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man:semtimedop

SEMOP(2) Linux Programmer's Manual SEMOP(2)

NAME

     semop, semtimedop - System V 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);
     int semtimedop(int semid, struct sembuf *sops, size_t nsops,
                    const struct timespec *timeout);
 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
     semtimedop(): _GNU_SOURCE

DESCRIPTION

     Each semaphore in a System V semaphore set has the following associated
     values:
         unsigned short  semval;   /*  semaphore  value  */  unsigned  short
         semzcnt;   /*  # waiting for zero */ unsigned short  semncnt;  /* #
         waiting for increase */ pid_t           sempid;   /* PID of process
         that last
     semop() performs operations on selected semaphores in the set indicated
     by semid.  Each of the nsops elements in the array pointed to  by  sops
     is  a structure that specifies an operation to be performed on a single
     semaphore.  The elements of this structure are of type  struct  sembuf,
     containing the following members:
         unsigned   short   sem_num;    /*   semaphore   number   */   short
         sem_op;   /* semaphore operation  */  short           sem_flg;   /*
         operation flags */
     Flags  recognized in sem_flg are IPC_NOWAIT and SEM_UNDO.  If an opera-
     tion specifies SEM_UNDO, it  will  be  automatically  undone  when  the
     process terminates.
     The  set  of  operations contained in sops is performed in array order,
     and atomically, that is, the operations are performed either as a  com-
     plete  unit, or not at all.  The behavior of the system call if not all
     operations can be performed immediately depends on the presence of  the
     IPC_NOWAIT flag in the individual sem_flg fields, as noted below.
     Each  operation  is  performed on the sem_num-th semaphore of the sema-
     phore set, where the first semaphore of the set is numbered  0.   There
     are three types of operation, distinguished by the value of sem_op.
     If  sem_op  is a positive integer, the operation adds this value to the
     semaphore value (semval).  Furthermore, if SEM_UNDO  is  specified  for
     this  operation,  the  system subtracts the value sem_op from the sema-
     phore adjustment (semadj) value for this semaphore.  This operation can
     always  proceed--it never forces a thread to wait.  The calling process
     must have alter permission on the semaphore set.
     If sem_op is zero, the process must have read permission on  the  sema-
     phore set.  This is a "wait-for-zero" operation: if semval is zero, the
     operation can immediately proceed.  Otherwise, if IPC_NOWAIT is  speci-
     fied  in  sem_flg,  semop() fails with errno set to EAGAIN (and none of
     the operations in sops is performed).  Otherwise, semzcnt (the count of
     threads  waiting  until  this semaphore's value becomes zero) is incre-
     mented by one and the thread sleeps until one of the following occurs:
     o  semval becomes 0, at which time the value of semzcnt is decremented.
     o  The  semaphore  set  is  removed:  semop()  fails, with errno set to
        EIDRM.
     o  The calling thread catches a signal: the value of semzcnt is  decre-
        mented and semop() fails, with errno set to EINTR.
     If  sem_op is less than zero, the process must have alter permission on
     the semaphore set.  If semval is greater than or equal to the  absolute
     value  of  sem_op,  the operation can proceed immediately: the absolute
     value of sem_op is subtracted from semval, and, if SEM_UNDO  is  speci-
     fied  for  this operation, the system adds the absolute value of sem_op
     to the semaphore adjustment (semadj) value for this semaphore.  If  the
     absolute  value  of  sem_op  is  greater than semval, and IPC_NOWAIT is
     specified in sem_flg, semop() fails, with errno set to EAGAIN (and none
     of  the  operations  in  sops  is  performed).  Otherwise, semncnt (the
     counter of threads waiting for this semaphore's value to  increase)  is
     incremented  by  one  and  the thread sleeps until one of the following
     occurs:
     o  semval becomes greater than  or  equal  to  the  absolute  value  of
        sem_op: the operation now proceeds, as described above.
     o  The  semaphore  set  is removed from the system: semop() fails, with
        errno set to EIDRM.
     o  The calling thread catches a signal: the value of semncnt is  decre-
        mented and semop() fails, with errno set to EINTR.
     On successful completion, the sempid value for each semaphore specified
     in the array pointed to by sops is set to the caller's process ID.   In
     addition, the sem_otime is set to the current time.
 semtimedop()
     semtimedop()  behaves identically to semop() except that in those cases
     where the calling thread would sleep, the duration  of  that  sleep  is
     limited  by the amount of elapsed time specified by the timespec struc-
     ture whose address is passed in  the  timeout  argument.   (This  sleep
     interval will be rounded up to the system clock granularity, and kernel
     scheduling delays mean  that  the  interval  may  overrun  by  a  small
     amount.)   If  the  specified time limit has been reached, semtimedop()
     fails with errno set to EAGAIN (and none of the operations in  sops  is
     performed).  If the timeout argument is NULL, then semtimedop() behaves
     exactly like semop().
     Note that if semtimedop() is interrupted by a signal, causing the  call
     to  fail  with  the  error  EINTR,  the  contents  of  timeout are left
     unchanged.

RETURN VALUE

     If successful, semop() and semtimedop() return 0; otherwise they return
     -1 with errno indicating the error.

