flock_demo.c

// ========================================================================
// title  : flock_demo.c
// author : Neil Rieck
// created: 2021-11-13
// notes  :
// 1) 'lockf' is published in the POSIX libraries
// 2) 'flock' is published in the BSD/Linux libraries
// 3) 'flock' and 'lockf' are not found in the CRTL libraries published
//    by HP for OpenVMS in 2005 (hopefully VSI can fix this) so you might
//    need to rely upon the ANSI function "void flockfile()" -OR- call
//    routines specifically written for the VMS "distributed lock manager"
// Linux  : "yum install moreutils" will enables commands like:
//                "errno 9" or "error -l"
// history:
// 20211113 NSR 1. original effort (this works on CentOS-7)
// ========================================================================
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <fcntl.h>
#include <sys/file.h>				// for strict use of flock
#define __NEW_STARLET   1  			// strict starlet (>= VMS7)
//
#define TESTFILENAME  "flock_demo.dat"		//
#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
extern int errno;				// is located elsewhere
//
void debug(int rc){
    if ((rc>0)||(errno>0)){
	printf("rc: %d err: %d\n", rc, errno);
    	errno = 0;
    }
}
//
void main(){
    FILE *fp;
    char *tmp;
    int rc;
    int seq;
    // =========================================
    printf("fopen\n");
    fp = fopen(TESTFILENAME,"r+");		// open for read-write
    if (fp == NULL){				// oops, handle first time
        printf("err: %d\n", errno);
	errno = 0;
        printf("fopen2\n");
        fp = fopen(TESTFILENAME,"w+");		// create the file
	if (fp != NULL){
	    seq = 1;				// init
	    fputc(seq, fp);			// write
	    fseek(fp, 0L, SEEK_SET);		// rewind
	}
    }
    if (fp == NULL){
        printf("err: %d\n", errno);
	perror("-e-cannot open file");
	exit(2);
    }
    printf("flock\n");
    rc = flock( fileno(fp), LOCK_EX );
    debug(rc);
    printf("now run this program on another session\n");
    printf("then hit <enter> to continue");
    getchar();
    printf("\n");
    printf("funlock\n");
    rc = flock( fileno(fp), LOCK_UN );
    debug(rc);
    printf("fclose\n");
    rc = fclose(fp);
    debug(rc);
}

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Neil Rieck
Waterloo, Ontario, Canada.