SHMGET (2) | System calls | Unix Manual Pages | :man▋
shmget - obtain a shared memory identifier
.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:
| || 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)
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