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Man Pages
SHMGET(2) FreeBSD System Calls Manual SHMGET(2)

shmget
obtain a shared memory identifier

Standard C Library (libc, -lc)

#include <sys/shm.h>
int
shmget(key_t key, size_t size, int flag);

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:
  • IPC_PRIVATE may be specified, in which case a new IPC object will be created.
  • 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.
  • The ftok(3) may be used to generate a key from a pathname.
The mode of a newly created IPC object is determined by which are set by ORing these constants into the flag argument:
Read access for owner.
Write access for owner.
Read access for group.
Write access for group.
Read access for other.
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).

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.

The shmget() system call will fail if:
[]
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.
[]
No shared memory segment was found matching key, and IPC_CREAT was not specified.
[]
The kernel was unable to allocate enough memory to satisfy the request.
[]
IPC_CREAT and IPC_EXCL were specified, and a shared memory segment corresponding to key already exists.

shmat(2), shmctl(2), shmdt(2), ftok(3)
March 4, 2018 FreeBSD 12.0-RELEASE

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