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

# NAME

shmat, shmdt — map/unmap shared memory

# LIBRARY

Standard C Library (libc, -lc)

# 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 0, 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 in existence 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)

# STANDARDS

The shmat and shmdt system calls conform 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.

June 17, 2002 | NetBSD 7.2