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mmap.c
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mmap.c
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/*
# mmapfile
Example of mmap on files.
Create a file, mmap to it, write to maped memory, close.
Then read the file and confirm it was written to.
Implemented in Linux by the mmap syscall.
*/
#include "common.h"
int main() {
char *filepath = TMPFILE();
enum Constexpr { NUMINTS = 4 };
int filesize = NUMINTS * sizeof(int);
int i;
int fd;
int result;
/* mmapped array of int's */
int *map;
/* Write to file with mmap. */
{
/* `O_WRONLY` is not sufficient when mmaping, need `O_RDWR`.*/
fd = open(filepath, O_RDWR | O_CREAT | O_TRUNC, S_IRUSR | S_IWUSR);
if (fd == -1) {
perror("open");
exit(EXIT_FAILURE);
}
/* Set fd position and write to it to strech the file. */
result = lseek(fd, filesize - 1, SEEK_SET);
if (result == -1) {
close(fd);
perror("lseek");
exit(EXIT_FAILURE);
}
/* Write something to the file to actually strech it. */
result = write(fd, "", 1);
if (result != 1) {
close(fd);
perror("write");
exit(EXIT_FAILURE);
}
/* Do the actual mapping call. */
map = mmap(NULL, filesize, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
if (map == MAP_FAILED) {
close(fd);
perror("mmap");
exit(EXIT_FAILURE);
}
/* Write int's to the file as if it were memory because MAP_SHARED was used. */
for (i = 0; i < NUMINTS; ++i) {
map[i] = i;
}
/* Free mmapped memory. */
if (munmap(map, filesize) == -1) {
perror("munmap");
exit(EXIT_FAILURE);
}
/* Un-mmaping doesn't close the file, so we still need to do that. */
if (close(fd) == -1) {
perror("close");
exit(EXIT_FAILURE);
}
}
/* Read result back in. */
{
fd = open(filepath, O_RDONLY, 0);
if (fd == -1) {
perror("open");
exit(EXIT_FAILURE);
}
map = mmap(0, filesize, PROT_READ, MAP_SHARED, fd, 0);
if (map == MAP_FAILED) {
close(fd);
perror("mmap");
exit(EXIT_FAILURE);
}
assert(map[1] == 1);
/* Segmentation fault because no `PROT_WRITE`: */
/*map[1] = 2;*/
if (munmap(map, filesize) == -1) {
perror("munmap");
exit(EXIT_FAILURE);
}
if (close(fd) == -1) {
perror("close");
exit(EXIT_FAILURE);
}
}
return EXIT_SUCCESS;
}