--- name: "shm_open" section: "2freebsd" platform: "linux" package: "freebsd-manpages" version: "12.2-2" release: "Debian 13 (Trixie)" platforms: ["linux"] url: "https://new.carta.tech/shm_open/2freebsd" --- # shm\_open(2freebsd) shared memory object operations ## NAME **shm\_open**, **shm\_unlink** – shared memory object operations ## LIBRARY Standard C Library (libc, -lc) ## SYNOPSIS **#include \** **#include \** **#include \** *int*\ **shm\_open**(*const char \*path*, *int flags*, *mode\_t mode*); *int*\ **shm\_unlink**(*const char \*path*); ## DESCRIPTION The **shm\_open**() 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)](/open/2.md). 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 **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)](/ftruncate/2.md) and queried via [fstat(2)](/fstat/2.md). The new descriptor is set to close during [execve(2)](/execve/2.md) system calls; see [close(2)](/close/2.md) and [fcntl(2)](/fcntl/2.md). As a FreeBSD extension, the constant SHM\_ANON may be used for the *path* argument to **shm\_open**(). 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)](/fork/2.md) or [sendmsg(2)](/sendmsg/2.md). Attempting to open an anonymous shared memory object with O\_RDONLY will fail with EINVAL. All other flags are ignored. 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. POSIX specifications state that the result of using [open(2)](/open/2.md), [read(2)](/read/2.md), or [write(2)](/write/2.md) 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)](/read/2.md) and [write(2)](/write/2.md). FreeBSD also supports zero-copy transmission of data from shared memory objects with [sendfile(2)](/sendfile/2.md). Neither shared memory objects nor their contents persist across reboots. Writes do not extend shared memory objects, so [ftruncate(2)](/ftruncate/2.md) must be called before any data can be written. See EXAMPLES. ## EXAMPLES This example fails without the call to [ftruncate(2)](/ftruncate/2.md): ``` 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__); ``` ## ERRORS **shm\_open**() fails with these error codes for these conditions: - \[EINVAL\] A flag other than O\_RDONLY, O\_RDWR, O\_CREAT, O\_EXCL, or O\_TRUNC was included in *flags*. - \[EMFILE\] The process has already reached its limit for open file descriptors. - \[ENFILE\] The system file table is full. - \[EINVAL\] O\_RDONLY was specified while creating an anonymous shared memory object via SHM\_ANON. - \[EFAULT\] The *path* argument points outside the process' allocated address space. - \[ENAMETOOLONG\] The entire pathname exceeded 1023 characters. - \[EINVAL\] The *path* does not begin with a slash (‘/’) character. - \[ENOENT\] O\_CREAT is specified and the named shared memory object does not exist. - \[EEXIST\] O\_CREAT and O\_EXCL are specified and the named shared memory object does exist. - \[EACCES\] The required permissions (for reading or reading and writing) are denied. **shm\_unlink**() fails with these error codes for these conditions: - \[EFAULT\] The *path* argument points outside the process' allocated address space. - \[ENAMETOOLONG\] The entire pathname exceeded 1023 characters. - \[ENOENT\] The named shared memory object does not exist. - \[EACCES\] The required permissions are denied. **shm\_unlink**() requires write permission to the shared memory object. ## SEE ALSO [close(2)](/close/2.md), [fstat(2)](/fstat/2.md), [ftruncate(2)](/ftruncate/2.md), [mmap(2)](/mmap/2.md), [munmap(2)](/munmap/2.md), [sendfile(2)](/sendfile/2.md) ## STANDARDS The **shm\_open**() and **shm\_unlink**() functions are believed to conform to IEEE Std 1003.1b-1993 (“POSIX.1b”). ## HISTORY 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. ## AUTHORS Garrett A. Wollman ⟨*wollman@FreeBSD.org*⟩ (C library support and this manual page) Matthew Dillon ⟨*dillon@FreeBSD.org*⟩ (MAP\_NOSYNC)