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SHMAT(2) System Calls Manual SHMAT(2)


shmat, shmdtattach or detach shared memory


Standard C Library (libc, -lc)


#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>

void *
shmat(int shmid, const void *addr, int flag);

shmdt(const void *addr);


The () system call attaches the shared memory segment identified by shmid to the calling process's address space. The address where the segment is attached is determined as follows:

  • If addr is 0, the segment is attached at an address selected by the kernel.
  • If addr is nonzero and SHM_RND is not specified in flag, the segment is attached the specified address.
  • If addr is specified and SHM_RND is specified, addr is rounded down to the nearest multiple of SHMLBA.

If the SHM_REMAP flag is specified and the passed addr is not NULL, any existing mappings in the virtual addresses range are cleared before the segment is attached. If the flag is not specified, addr is not NULL, and the virtual address range contains some pre-existing mappings, the () call fails.

The () system call detaches the shared memory segment at the address specified by addr from the calling process's address space.


Upon success, shmat() returns the address where the segment is attached; otherwise, -1 is returned and errno is set to indicate the error.

The shmdt() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.


The shmat() system call will fail if:

No shared memory segment was found corresponding to shmid.
The addr argument was not an acceptable address.
The specified addr cannot be used for mapping, for instance due to the amount of available space being smaller than the segment size, or because pre-existing mappings are in the range and no SHM_REMAP flag was provided.
Failed to attach the shared memory segment because the per-process kern.ipc.shmseg sysctl(3) limit was reached.

The shmdt() system call will fail if:

The addr argument does not point to a shared memory segment.


shmctl(2), shmget(2)

January 14, 2019 Debian