bionic (2) semget.2.gz

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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.
       A new set of nsems semaphores is created if key has the value IPC_PRIVATE 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 meaningful for semaphores, and write
       permissions mean permission to alter semaphore values).

       When creating a new semaphore set, semget() initializes the set's  associated  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
       initialize 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
       structure 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 semaphores
              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

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