SHM_OPEN(3) | Library Functions Manual | SHM_OPEN(3)

# 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.

December 19, 2013 | NetBSD 9.2