The Source

/*************************************************
 Excerpt from "Linux Programmer's Guide - Chapter 6"
 (C)opyright 1994-1995, Scott Burkett
 ************************************************* 
 MODULE: semtool.c
 *************************************************
A command line tool for tinkering with
SysV style Semaphore Sets

 *************************************************/

#include <stdio.h>
#include <ctype.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>

#define SEM_RESOURCE_MAX        1
/* Initial value of all semaphores */

void opensem(int *sid, key_t key);
void createsem(int *sid, key_t key, int members);
void locksem(int sid, int member);
void unlocksem(int sid, int member);
void removesem(int sid);
unsigned short get_member_count(int sid);
int getval(int sid, int member);
void dispval(int sid, int member);
void changemode(int sid, char *mode);
void usage(void);

int main(int argc, char *argv[])
{
key_t key;
int   semset_id;

if(argc == 1)
usage();

/* Create unique key via call to ftok() */
key = ftok(".", 's');

switch(tolower(argv[1][0]))
{
case 'c': if(argc != 3)
usage();
  createsem(&semset_id, key,  atoi(argv[2]));
  break;
case 'l': if(argc != 3)
usage();
  opensem(&semset_id, key);
  locksem(semset_id, atoi(argv[2]));
  break;
case 'u': if(argc != 3)
usage();
  opensem(&semset_id, key);
  unlocksem(semset_id, atoi(argv[2]));
  break;
case 'd': opensem(&semset_id, key);
  removesem(semset_id);
  break;
case 'm': opensem(&semset_id, key);
  changemode(semset_id, argv[2]);
  break;
 default: usage();

}

return(0);
}

void opensem(int *sid, key_t key)
{
/* Open the semaphore set - do not create! */

if((*sid = semget(key, 0, 0666)) == -1) 
{
printf("Semaphore set does not exist!\n");
exit(1);
}

}

void createsem(int *sid, key_t key, int members)
{
int cntr;
union semun semopts;

if(members > SEMMSL) {
printf("Sorry, max number of semaphores in a set is %d\n",
SEMMSL);
exit(1);
}

printf("Attempting to create new semaphore
set with %d members\n",
members);

if((*sid = semget(key, members, IPC_CREAT|IPC_EXCL|0666))
== -1) 
{
fprintf(stderr, "Semaphore set already exists!\n");
exit(1);
}

semopts.val = SEM_RESOURCE_MAX;

/* Initialize all members (could be done with SETALL) */
for(cntr=0; cntr<members; cntr++)
semctl(*sid, cntr, SETVAL, semopts);
}

void locksem(int sid, int member)
{
struct sembuf sem_lock={ 0, -1, IPC_NOWAIT};

if( member<0 || member>(get_member_count(sid)-1))
{
fprintf(stderr, "semaphore member %d out
of range\n", member);
return;
}

/* Attempt to lock the semaphore set */
if(!getval(sid, member))
{
fprintf(stderr, "Semaphore resources
exhausted (no lock)!\n");
exit(1);
}

sem_lock.sem_num = member;

if((semop(sid, &sem_lock, 1)) == -1)
{
fprintf(stderr, "Lock failed\n");
exit(1);
}
else
printf("Semaphore resources decremented
by one (locked)\n");

dispval(sid, member);
}

void unlocksem(int sid, int member)
{
struct sembuf sem_unlock={ member, 1, IPC_NOWAIT};
int semval;

if( member<0 || member>(get_member_count(sid)-1))
{
fprintf(stderr, "semaphore member %d out
of range\n", member);
return;
}

/* Is the semaphore set locked? */
semval = getval(sid, member);
if(semval == SEM_RESOURCE_MAX) {
fprintf(stderr, "Semaphore not locked!\n");
exit(1);
}

sem_unlock.sem_num = member;

/* Attempt to lock the semaphore set */
if((semop(sid, &sem_unlock, 1)) == -1)
{
fprintf(stderr, "Unlock failed\n");
exit(1);
}
else
printf("Semaphore resources incremented
by one (unlocked)\n");

dispval(sid, member);
}

void removesem(int sid)
{
semctl(sid, 0, IPC_RMID, 0);
printf("Semaphore removed\n");
}

unsigned short get_member_count(int sid)
{
union semun semopts;
struct semid_ds mysemds;

semopts.buf = &mysemds;

/* Return number of members in the semaphore set */
return(semopts.buf->sem_nsems);
}

int getval(int sid, int member)
{
int semval;

semval = semctl(sid, member, GETVAL, 0);
return(semval);
}

void changemode(int sid, char *mode)
{
int rc;
union semun semopts;
struct semid_ds mysemds;

/* Get current values for internal data structure */
semopts.buf = &mysemds;

rc = semctl(sid, 0, IPC_STAT, semopts);

if (rc == -1) {
perror("semctl");
exit(1);
}

printf("Old permissions were %o\n",
semopts.buf->sem_perm.mode);

/* Change the permissions on the semaphore */
sscanf(mode, "%ho",
&semopts.buf->sem_perm.mode);

/* Update the internal data structure */
semctl(sid, 0, IPC_SET, semopts);

printf("Updated...\n");

}

void dispval(int sid, int member)
{
int semval;

semval = semctl(sid, member, GETVAL, 0);
printf("semval for member %d is %d\n", member, semval);
}

void usage(void)
{
fprintf(stderr, "semtool - A utility for tinkering with
semaphores\n");
fprintf(stderr, "\nUSAGE:  semtool4 (c)reate <semcount>\n");
fprintf(stderr, " (l)ock <sem #>\n");
fprintf(stderr, " (u)nlock <sem #>\n");
fprintf(stderr, " (d)elete\n");
fprintf(stderr, " (m)ode <mode>\n");
exit(1);
}