NAME

shm_open, shm_unlink — shared memory object operations

LIBRARY

POSIX Real-time Library (librt, -lrt)

SYNOPSIS

#include <sys/types.h>
#include <sys/mman.h>
#include <fcntl.h>

int
shm_open(const char *name, int flags, mode_t mode);

int
shm_unlink(const char *name);

DESCRIPTION

The shm_open() function creates or opens a POSIX shared memory object named name. The flags argument contains a subset of the flags used by open(2). An access mode of either O_RDONLY or O_RDWR must be included in flags. The optional flags O_CREAT, O_EXCL, and O_TRUNC may also be specified.

If O_CREAT is specified, then a new shared memory object named name will be created if it does not exist. In this case, the shared memory object is created with mode mode subject to the process' umask value. If both the O_CREAT and O_EXCL flags are specified and a shared memory object named path already exists, then shm_open() will fail with EEXIST.

Newly created objects start off with a size of zero. If an existing shared memory object is opened with O_RDWR and the O_TRUNC flag is specified, then the shared memory object will be truncated to a size of zero. The size of the object can be adjusted via ftruncate(2) and queried via fstat(2).

The new descriptor is set to close during execve(2) system calls; see close(2) and fcntl(2).

The shm_unlink() system call removes a shared memory object named path.

RETURN VALUES

If successful, shm_open() returns a non-negative integer, and shm_unlink() returns zero. Both functions return -1 on failure, and set errno to indicate the error.

COMPATIBILITY

The path argument does not necessarily represent a pathname (although it does in most other implementations). Two processes opening the same path are guaranteed to access the same shared memory object if and only if path begins with a slash (‘/’) character.

Only the O_RDONLY, O_RDWR, O_CREAT, O_EXCL, and O_TRUNC flags may be used in portable programs.

The result of using open(2), read(2), or write(2) on a shared memory object, or on the descriptor returned by shm_open(), is undefined. It is also undefined whether the shared memory object itself, or its contents, persist across reboots.

ERRORS

The following errors are defined for shm_open():

[EACCES]

The required permissions (for reading or reading and writing) are denied.

[EEXIST]

O_CREAT and O_EXCL are specified and the named shared memory object does exist.

[EFAULT]

The path argument points outside the process' allocated address space.

[EINVAL]

A flag other than O_RDONLY, O_RDWR, O_CREAT, O_EXCL, or O_TRUNC was included in flags; or the path does not begin with a slash (‘/’) character.

[EMFILE]

The process has already reached its limit for open file descriptors.

[ENAMETOOLONG]

The entire pathname exceeded {PATH_MAX} characters.

[ENFILE]

The system file table is full.

[ENOENT]

O_CREAT is specified and the named shared memory object does not exist.

[ENOTSUP]

Not supported, most likely due to missing or incorrect /var/shm mount.

The following errors are defined for shm_unlink():

[EACCES]

The required permissions are denied. shm_unlink() requires write permission to the shared memory object.

[EFAULT]

The path argument points outside the process' allocated address space.

[ENAMETOOLONG]

The entire pathname exceeded {PATH_MAX} characters.

[ENOENT]

The named shared memory object does not exist.

SEE ALSO

close(2), fstat(2), ftruncate(2), mmap(2), munmap(2)

STANDARDS

The shm_open() and shm_unlink() functions are expected to conform to IEEE Std 1003.1b-1993 (“POSIX.1b”).

HISTORY

These functions first appeared in NetBSD 7.0.