ERRORS

     On failure, errno is set to one of the following:
     E2BIG  The argument nsops is greater than SEMOPM, the maximum number of
            operations allowed per system call.
     EACCES The calling process does not have the  permissions  required  to
            perform  the  specified  semaphore operations, and does not have
            the CAP_IPC_OWNER capability in the user namespace that  governs
            its IPC namespace.
     EAGAIN An operation could not proceed immediately and either IPC_NOWAIT
            was specified in sem_flg or the time limit specified in  timeout
            expired.
     EFAULT An  address specified in either the sops or the timeout argument
            isn't accessible.
     EFBIG  For some operation the value  of  sem_num  is  less  than  0  or
            greater than or equal to the number of semaphores in the set.
     EIDRM  The semaphore set was removed.
     EINTR  While  blocked  in this system call, the thread caught a signal;
            see signal(7).
     EINVAL The semaphore set doesn't exist, or semid is less than zero,  or
            nsops has a nonpositive value.
     ENOMEM The  sem_flg of some operation specified SEM_UNDO and the system
            does not have enough memory to allocate the undo structure.
     ERANGE For some operation sem_op+semval is  greater  than  SEMVMX,  the
            implementation dependent maximum value for semval.

VERSIONS

     semtimedop() first appeared in Linux 2.5.52, and was subsequently back-
     ported into  kernel  2.4.22.   Glibc  support  for  semtimedop()  first
     appeared in version 2.3.3.

CONFORMING TO

     POSIX.1-2001, POSIX.1-2008, SVr4.

NOTES

     The  inclusion of <sys/types.h> and <sys/ipc.h> isn't required on Linux
     or by any version of POSIX.  However, some old implementations required
     the inclusion of these header files, and the SVID also documented their
     inclusion.  Applications intended to be portable to  such  old  systems
     may need to include these header files.
     The sem_undo structures of a process aren't inherited by the child pro-
     duced by fork(2), but they are inherited  across  an  execve(2)  system
     call.
     semop()  is  never automatically restarted after being interrupted by a
     signal handler, regardless of the setting of the SA_RESTART  flag  when
     establishing a signal handler.
     A  semaphore  adjustment (semadj) value is a per-process, per-semaphore
     integer that is the negated sum of all operations performed on a  sema-
     phore  specifying the SEM_UNDO flag.  Each process has a list of semadj
     values--one value for each semaphore on which  it  has  operated  using
     SEM_UNDO.   When a process terminates, each of its per-semaphore semadj
     values is added to the corresponding semaphore, thus undoing the effect
     of  that  process's  operations  on the semaphore (but see BUGS below).
     When a semaphore's value is directly set using  the  SETVAL  or  SETALL
     request  to semctl(2), the corresponding semadj values in all processes
     are cleared.  The clone(2) CLONE_SYSVSEM  flag  allows  more  than  one
     process to share a semadj list; see clone(2) for details.
     The  semval, sempid, semzcnt, and semnct values for a semaphore can all
     be retrieved using appropriate semctl(2) calls.
 Semaphore limits
     The following limits on semaphore  set  resources  affect  the  semop()
     call:
     SEMOPM Maximum  number  of  operations  allowed  for  one semop() call.
            Before Linux 3.19, the default value  for  this  limit  was  32.
            Since  Linux  3.19,  the  default  value is 500.  On Linux, this
            limit  can  be  read  and  modified  via  the  third  field   of
            /proc/sys/kernel/sem.   Note:  this  limit  should not be raised
            above 1000, because of the risk of that  semop()  fails  due  to
            kernel  memory  fragmentation when allocating memory to copy the
            sops array.
     SEMVMX Maximum allowable value  for  semval:  implementation  dependent
            (32767).
     The implementation has no intrinsic limits for the adjust on exit maxi-
     mum value (SEMAEM), the system wide maximum number of  undo  structures
     (SEMMNU)  and  the  per-process  maximum  number of undo entries system
     parameters.

BUGS

     When a process terminates, its set of associated semadj  structures  is
     used to undo the effect of all of the semaphore operations it performed
     with the SEM_UNDO flag.  This raises a difficulty: if one (or more)  of
     these  semaphore  adjustments  would result in an attempt to decrease a
     semaphore's value below zero, what should an  implementation  do?   One
     possible approach would be to block until all the semaphore adjustments
     could be performed.  This is however undesirable since it  could  force
     process  termination  to  block  for arbitrarily long periods.  Another
     possibility is that such semaphore adjustments could be  ignored  alto-
     gether  (somewhat  analogously  to failing when IPC_NOWAIT is specified
     for a semaphore operation).  Linux adopts a third approach:  decreasing
     the  semaphore  value  as  far as possible (i.e., to zero) and allowing
     process termination to proceed immediately.
     In kernels 2.6.x, x <= 10, there is a bug that  in  some  circumstances
     prevents  a thread that is waiting for a semaphore value to become zero
     from being woken up when the value does actually become zero.  This bug
     is fixed in kernel 2.6.11.

EXAMPLE

     The  following  code  segment  uses  semop() to atomically wait for the
     value of semaphore 0 to become zero, and then increment  the  semaphore
     value by one.
         struct sembuf sops[2]; int semid;
         /* Code to set semid omitted */
         sops[0].sem_num   =   0;          /*  Operate  on  semaphore  0  */
         sops[0].sem_op = 0;          /*  Wait  for  value  to  equal  0  */
         sops[0].sem_flg = 0;
         sops[1].sem_num   =   0;          /*  Operate  on  semaphore  0  */
         sops[1].sem_op  =  1;          /*  Increment  value   by   one   */
         sops[1].sem_flg = 0;
         if (semop(semid, sops, 2) == -1) {
             perror("semop");
             exit(EXIT_FAILURE); }

SEE ALSO

     clone(2),    semctl(2),   semget(2),   sigaction(2),   capabilities(7),
     sem_overview(7), svipc(7), time(7)

COLOPHON

     This page is part of release 4.16 of the Linux  man-pages  project.   A
     description  of  the project, information about reporting bugs, and the
     latest    version    of    this    page,    can     be     found     at
     https://www.kernel.org/doc/man-pages/.

Linux 2017-09-15 SEMOP(2)

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