sequencer.c

A simple program that exploded with too many features

// =========================================================================================
// title  : sequencer_203.c
// author : Neil Rieck 
// created: 2021-11-11
// history:
// 200 20211111 NSR 1. original effort (hard-coded parameters)				DVLP
// 201 20211112 NSR 1. added support for soft-coded parameters				DVLP
// 202 20211115 NSR 1. added support for flock						DVLP
// ------------
// 203 20211202 NSR 1. fixed hacks; replaced fgetc with fread (and fputc with fwrite)	PROD
// notes  : 
// 1) a tiny little program for use with email interfaces on CentOS (espp, radiovideo, etc).
//    Call this program when trying to determine the next WORKBENCH folder (eg. WORKBENCH9)
// 2) when used from OpenVMS, getenv reads information from either logical names or shell
//    vars. Logical names do not exist on CentOS
// 3) setenv and putenv do not work the way most people think. On any UNIX or Linux system
// they only work on the forked process but cannot directly update anything in the calling
// process. Even though OpenVMS does not fork, these routines still only affect the shell
// vars of the active process and are lost when that program exits. OpenVMS programs
// requiring this functionality can do so by calling lib$set_symbol. Linux systems with BASH
// can copy the output of this program into a shell variable like so: SEQ_NUM=$(./sequencer)
// 4) all versions before 203 will only work on little endian systems where seq <= 127
// 5) CentOS build: gcc sequencer_203.c -o sequencer
// =========================================================================================
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#define __NEW_STARLET   1  			// enable new-strict starlet (>= OpenVMS7)
#define SMALLER	1				// disable optional parameter support
//
#define DEFSEQMIN 1				// default MIN
#define DEFSEQMAX 9				// default MAX
#define NAMSEQMIN "SEQ_MIN"			// optional parameter (define in your shell)
#define NAMSEQMAX "SEQ_MAX"			// optional parameter (define in your shell)
#define NAMSEQDEB "SEQ_DEBUG"			// optional parameter (define in your shell)
#define NAMSEQNUM "SEQ_NUM"			// written to your VMS shell (must already
//
#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;				// this is located elsewhere
//
int main(){
    // =========================================
    //	init
    // =========================================
    FILE *fp;
    char *tmp;
    int seqmin, seqmax;
    int debug;
    int seq;
    int rc;
    // =========================================
    //	get params
    // =========================================
#if (SMALLER==1)
    debug = 0;
    seqmin = DEFSEQMIN;
    seqmax = DEFSEQMAX;
#else
    tmp = getenv(NAMSEQDEB);
    if (tmp==NULL){
	debug = 0;
    }else{
	debug = atoi(tmp);
    }
    //
    tmp = getenv(NAMSEQMIN);
    if (tmp==NULL){
	seqmin = DEFSEQMIN;
    }else{
	seqmin = atoi(tmp);
	if (debug>0) printf("-i-param seqmin: %d\n",seqmin);
    }
    //
    tmp = getenv(NAMSEQMAX);
    if (tmp==NULL){
	seqmax = DEFSEQMAX;
    }else{
	seqmax = atoi(tmp);
	if (debug>0) printf("-i-param seqmax: %d\n",seqmax);
    }
#endif
    //
    //	minimal tests of parameter data (KISS)
    //
    if (seqmin<DEFSEQMIN){			//
	seqmin = DEFSEQMIN;			//
    }
    if (seqmin>=seqmax){			//
	seqmax = seqmin + 1;			//
    }
    if (debug>0){
	printf("-i-data seqmin: %d\n",seqmin);
	printf("-i-data seqmax: %d\n",seqmax);
    }
    // =========================================
    //	support the main task
    // =========================================
    // open the file then place a lock
    fp = fopen("sequencer.dat","r+");		// open for read-write
    if ((fp == NULL)&&(errno == 2)){		// oops, handle first time
        fp = fopen("sequencer.dat","w+");	// create the file
	if (fp != NULL){
	    seq = seqmin;			// init
	    fwrite(&seq, sizeof(seq), 1L, fp);
	    fseek(fp, 0L, SEEK_SET);
	    errno = 1;
	}
    }
    if (fp == NULL){
	perror("-e-the data file cannot be opened");
	exit (2);
    }
    rc = flock(fileno(fp), LOCK_EX);		// place an exclusive lock
    //
    //	get down to work
    //
    seq = -1;					// init
    fread(&seq, sizeof(seq), 1L, fp);		// read
    if (seq<seqmin){				// oops, handle first time
	seq = seqmin;
    };
    seq = seq + 1;				// okay, get down to business
    if (seq>seqmax){
	seq = seqmin;
    };
    fseek(fp, 0L, SEEK_SET);			// prep to write back
    fwrite(&seq, sizeof(seq), 1, fp);		// write
    fflush(fp);					// 
    rc = flock(fileno(fp), LOCK_UN);		//
    fclose(fp);					// close
    if (debug>0){				// DVLP
	printf("-i-new sequence data: %d\n", seq);
    }else{					// PROD
	printf("%d", seq);			// send stdout (then a BASH variable?) 
    }
    exit(0);					// okay
} 

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Neil Rieck
Waterloo, Ontario, Canada.