Listing 4: maxtab.c -- C source to create /usr/bin/maxtab

/* ================================================ */
/*
maxtab.c
Steve Isaacson

chown bin maxtab
chgrp cron maxtab
chmod 2111 maxtab
mv maxtab /usr/bin/maxtab

This is the maxtab program similiar to crontab.
*/
/* ================================================ */

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

#define BUFSIZE 200
#define PERMS 0600
extern int errno;

char *tdir="/tmp";

/* should be owned by, and only writeable by bin */
char *dir="/usr/spool/cron/maxtabs";

main(argc, argv)
int argc;
char *argv[];
{
int fd, tempfd, n;
char *pfn;
char buf[BUFSIZE], tabname[BUFSIZE];
char tempname[BUFSIZE];

/* create the filenames */
sprintf(tabname, "%s/%s", dir, cuserid(NULL) );
sprintf(tempname, "%s/tmp.%d", tdir, getuid());

switch ( argc ) {

case 1: /* take input from stdin */
fd=0;
break;

case 2: /* if it's not a command-line option,
then it's a file */
if ( argv[1][0] != '-' ) {
pfn=argv[1];
if ( (fd=open(pfn, O_RDONLY)) == -1 ) {
perror(pfn);
exit(errno);
}
}
else switch ( argv[1][1] ) {
case 'l':
/* list existing maxtab file */
if ( (fd=open(tabname,
O_RDONLY)) == -1 ) {
perror(tabname);
exit(errno);
}
while ((n=read(fd, buf,
sizeof(buf))) > 0)
write(1, buf, n);
exit(0);
break;

case 'r':
/* remove existing maxtab file */
if ( unlink(tabname) == -1 )
perror(tabname);
exit(errno);
break;

default:
fprintf(stderr,
"Error: illegal argument: %s\n",argv[1]);
usage();
}
break;

default:
fprintf(stderr, "Error: Too many args\n");
usage();
break;
}

/* open a temp file to work in. This is required
because you don't want to wipe out an
existing maxtab file until you're sure
you have a new one to replace it with. */
if ( (tempfd=open(tempname,
O_TRUNC|O_RDWR|O_CREAT, PERMS)) == -1 ) {
perror(tempname);
exit(errno);
}

chown(tempname, geteuid(), getegid());
chmod (tempname, PERMS);

/* write to it. */
while ((n=read(fd, buf, sizeof(buf))) > 0)
write(tempfd, buf, n);

unlink(tabname); /* ignore error return */

if ( link(tempname, tabname) ) {
perror("link");
exit(errno);
}

close (fd);
close (tempfd);
if ( unlink(tempname) == -1 )
perror(tempname);
exit(errno);
}

/* ================================================ */

usage() {

fprintf(stderr, "Usage: maxtab [filename]\n");
fprintf(stderr, "       maxtab [-r|-l]\n");
exit(1);
}



/* ================================================ */
/* End of File */