[Go to CFHT Home Page] Man Pages
Back to Software Index  BORDER=0Manpage Top Level
    semop(2) manual page Table of Contents

Name

semop - semaphore operations

Synopsis


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

int semop(int semid, struct sembuf *sops, size_t nsops);

Description

semop() is used to perform atomically an array of semaphore operations on the set of semaphores associated with the semaphore identifier specified by semid. sops is a pointer to the array of semaphore-operation structures. nsops is the number of such structures in the array. The contents of each structure includes the following members: 


short    sem_num;    /* semaphore number */
short    sem_op;    /* semaphore operation */
short    sem_flg;    /* operation flags */

Each semaphore operation specified by sem_op is performed on the corresponding semaphore specified by semid and sem_num. The permission required for a semaphore operation is given as {token}, where token is the type of permission needed. The types of permission are interpreted as follows: 


00400    READ by user
00200    ALTER by user
00040    READ by group
00020    ALTER by group
00004    READ by others
00002    ALTER by others
See the Semaphore Operation Permissions section of intro(2) for more information.

sem_op specifies the {ALTER} token if its value is negative or positive, and the {READ} token if its value is zero. Depending on the value of sem_op, the following may occur:

sem_op is a negative integer; {ALTER}
  • If semval (see intro(2) ) is greater than or equal to the absolute value of sem_op, the absolute value of sem_op is subtracted from semval. Also, if (sem_flg&SEM_UNDO) is true, the absolute value of sem_op is added to the calling process’s semadj value (see exit(2) ) for the specified semaphore.
  • If semval is less than the absolute value of sem_op and (sem_flg&IPC_NOWAIT) is true, semop() returns immediately.
  • If semval is less than the absolute value of sem_op and (sem_flg&IPC_NOWAIT) is false, semop() increments the semncnt associated with the specified semaphore and suspends execution of the calling process until one of the following conditions occur:
  • semval becomes greater than or equal to the absolute value of sem_op. When this occurs, the value of semncnt associated with the specified semaphore is decremented, the absolute value of sem_op is subtracted from semval and, if (sem_flg&SEM_UNDO) is true, the absolute value of sem_op is added to the calling process’s semadj value for the specified semaphore.
  • The semid for which the calling process is awaiting action is removed from the system (see semctl(2) ). When this occurs, errno is set equal to EIDRM, and a value of -1 is returned.
  • The calling process receives a signal that is to be caught. When this occurs, the value of semncnt associated with the specified semaphore is decremented, and the calling process resumes execution in the manner prescribed in signal(3C) .
    • sem_op is a positive integer; {ALTER}
  • The value of sem_op is added to semval and, if (sem_flg&SEM_UNDO) is true, the value of sem_op is subtracted from the calling process’s semadj value for the specified semaphore.
    • sem_op is zero; {READ}
  • If semval is zero, semop() returns immediately.
  • If semval is not equal to zero and (sem_flg&IPC_NOWAIT) is true, semop() returns immediately.
  • If semval is not equal to zero and (sem_flg&IPC_NOWAIT) is false, semop() increments the semzcnt associated with the specified semaphore and suspends execution of the calling process until one of the following occurs:
  • semval becomes zero, at which time the value of semzcnt associated with the specified semaphore is decremented.
  • The semid for which the calling process is awaiting action is removed from the system. When this occurs, errno is set equal to EIDRM, and a value of -1 is returned.
  • The calling process receives a signal that is to be caught. When this occurs, the value of semzcnt associated with the specified semaphore is decremented, and the calling process resumes execution in the manner prescribed in signal(3C) .

    • Return Values

    Upon successful completion, a value of zero is returned. Otherwise, a value of -1 is returned and errno is set to indicate the error.

    Errors

    semop() fails if one or more of the following are true for any of the semaphore operations specified by sops:

    E2BIG
    nsops is greater than the system-imposed maximum.
    EACCES
    Operation permission is denied to the calling process (see intro(2) ).
    EAGAIN
    The operation would result in suspension of the calling process but (sem_flg&IPC_NOWAIT) is true.
    EFAULT
    sops points to an illegal address.
    EFBIG
    sem_num is less than zero or greater than or equal to the number of semaphores in the set associated with semid.
    EIDRM
    semop() A semid was removed from the system.
    EINTR
    A signal was received.
    EINVAL
    semid is not a valid semaphore identifier, or the number of individual semaphores for which the calling process requests a SEM_UNDO would exceed the limit.
    ENOSPC
    The limit on the number of individual processes requesting an SEM_UNDO would be exceeded.
    ERANGE
    An operation would cause a semval or a semadj value to overflow the system-imposed limit.

    Upon successful completion, the value of sempid for each semaphore specified in the array pointed to by sops is set equal to the process ID of the calling process.

    See Also

    ipcs(1) , intro(2) , exec(2) , exit(2) , fork(2) , semctl(2) , semget(2)

    Table of Contents