Locking Files in Linux with C/C++

Locking files in linux with c/c++

Yes, this is possible.

The Unix way to do this is via fcntl or lockf.
Whatever you choose, make sure to use only it and not mix the two. Have a look at this question (with answer) about it: fcntl, lockf, which is better to use for file locking?.

If you can, have a look at section 14.3 in Advanced Programming in the UNIX Environment.

Linux file locking

Using open and the advisory flock locking, the writers need to hold a LOCK_EX exclusive lock. The readers must not hold any lock and they then might see the file being truncated abruptly.

Plain fopen can be used for readers. For well-behaving writers,

#include <sys/types.h>
#include <sys/stat.h>
#include <sys/file.h>
#include <fcntl.h>
#include <stdio.h>

// never truncates
int fd = open("foo.log", O_CREAT|O_WRONLY /*or O_RDWR */, 0644 /* mode */);
if (flock(fd, LOCK_EX|LOCK_NB) != 0) {
    perror("Locking failed - I am not an exclusive writer");
    exit(1);
}

// I hold the exclusive lock - now, truncate the file to 0 bytes
ftruncate(fd, 0);

(other error checking omitted for brevity)

If you wish to use <stdio.h> routines with the file from thereon, you can use fdopen:

// "w" will not truncate!
FILE *f = fdopen(fd, "w"); // or `r+` for O_RDWR...

Note that a reader must not set a LOCK_SH lock on the file, otherwise it cannot be opened by a writer.

lock file on linux and if lock successful become daemon?

Lock files don't require any file locking such as flock. The existence of the file is used as the lock. Add O_EXCL to the open call. If open succeeds, you have the lock; then to release the lock you remove or unlink the file.
You need some handling for stale lock files in the case of a crash. That's one reason they are typically created in a particular directory, because the system startup scripts know about it and delete the lock files.

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