int main() {
 char *fn = "/tmp/test.txt"; //the test file
 FILE *F = fopen(fn,"w"); //this simulates another process writing to a file but not closing it yet

 //We now attempt to detect that the file is still open
 int fd = open(fn,O_RDONLY);
 struct flock fl;
  fl.l_type   = F_WRLCK;  /* F_RDLCK, F_WRLCK, F_UNLCK */
  fl.l_whence = SEEK_SET; /* SEEK_SET, SEEK_CUR, SEEK_END */
  fl.l_start  = 0; /* Offset from l_whence */
  fl.l_len    = 0; /* length, 0 = to EOF */
  fl.l_pid    = getpid();

 printf("%d %d %d %d %d\n",fl.l_type,fl.l_whence,fl.l_start,fl.l_len,fl.l_pid);
 //1 0 0 0 19865
 int ret = fcntl(fd,F_GETLK,&fl);
 printf("%d %d %d %d %d\n",fl.l_type,fl.l_whence,fl.l_start,fl.l_len,fl.l_pid);
 //2 0 0 0 19865

 sleep(3); //this gives me time to manually run fuser/lsof to observe that the file is indeed open

 //finalize
 fclose(F);
 return 0;
}

struct flock fl;
int fd = open(fn, O_RDWR);
fl.l_type   = F_WRLCK;  /* F_RDLCK, F_WRLCK, F_UNLCK */
fl.l_whence = SEEK_SET; /* SEEK_SET, SEEK_CUR, SEEK_END */
fl.l_start  = 0; /* Offset from l_whence */
fl.l_len    = 0; /* length, 0 = to EOF */

if (fcntl(fd, F_SETLK, &fl) == -1)
    printf("%s\n", errno == EACCESS||errno == EAGAIN ? "In use by another process":"another error occurred");
else
    /* lock granted so do I/O on the file */

#include <sys/types.h>
#include <dirent.h>
#include <sys/stat.h>
#include <unistd.h>
#include <stdio.h>
#include <errno.h>
#include <stdlib.h>

int main(void)
{
	struct stat statbuf;
	DIR* dir = opendir("/proc/14867/fd");
	struct dirent* ent;

	// foo.txt is the file that pid 14867 is producing
	if (stat("/usr/home/foo.txt",  &statbuf) < 0)
	{
		perror("Cannot stat");
		exit(1);
	}

	while ((ent = readdir(dir)) != NULL)
	{
		char buf[128];
		struct stat statbuf2;
		sprintf(buf, "/proc/14867/fd/%s", ent->d_name);
		stat(buf, &statbuf2);

		if (statbuf2.st_ino == statbuf.st_ino)
		{
			// This means that pid 14867 still has foo.txt open
			fprintf(stderr, "YOU GOT IT\n");
			break;
		}
	}

	closedir(dir);
}