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shmget(2) [freebsd man page]

SHMGET(2)						      BSD System Calls Manual							 SHMGET(2)

NAME
shmget -- obtain a shared memory identifier LIBRARY
Standard C Library (libc, -lc) SYNOPSIS
#include <sys/types.h> #include <sys/ipc.h> #include <sys/shm.h> int shmget(key_t key, size_t size, int flag); DESCRIPTION
Based on the values of key and flag, shmget() returns the identifier of a newly created or previously existing shared memory segment. The key is analogous to a filename: it provides a handle that names an IPC object. There are three ways to specify a key: o IPC_PRIVATE may be specified, in which case a new IPC object will be created. o An integer constant may be specified. If no IPC object corresponding to key is specified and the IPC_CREAT bit is set in flag, a new one will be created. o The ftok(3) may be used to generate a key from a pathname. The mode of a newly created IPC object is determined by OR'ing the following constants into the flag argument: S_IRUSR Read access for owner. S_IWUSR Write access for owner. S_IRGRP Read access for group. S_IWGRP Write access for group. S_IROTH Read access for other. S_IWOTH Write access for other. When creating a new shared memory segment, size indicates the desired size of the new segment in bytes. The size of the segment may be rounded up to a multiple convenient to the kernel (i.e., the page size). RETURN VALUES
Upon successful completion, shmget() returns the positive integer identifier of a shared memory segment. Otherwise, -1 is returned and errno set to indicate the error. ERRORS
The shmget() system call will fail if: [EINVAL] Size specified is greater than the size of the previously existing segment. Size specified is less than the system imposed minimum, or greater than the system imposed maximum. [ENOENT] No shared memory segment was found matching key, and IPC_CREAT was not specified. [ENOSPC] The kernel was unable to allocate enough memory to satisfy the request. [EEXIST] IPC_CREAT and IPC_EXCL were specified, and a shared memory segment corresponding to key already exists. SEE ALSO
shmat(2), shmctl(2), shmdt(2), stat(2), ftok(3) BSD
December 17, 2010 BSD

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SHMGET(2)						      BSD System Calls Manual							 SHMGET(2)

NAME
shmget -- get shared memory segment LIBRARY
Standard C Library (libc, -lc) SYNOPSIS
#include <sys/shm.h> int shmget(key_t key, size_t size, int shmflg); DESCRIPTION
shmget() returns the shared memory identifier associated with the key key. A shared memory segment is created if either key is equal to IPC_PRIVATE, or key does not have a shared memory segment identifier associated with it, and the IPC_CREAT bit is set in shmflg. If both the IPC_CREAT bit and the IPC_EXCL bit are set in shmflg, and key has a shared mem- ory segment identifier associated with it already, the operation will fail. If a new shared memory segment is created, the data structure associated with it (the shmid_ds structure, see shmctl(2)) is initialized as follows: o shm_perm.cuid and shm_perm.uid are set to the effective uid of the calling process. o shm_perm.gid and shm_perm.cgid are set to the effective gid of the calling process. o shm_perm.mode is set to the lower 9 bits of shmflg. o shm_lpid, shm_nattch, shm_atime, and shm_dtime are set to 0. o shm_ctime is set to the current time. o shm_segsz is set to the value of size. RETURN VALUES
Upon successful completion a positive shared memory segment identifier is returned. Otherwise, -1 is returned and the global variable errno is set to indicate the error. ERRORS
[EACCES] A shared memory segment is already associated with key and the caller has no permission to access it. [EEXIST] Both IPC_CREAT and IPC_EXCL are set in shmflg, and a shared memory segment is already associated with key. [EINVAL] No shared memory segment is to be created, and a shared memory segment exists for key, but the size of the segment associ- ated with it is less than size, which is non-zero. A shared memory segment is to be created, and size is less than the system imposed minimum, or greater than the system imposed maximum (refer to the kern.ipc values in sysctl(7)). [ENOENT] IPC_CREAT is not set in shmflg and no shared memory segment associated with key was found. [ENOMEM] There is not enough memory left to create a shared memory segment of the requested size. [ENOSPC] A new shared memory identifier could not be created because the system limit for the number of shared memory identifiers has been reached. SEE ALSO
ipcrm(1), ipcs(1), mmap(2), shmat(2), shmctl(2), ftok(3), sysctl(7) STANDARDS
The shmget system call conforms to X/Open System Interfaces and Headers Issue 5 (``XSH5''). HISTORY
Shared memory segments appeared in the first release of AT&T System V UNIX. BSD
October 27, 2008 BSD
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