SHMAT(2) | System Calls Manual | SHMAT(2)

# NAME

`shmat`, `shmdt` — map/unmap shared memory

# SYNOPSIS

`#include <sys/shm.h>`

void *
`shmat`(int shmid, const void *shmaddr, int shmflg);

int
`shmdt`(const void *shmaddr);

# DESCRIPTION

`shmat`() maps the shared memory segment associated with the shared memory identifier shmid into the address space of the calling process. The address at which the segment is mapped is determined by the shmaddr parameter. If it is equal to NULL, the system will pick an address itself. Otherwise, an attempt is made to map the shared memory segment at the address shmaddr specifies. If `SHM_RND` is set in shmflg, the system will round the address down to a multiple of `SHMLBA` bytes (`SHMLBA` is defined in `<sys/shm.h>`).

A shared memory segment can be mapped read-only by specifying the `SHM_RDONLY` flag in shmflg.

`shmdt`() unmaps the shared memory segment that is currently mapped at shmaddr from the calling process' address space. shmaddr must be a value returned by a prior `shmat`() call. A shared memory segment will remain existent until it is removed by a call to shmctl(2) with the `IPC_RMID` command.

# RETURN VALUES

`shmat`() returns the address at which the shared memory segment has been mapped into the calling process' address space when successful, `shmdt`() returns 0 on successful completion. Otherwise, a value of -1 is returned, and the global variable errno is set to indicate the error.

# ERRORS

`shmat`() will fail if:

[`EACCES`]
  The calling process has no permission to access this shared memory segment.
[`ENOMEM`]
  There is not enough available data space for the calling process to map the shared memory segment.
[`EINVAL`]
  shmid is not a valid shared memory identifier.

  shmaddr specifies an illegal address.

[`EMFILE`]
  The number of shared memory segments has reached the system-wide limit.

`shmdt`() will fail if:

[`EINVAL`]
  shmaddr is not the start address of a mapped shared memory segment.

# SEE ALSO

ipcrm(1), ipcs(1), mmap(2), shmctl(2), shmget(2)

June 16, 2022 | Debian