Unix semaphore example
/* semabinit.c - initialize a semaphore for use by programs sema and semb */
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <stdio.h>
/* The semaphore key is an arbitrary long integer which serves as an
external identifier by which the semaphore is known to any program
that wishes to use it. */
#define KEY (1492)
void main()
{
int id; /* Number by which the semaphore is known within a program */
/* The next thing is an argument to the semctl() function. Semctl()
does various things to the semaphore depending on which arguments
are passed. We will use it to make sure that the value of the
semaphore is initially 0. */
union semun {
int val;
struct semid_ds *buf;
ushort * array;
} argument;
argument.val = 0;
/* Create the semaphore with external key KEY if it doesn't already
exists. Give permissions to the world. */
id = semget(KEY, 1, 0666 | IPC_CREAT);
/* Always check system returns. */
if(id < 0)
{
fprintf(stderr, "Unable to obtain semaphore. ");
exit(0);
}
/* What we actually get is an array of semaphores. The second
argument to semget() was the array dimension - in our case
1. */
/* Set the value of the number 0 semaphore in semaphore array
# id to the value 0. */
if( semctl(id, 0, SETVAL, argument) < 0)
{
fprintf( stderr, "Cannot set semaphore value. ");
}
else
{
fprintf(stderr, "Semaphore %d initialized. ", KEY);
}
}
/* Semaphore example program a (sema.c) */
/* We have two programs, sema and semb. Semb may be initiated at any
time, but will be forced to wait until sema is executed. Sema and
semb do not have to be executed by the same user! */
#include <stdio.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#define KEY (1492)
/* This is the external name by which the semaphore is known to any
program that wishes to access it. */
void main()
{
int id; /* Internal identifier of the semaphore. */
struct sembuf operations[1];
/* An "array" of one operation to perform on the semaphore. */
int retval; /* Return value from semop() */
/* Get the index for the semaphore with external name KEY. */
id = semget(KEY, 1, 0666);
if(id < 0)
/* Semaphore does not exist. */
{
fprintf(stderr, "Program sema cannot find semaphore, exiting. ");
exit(0);
}
/* Do a semaphore V-operation. */
printf("Program sema about to do a V-operation. ");
/* Set up the sembuf structure. */
/* Which semaphore in the semaphore array : */
operations[0].sem_num = 0;
/* Which operation? Add 1 to semaphore value : */
operations[0].sem_op = 1;
/* Set the flag so we will wait : */
operations[0].sem_flg = 0;
/* So do the operation! */
retval = semop(id, operations, 1);
if(retval == 0)
{
printf("Successful V-operation by program sema. ");
}
else
{
printf("sema: V-operation did not succeed. ");
perror("REASON");
}
}
/* Think carefully about what the V-operation does. If sema is executed
twice, then semb can execute twice. */
/* Semaphore example program b (semb.c) */
/* We have two programs, sema and semb. Semb may be initiated at any
time, but will be forced to wait until sema is executed. Sema and
semb do not have to be executed by the same user! */
/* HOW TO TEST:
Execute semb &
The & is important - otherwise you would have have to move to
a different terminal to execute sema.
Then execute sema.
*/
#include <stdio.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#define KEY (1492)
/* This is the external name by which the semaphore is known to any
program that wishes to access it. */
void main()
{
int id; /* Internal identifier of the semaphore. */
struct sembuf operations[1];
/* An "array" of one operation to perform on the semaphore. */
int retval; /* Return value from semop() */
/* Get the index for the semaphore with external name KEY. */
id = semget(KEY, 1, 0666);
if(id < 0)
/* Semaphore does not exist. */
{
fprintf(stderr, "Program semb cannot find semaphore, exiting. ");
exit(0);
}
/* Do a semaphore P-operation. */
printf("Program semb about to do a P-operation. ");
printf("Process id is %d ", getpid());
/* Set up the sembuf structure. */
/* Which semaphore in the semaphore array : */
operations[0].sem_num = 0;
/* Which operation? Subtract 1 from semaphore value : */
operations[0].sem_op = -1;
/* Set the flag so we will wait : */
operations[0].sem_flg = 0;
/* So do the operation! */
retval = semop(id, operations, 1);
if(retval == 0)
{
printf("Successful P-operation by program semb. ");
printf("Process id is %d ", getpid());
}
else
{
printf("semb: P-operation did not succeed. ");
}
}
/* Think carefully about what the V-operation does. If sema is executed
twice, then semb can execute twice. */
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