How do I lock files using fopen()?

Question

I wonder if there is any way to lock and unlock a file in Linux when I open a file using fopen (not open)?

Based on Stack Overflow question

Answer 1

Files can be locked by using flock(). Its syntax is

 #include 
 #define   LOCK_SH   1    /* shared lock */
 #define   LOCK_EX   2    /* exclusive lock */
 #define   LOCK_NB   4    /* don't block when locking */
 #define   LOCK_UN   8    /* unlock */

int flock(int fd, int operation);

First file is opened using fopen() or open(). Then this opened file is locked using flock() as given below

int fd = open("test.txt","r");
int lock = flock(fd, LOCK_SH);  // Lock the file . . .
// . . . .
// Locked file in use 
// . . . .
int release = flock(fd, LOCK_UN);  // Unlock the file . . .