SHMGET (2) | System calls | Unix Manual Pages | :man ▋
shmget - obtain a shared memory identifier
Library Synopsis Description Return Values Errors
.In machine/param.h .In sys/types.h .In sys/ipc.h .In 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
OR ing the following constants into the flag argument:
SHM_R Read access for user. SHM_W Write access for user. ( SHM_R>>3) Read access for group. ( SHM_W>>3) Write access for group. ( SHM_R>>6) Read access for other. ( SHM_W>>6) 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:
[ 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.
shmat(2), shmctl(2), shmdt(2),