--- name: "open" section: "2freebsd" platform: "linux" package: "freebsd-manpages" version: "12.2-2" release: "Debian 13 (Trixie)" platforms: ["linux"] url: "https://new.carta.tech/open/2freebsd" --- # open(2freebsd) open or create a file for reading, writing or executing ## NAME **open**, **openat** – open or create a file for reading, writing or executing ## LIBRARY Standard C Library (libc, -lc) ## SYNOPSIS **#include \** *int*\ **open**(*const char \*path*, *int flags*, *...*); *int*\ **openat**(*int fd*, *const char \*path*, *int flags*, *...*); ## DESCRIPTION The file name specified by *path* is opened for either execution or reading and/or writing as specified by the argument *flags* and the file descriptor returned to the calling process. The *flags* argument may indicate the file is to be created if it does not exist (by specifying the O\_CREAT flag). In this case **open**() and **openat**() require an additional argument *mode\_t mode*, and the file is created with mode *mode* as described in [chmod(2)](/chmod/2.md) and modified by the process' umask value (see [umask(2)](/umask/2.md)). The **openat**() function is equivalent to the **open**() function except in the case where the *path* specifies a relative path. In this case the file to be opened is determined relative to the directory associated with the file descriptor *fd* instead of the current working directory. The *flag* parameter and the optional fourth parameter correspond exactly to the parameters of **open**(). If **openat**() is passed the special value AT\_FDCWD in the *fd* parameter, the current working directory is used and the behavior is identical to a call to **open**(). In [capsicum(4)](/capsicum/4.md) capability mode, **open**() is not permitted. The *path* argument to **openat**() must be strictly relative to a file descriptor *fd*, as defined in *sys/kern/vfs\_lookup.c*. *path* must not be an absolute path and must not contain ".." components. Additionally, no symbolic link in *path* may contain ".." components either. *fd* must not be AT\_FDCWD. The flags specified are formed by *or*'ing the following values ``` O_RDONLY open for reading only O_WRONLY open for writing only O_RDWR open for reading and writing O_EXEC open for execute only O_SEARCH open for search only, an alias for O_EXEC O_NONBLOCK do not block on open O_APPEND append on each write O_CREAT create file if it does not exist O_TRUNC truncate size to 0 O_EXCL error if create and file exists O_SHLOCK atomically obtain a shared lock O_EXLOCK atomically obtain an exclusive lock O_DIRECT eliminate or reduce cache effects O_FSYNC synchronous writes O_SYNC synchronous writes O_NOFOLLOW do not follow symlinks O_NOCTTY ignored O_TTY_INIT ignored O_DIRECTORY error if file is not a directory O_CLOEXEC set FD_CLOEXEC upon open O_VERIFY verify the contents of the file ``` Opening a file with O\_APPEND set causes each write on the file to be appended to the end. If O\_TRUNC is specified and the file exists, the file is truncated to zero length. If O\_EXCL is set with O\_CREAT and the file already exists, **open**() returns an error. This may be used to implement a simple exclusive access locking mechanism. If O\_EXCL is set and the last component of the pathname is a symbolic link, **open**() will fail even if the symbolic link points to a non-existent name. If the O\_NONBLOCK flag is specified and the **open**() system call would result in the process being blocked for some reason (e.g., waiting for carrier on a dialup line), **open**() returns immediately. The descriptor remains in non-blocking mode for subsequent operations. If O\_FSYNC is used in the mask, all writes will immediately and synchronously be written to disk. O\_SYNC is a synonym for O\_FSYNC required by POSIX. If O\_NOFOLLOW is used in the mask and the target file passed to **open**() is a symbolic link then the **open**() will fail. When opening a file, a lock with [flock(2)](/flock/2.md) semantics can be obtained by setting O\_SHLOCK for a shared lock, or O\_EXLOCK for an exclusive lock. If creating a file with O\_CREAT, the request for the lock will never fail (provided that the underlying file system supports locking). O\_DIRECT may be used to minimize or eliminate the cache effects of reading and writing. The system will attempt to avoid caching the data you read or write. If it cannot avoid caching the data, it will minimize the impact the data has on the cache. Use of this flag can drastically reduce performance if not used with care. O\_NOCTTY may be used to ensure the OS does not assign this file as the controlling terminal when it opens a tty device. This is the default on FreeBSD, but is present for POSIX compatibility. The **open**() system call will not assign controlling terminals on FreeBSD. O\_TTY\_INIT may be used to ensure the OS restores the terminal attributes when initially opening a TTY. This is the default on FreeBSD, but is present for POSIX compatibility. The initial call to **open**() on a TTY will always restore default terminal attributes on FreeBSD. O\_DIRECTORY may be used to ensure the resulting file descriptor refers to a directory. This flag can be used to prevent applications with elevated privileges from opening files which are even unsafe to open with O\_RDONLY, such as device nodes. O\_CLOEXEC may be used to set FD\_CLOEXEC flag for the newly returned file descriptor. O\_VERIFY may be used to indicate to the kernel that the contents of the file should be verified before