Full Discussion: Hi...Need Help
Top Forums Programming Hi...Need Help Post 17972 by rwb1959 on Thursday 21st of March 2002 03:09:35 PM
Old 03-21-2002
I believe that when you are "setting" a semaphore,
the argument (4th parameter in semctl() ) needs to be
a union...

union semun {
int val;
struct semid_ds *buf;
unsigned short *array;
} arg;

...check out sem.h for complete union def. for your system.
In which case you do...

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

main()
{
int semid,retval;
union semun args;
semid=semget(0x20,1,IPC_CREAT|0666);
retval= semctl(semid,0,GETVAL,0);
printf("The value returned by the Getval is %d\n",retval);
args.val = 1;
semctl(semid,0,SETVAL,args);
printf("HI2\n");
retval= semctl(semid,0,GETVAL,0);
printf("The value returned by the Getval is %d\n",retval);
}
 
SEMCTL(2)						      BSD System Calls Manual							 SEMCTL(2)

NAME
semctl -- control operations on a semaphore set SYNOPSIS
#include <sys/sem.h> int semctl(int semid, int semnum, int cmd, ...); DESCRIPTION
The semctl() system call performs the operation indicated by cmd on the semaphore set indicated by semid. A fourth argument, a union semun arg, is required for certain values of cmd. For the commands that use the arg argument, union semun is defined as follows: union semun { int val; /* value for SETVAL */ struct semid_ds *buf; /* buffer for IPC_STAT & IPC_SET */ u_short *array; /* array for GETALL & SETALL */ }; Commands are performed as follows: IPC_STAT Fetch the semaphore set's struct semid_ds, storing it in the memory pointed to by arg.buf. IPC_SET Changes the sem_perm.uid, sem_perm.gid, and sem_perm.mode members of the semaphore set's struct semid_ds to match those of the struct pointed to by arg.buf. The calling process's effective uid must match either sem_perm.uid or sem_perm.cuid, or it must have superuser privileges. IPC_RMID Immediately removes the semaphore set from the system. The calling process's effective uid must equal the semaphore set's sem_perm.uid or sem_perm.cuid, or the process must have superuser privileges. GETVAL Return the value of semaphore number semnum. SETVAL Set the value of semaphore number semnum to arg.val. Outstanding adjust on exit values for this semaphore in any process are cleared. GETPID Return the pid of the last process to perform an operation on semaphore number semnum. GETNCNT Return the number of processes waiting for semaphore number semnum's value to become greater than its current value. GETZCNT Return the number of processes waiting for semaphore number semnum's value to become 0. GETALL Fetch the value of all of the semaphores in the set into the array pointed to by arg.array. SETALL Set the values of all of the semaphores in the set to the values in the array pointed to by arg.array. Outstanding adjust on exit values for all semaphores in this set, in any process are cleared. The struct semid_ds is defined as follows: struct semid_ds { struct ipc_perm sem_perm; /* operation permission struct */ struct sem *sem_base; /* pointer to first semaphore in set */ u_short sem_nsems; /* number of sems in set */ time_t sem_otime; /* last operation time */ long sem_pad1; /* SVABI/386 says I need this here */ time_t sem_ctime; /* last change time */ /* Times measured in secs since */ /* 00:00:00 GMT, Jan. 1, 1970 */ long sem_pad2; /* SVABI/386 says I need this here */ long sem_pad3[4]; /* SVABI/386 says I need this here */ }; RETURN VALUES
On success, when cmd is one of GETVAL, GETPID, GETNCNT or GETZCNT, semctl() returns the corresponding value; otherwise, 0 is returned. On failure, -1 is returned, and errno is set to indicate the error. ERRORS
The semctl() system call will fail if: [EACCES] Permission denied due to mismatch between operation and mode of semaphore set. [EINVAL] No semaphore set corresponds to semid. [EINVAL] The semnum argument is not in the range of valid semaphores for given semaphore set. [EPERM] The calling process's effective uid does not match the uid of the semaphore set's owner or creator. [ERANGE] SETVAL or SETALL attempted to set a semaphore outside the allowable range [0 .. SEMVMX]. LEGACY SYNOPSIS
#include <sys/types.h> #include <sys/ipc.h> #include <sys/sem.h> The include files <sys/types.h> and <sys/ipc.h> are necessary. SEE ALSO
semget(2), semop(2), compat(5) BUGS
SETALL may update some semaphore elements before returning an error. BSD
September 12, 1995 BSD

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