "this shall be written to file\n";
  19     char *map;
  20     int fd;
  21
  22     /* Check argument count */
  23     if (argc != 2) {
  24         fprintf(stderr, "Usage: %s\n", argv[0]);
  25         exit(EXIT_FAILURE);
  26     }
  27
  28     /* Open file */
  29     if ((fd = open(argv[1], O_RDWR | O_CREAT | O_TRUNC, (mode_t)0600)) == -1)
  30         diep("open()");
  31
  32     /* Stretch the file size to the size of the (mmapped) array of chars */
  33     if (lseek(fd, strlen(message) - 1, SEEK_SET) == -1) {
  34         close(fd);
  35         diep("lseek()");
  36     }
  37
  38     /*
  39      * Something needs to be written at the end of the file to
  40      * have the file actually have the new size.
  41      */
  42     if (write(fd, "", 1) != 1) {
  43         close(fd);
  44         diep("write()");
  45     }
  46
  47     /* mmap() the file */
  48     if ((map = (char *)mmap(0, strlen(message),
  49                             PROT_WRITE,
  50                             MAP_SHARED, fd, 0)) == MAP_FAILED) {
  51         close(fd);
  52         diep("mmap()");
  53     }
  54
  55     /* Copy message to mmapped() memory  */
  56     memcpy(map, message, strlen(message));
  57
  58     /* Free the mmaped() memory */
  59     if (munmap(map, strlen(message)) == -1) {
  60         close(fd);
  61         exit(EXIT_FAILURE);
  62     }
  63
  64     /* Close file */
  65     close(fd);
  66
  67     return EXIT_SUCCESS;
  68 }
  69
  70 void diep(const char *s)
  71 {
  72     perror(s);
  73     exit(EXIT_FAILURE);
  74 }