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man:semget

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

NAME

     semget - get a System V semaphore set identifier

SYNOPSIS

     #include <sys/types.h>
     #include <sys/ipc.h>
     #include <sys/sem.h>
     int semget(key_t key, int nsems, int semflg);

DESCRIPTION

     The  semget() system call returns the System V semaphore set identifier
     associated with the argument key.  It may be used either to obtain  the
     identifier  of  a previously created semaphore set (when semflg is zero
     and key does not have the value IPC_PRIVATE), or to create a new set.
     A new set of nsems semaphores is created if key has the value  IPC_PRI-
     VATE  or  if  no  existing  semaphore  set  is  associated with key and
     IPC_CREAT is specified in semflg.
     If semflg specifies both IPC_CREAT and IPC_EXCL  and  a  semaphore  set
     already  exists  for key, then semget() fails with errno set to EEXIST.
     (This is analogous to the effect of the combination  O_CREAT  |  O_EXCL
     for open(2).)
     Upon  creation,  the  least  significant  9 bits of the argument semflg
     define the permissions (for owner, group and others) for the  semaphore
     set.   These  bits  have  the same format, and the same meaning, as the
     mode argument of open(2) (though the execute permissions are not  mean-
     ingful  for  semaphores, and write permissions mean permission to alter
     semaphore values).
     When creating a new semaphore set, semget() initializes the set's asso-
     ciated data structure, semid_ds (see semctl(2)), as follows:
            sem_perm.cuid  and sem_perm.uid are set to the effective user ID
            of the calling process.
            sem_perm.cgid and sem_perm.gid are set to the effective group ID
            of the calling process.
            The  least  significant  9  bits of sem_perm.mode are set to the
            least significant 9 bits of semflg.
            sem_nsems is set to the value of nsems.
            sem_otime is set to 0.
            sem_ctime is set to the current time.
     The argument nsems can be 0 (a don't care) when a semaphore set is  not
     being  created.   Otherwise, nsems must be greater than 0 and less than
     or equal  to  the  maximum  number  of  semaphores  per  semaphore  set
     (SEMMSL).
     If the semaphore set already exists, the permissions are verified.

RETURN VALUE

     If successful, the return value will be the semaphore set identifier (a
     nonnegative integer), otherwise, -1 is returned, with errno  indicating
     the error.

ERRORS

     On failure, errno will be set to one of the following:
     EACCES A semaphore set exists for key, but the calling process does not
            have permission to  access  the  set,  and  does  not  have  the
            CAP_IPC_OWNER  capability in the user namespace that governs its
            IPC namespace.
     EEXIST IPC_CREAT and IPC_EXCL were specified in semflg, but a semaphore
            set already exists for key.
     EINVAL nsems  is less than 0 or greater than the limit on the number of
            semaphores per semaphore set (SEMMSL).
     EINVAL A semaphore set corresponding to key already exists,  but  nsems
            is larger than the number of semaphores in that set.
     ENOENT No  semaphore  set  exists  for  key  and semflg did not specify
            IPC_CREAT.
     ENOMEM A semaphore set has to be created but the system does  not  have
            enough memory for the new data structure.
     ENOSPC A  semaphore  set has to be created but the system limit for the
            maximum number of semaphore sets (SEMMNI), or  the  system  wide
            maximum number of semaphores (SEMMNS), would be exceeded.

CONFORMING TO

     SVr4, POSIX.1-2001.

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.
     IPC_PRIVATE isn't a flag field but a key_t type.  If this special value
     is used for key, the system call ignores all but the least  significant
     9 bits of semflg and creates a new semaphore set (on success).
 Semaphore initialization
     The  values of the semaphores in a newly created set are indeterminate.
     (POSIX.1-2001 and POSIX.1-2008 are explicit  on  this  point,  although
     POSIX.1-2008 notes that a future version of the standard may require an
     implementation to initialize the semaphores  to  0.)   Although  Linux,
     like many other implementations, initializes the semaphore values to 0,
     a portable application cannot rely on this: it should  explicitly  ini-
     tialize the semaphores to the desired values.
     Initialization  can be done using semctl(2) SETVAL or SETALL operation.
     Where multiple peers do not know who will be the  first  to  initialize
     the set, checking for a nonzero sem_otime in the associated data struc-
     ture retrieved by a semctl(2) IPC_STAT operation can be used  to  avoid
     races.
 Semaphore limits
     The  following  limits  on  semaphore set resources affect the semget()
     call:
     SEMMNI System-wide limit on the number of  semaphore  sets.   On  Linux
            systems  before  version  3.19, the default value for this limit
            was 128.  Since Linux 3.19, the default  value  is  32,000.   On
            Linux,  this limit can be read and modified via the fourth field
            of /proc/sys/kernel/sem.
     SEMMSL Maximum number of semaphores per semaphore ID.  On Linux systems
            before  version  3.19, the default value for this limit was 250.
            Since Linux 3.19, the default value is 32,000.  On  Linux,  this
            limit   can  be  read  and  modified  via  the  first  field  of
            /proc/sys/kernel/sem.
     SEMMNS System-wide limit on the number of semaphores: policy  dependent
            (on  Linux,  this  limit can be read and modified via the second
            field of /proc/sys/kernel/sem).  Note that the number  of  sema-
            phores  system-wide is also limited by the product of SEMMSL and
            SEMMNI.

BUGS

     The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW would more
     clearly show its function.

SEE ALSO

     semctl(2),   semop(2),   ftok(3),   capabilities(7),   sem_overview(7),
     svipc(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 2018-04-30 SEMGET(2)

/data/webs/external/dokuwiki/data/pages/man/semget.txt · Last modified: 2019/05/17 09:47 by 127.0.0.1

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