fcntl lock file example

Related analysis:  http://blog.chinaunix.net/uid-20775448-id-4009263.html

Linux provides two system call interfaces for file locks: fcntl and flock; the library function lockf is actually an encapsulation of fcntl. 

In addition, there is a bug in the mandatory lock of fcntl.

#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>

int main(int argc, char * argv[])
{
	pid_t pid;
	int ret;
	struct flock lock;
	int fd;

	pid = getpid();

	fd = open("temp.txt", O_CREAT | O_RDWR, 0644);
	if( fd < 0 ){
		perror("open file failed\n");
		return -1;
	}

	lock.l_len = 0;
	lock.l_start = 0;
	lock.l_type = F_WRLCK;
	lock.l_whence = SEEK_SET;

	printf("%d try to lock file\n", pid);

	ret = fcntl(fd, F_SETLKW, &lock);
	if(ret<0){
		perror("lock failed\n");
		return -1;
	}

	printf("%d locking file\n", pid);

	sleep(5);

	lock.l_type = F_ULOCK;
	ret = fcntl(fd, F_SETLK, &lock);
	if(ret<0){
		perror("unlock failed\n");
		return -1;
	}

	printf("%d unlock file\n", pid);
	
	
	
	return 0;
}

After compiling, run two instance tests at the same time, such as:

./a.out &;  ./a.out

flock example

#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/file.h>

int main(int argc, char * argv[])
{
	pid_t pid;
	int ret;
	int fd;

	pid = getpid();

	fd = open("temp.txt", O_CREAT | O_RDWR, 0644);
	if( fd < 0 ){
		perror("open file failed\n");
		return -1;
	}


	printf("%d try to lock file\n", pid);

	ret = flock(fd, LOCK_EX);
	if(ret<0){
		perror("lock failed\n");
		return -1;
	}

	printf("%d locking file\n", pid);

	sleep(5);

	ret = flock(fd, LOCK_EX);
	if(ret<0){
		perror("unlock failed\n");
		return -1;
	}

	printf("%d unlock file\n", pid);
		
	
	return 0;
}