The intent of this code is to create a posix shared memory segment, open it in RW mode using mmap(), then open the same segment in private mode so I get the copy-on-write functionality.
Can anyone see what I'm doing wrong in this code? It compiles and runs fine under Linux but after outputting "debug 1" I get
(It's mostly just error-checking code below
Can anyone see what I'm doing wrong in this code? It compiles and runs fine under Linux but after outputting "debug 1" I get
(It's mostly just error-checking code below
Code:
#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <errno.h>
#define SIZE 4*4096
#define HANDLE "/tmpmem2"
int main(void) {
int err = EXIT_SUCCESS;
int fd = shm_open(HANDLE, O_RDWR | O_CREAT, 0777);
if (fd == -1) {
perror(NULL);
err = -1;
goto end;
}
printf("Page size is %ld\n", sysconf(_SC_PAGESIZE));
int r = ftruncate(fd, SIZE);
if (r == -1) {
printf("errno is %d\n", errno);
perror(NULL);
err = -1;
goto unlink;
}
printf("fd: %i, r: %i\n", fd, r);
char *buf = mmap(0, SIZE, PROT_READ | PROT_WRITE,
MAP_SHARED, fd, 0);
if (buf == MAP_FAILED) {
printf("errno is %d\n", errno);
perror(NULL);
err = -1;
goto unlink;
}
printf("buf is %p\n", buf);
printf("debug 0\n");
buf[SIZE - 2] = 41;
buf[SIZE - 1] = 42;
printf("debug 1\n");
char *buf2 = mmap(0, SIZE, PROT_READ | PROT_WRITE,
MAP_PRIVATE, fd, 0);
if (buf2 == MAP_FAILED) {
printf("errno is %d\n", errno);
perror(NULL);
err = -1;
goto munmap1;
}
printf("buf2: %p\n", buf2);
buf2[SIZE - 1] = 43;
buf[SIZE - 2] = 40;
printf("buf[-2]: %i, buf[-1]: %i, buf2[-2]: %i, buf2[-1]: %i\n",
buf[SIZE - 2],
buf[SIZE - 1],
buf2[SIZE - 2],
buf2[SIZE - 1]);
munmap2:
munmap(buf2, SIZE);
munmap1:
munmap(buf, SIZE);
unlink:
shm_unlink(HANDLE);
end:
return err;
}