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SHM_OPEN(2) System Calls Manual SHM_OPEN(2)

shm_open, shm_unlink — shared memory object operations

library “libc”

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

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

int
shm_unlink(const char *path);

The () system call opens (or optionally creates) a POSIX shared memory object named path. 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 path 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 () 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).

As a FreeBSD extension, the constant SHM_ANON may be used for the path argument to (). In this case, an anonymous, unnamed shared memory object is created. Since the object has no name, it cannot be removed via a subsequent call to shm_unlink(). Instead, the shared memory object will be garbage collected when the last reference to the shared memory object is removed. The shared memory object may be shared with other processes by sharing the file descriptor via fork(2) or sendmsg(2). Attempting to open an anonymous shared memory object with O_RDONLY will fail with EINVAL. All other flags are ignored.

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.

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.

POSIX specifications state that 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. However, the FreeBSD kernel implementation explicitly includes support for read(2) and write(2).

FreeBSD also supports zero-copy transmission of data from shared memory objects with sendfile(2).

Neither shared memory objects nor their contents persist across reboots.

Writes do not extend shared memory objects, so ftruncate(2) must be called before any data can be written. See EXAMPLES.

This example fails without the call to ftruncate(2):

        uint8_t buffer[getpagesize()];
        ssize_t len;
        int fd;

        fd = shm_open(SHM_ANON, O_RDWR | O_CREAT, 0600);
        if (fd < 0)
                err(EX_OSERR, "%s: shm_open", __func__);
        if (ftruncate(fd, getpagesize()) < 0)
                err(EX_IOERR, "%s: ftruncate", __func__);
        len = pwrite(fd, buffer, getpagesize(), 0);
        if (len < 0)
                err(EX_IOERR, "%s: pwrite", __func__);
        if (len != getpagesize())
                errx(EX_IOERR, "%s: pwrite length mismatch", __func__);

shm_open() fails with these error codes for these conditions:

[]
A flag other than O_RDONLY, O_RDWR, O_CREAT, O_EXCL, or O_TRUNC was included in flags.
[]
The process has already reached its limit for open file descriptors.
[]
The system file table is full.
[]
was specified while creating an anonymous shared memory object via SHM_ANON.
[]
The path argument points outside the process' allocated address space.
[]
The entire pathname exceeded 1023 characters.
[]
The path does not begin with a slash (‘/’) character.
[]
is specified and the named shared memory object does not exist.
[]
and O_EXCL are specified and the named shared memory object does exist.
[]
The required permissions (for reading or reading and writing) are denied.

shm_unlink() fails with these error codes for these conditions:

[]
The path argument points outside the process' allocated address space.
[]
The entire pathname exceeded 1023 characters.
[]
The named shared memory object does not exist.
[]
The required permissions are denied. shm_unlink() requires write permission to the shared memory object.

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

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

The shm_open() and shm_unlink() functions first appeared in FreeBSD 4.3. The functions were reimplemented as system calls using shared memory objects directly rather than files in FreeBSD 8.0.

Garrett A. Wollman <wollman@FreeBSD.org> (C library support and this manual page)


Matthew Dillon <dillon@FreeBSD.org> (MAP_NOSYNC)

January 20, 2017 FreeBSD-12.0