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// Takes two files and exits with 0 if fcntl-locking one fcntl-locks the other.
// If the files don't fcntl-lock each other, returns 1.
// If any other error occurs, returns 2.
#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
int main(int argc, const char* argv[]) {
if (argc != 3) {
fprintf(stderr, "expecting exactly two arguments\n");
return 2;
}
int fd1 = -1;
int fd2 = -1;
int result = 2;
fd1 = open(argv[1], O_RDWR);
if (fd1 == -1) {
perror("failed to open the first file");
goto exit;
}
fd2 = open(argv[2], O_RDWR);
if (fd2 == -1) {
perror("failed to open the second file");
goto exit;
}
struct flock lock;
memset(&lock, 0, sizeof(lock));
lock.l_type = F_WRLCK;
lock.l_whence = SEEK_SET;
lock.l_start = 0;
lock.l_len = 0;
if (fcntl(fd1, F_SETLK, &lock) == -1) {
perror("fcntl F_SETLK on the first file failed");
goto exit;
}
memset(&lock, 0, sizeof(lock));
lock.l_type = F_WRLCK;
lock.l_whence = SEEK_SET;
lock.l_start = 0;
lock.l_len = 0;
if (fcntl(fd2, F_SETLK, &lock) == -1) {
if (errno == EACCES || errno == EAGAIN) {
result = 0;
goto exit;
} else {
perror("fcntl F_SETLK on the second file failed");
goto exit;
}
} else {
result = 1;
goto exit;
}
exit:
close(fd1);
close(fd2);
return result;
}
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