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<B> Next:</B> <A NAME="tex2html1156" HREF="node64.html">semstat: A semtool companion </A>
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<H4><A NAME="SECTION00743640000000000000">The Source</A></H4>
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<HR><PRE>/*****************************************************************************
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);
}</PRE>
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