allowing the open to proceed. The details of what “verified” means is implementation specific. The run-time linker (rtld) uses this flag to ensure shared objects have been verified before operating on them. When *fd* is opened with O\_SEARCH, execute permissions are checked at open time. The *fd* may not be used for any read operations like [getdirentries(2)](/getdirentries/2.md). The primary use for this descriptor will be as the lookup descriptor for the **\*at**() family of functions. If successful, **open**() returns a non-negative integer, termed a file descriptor. It returns -1 on failure. The file pointer used to mark the current position within the file is set to the beginning of the file. If a sleeping open of a device node from devfs(5) is interrupted by a signal, the call always fails with EINTR, even if the SA\_RESTART flag is set for the signal. A sleeping open of a fifo (see [mkfifo(2)](/mkfifo/2.md)) is restarted as normal. When a new file is created it is given the group of the directory which contains it. Unless O\_CLOEXEC flag was specified, the new descriptor is set to remain open across [execve(2)](/execve/2.md) system calls; see [close(2)](/close/2.md), [fcntl(2)](/fcntl/2.md) and O\_CLOEXEC description. The system imposes a limit on the number of file descriptors open simultaneously by one process. The [getdtablesize(2)](/getdtablesize/2.md) system call returns the current system limit. ## RETURN VALUES If successful, **open**() and **openat**() return a non-negative integer, termed a file descriptor. They return -1 on failure, and set *errno* to indicate the error. ## ERRORS The named file is opened unless: - \[ENOTDIR\] A component of the path prefix is not a directory. - \[ENAMETOOLONG\] A component of a pathname exceeded 255 characters, or an entire path name exceeded 1023 characters. - \[ENOENT\] O\_CREAT is not set and the named file does not exist. - \[ENOENT\] A component of the path name that must exist does not exist. - \[EACCES\] Search permission is denied for a component of the path prefix. - \[EACCES\] The required permissions (for reading and/or writing) are denied for the given flags. - \[EACCES\] O\_TRUNC is specified and write permission is denied. - \[EACCES\] O\_CREAT is specified, the file does not exist, and the directory in which it is to be created does not permit writing. - \[EPERM\] O\_CREAT is specified, the file does not exist, and the directory in which it is to be created has its immutable flag set, see the [chflags(2)](/chflags/2.md) manual page for more information. - \[EPERM\] The named file has its immutable flag set and the file is to be modified. - \[EPERM\] The named file has its append-only flag set, the file is to be modified, and O\_TRUNC is specified or O\_APPEND is not specified. - \[ELOOP\] Too many symbolic links were encountered in translating the pathname. - \[EISDIR\] The named file is a directory, and the arguments specify it is to be modified. - \[EISDIR\] The named file is a directory, and the flags specified O\_CREAT without O\_DIRECTORY. - \[EROFS\] The named file resides on a read-only file system, and the file is to be modified. - \[EROFS\] O\_CREAT is specified and the named file would reside on a read-only file system. - \[EMFILE\] The process has already reached its limit for open file descriptors. - \[ENFILE\] The system file table is full. - \[EMLINK\] O\_NOFOLLOW was specified and the target is a symbolic link. - \[ENXIO\] The named file is a character special or block special file, and the device associated with this special file does not exist. - \[ENXIO\] O\_NONBLOCK is set, the named file is a fifo, O\_WRONLY is set, and no process has the file open for reading. - \[EINTR\] The **open**() operation was interrupted by a signal. - \[EOPNOTSUPP\] O\_SHLOCK or O\_EXLOCK is specified but the underlying file system does not support locking. - \[EOPNOTSUPP\] The named file is a special file mounted through a file system that does not support access to it (e.g. NFS). - \[EWOULDBLOCK\] O\_NONBLOCK and one of O\_SHLOCK or O\_EXLOCK is specified and the file is locked. - \[ENOSPC\] O\_CREAT is specified, the file does not exist, and the directory in which the entry for the new file is being placed cannot be extended because there is no space left on the file system containing the directory. - \[ENOSPC\] O\_CREAT is specified, the file does not exist, and there are no free inodes on the file system on which the file is being created. - \[EDQUOT\] O\_CREAT is specified, the file does not exist, and the directory in which the entry for the new file is being placed cannot be extended because the user's quota of disk blocks on the file system containing the directory has been exhausted. - \[EDQUOT\] O\_CREAT is specified, the file does not exist, and the user's quota of inodes on the file system on which the file is being created has been exhausted. - \[EIO\] An I/O error occurred while making the directory entry or allocating the inode for O\_CREAT. - \[EINTEGRITY\] Corrupted data was detected while reading from the file system. - \[ETXTBSY\] The file is a pure procedure (shared text) file that is being executed and the **open**() system call requests write access. - \[EFAULT\] The *path* argument points outside the process's allocated address space. - \[EEXIST\] O\_CREAT and O\_EXCL were specified and the file exists. - \[EOPNOTSUPP\] An attempt was made to open a socket (not currently implemented). - \[EINVAL\] An attempt was made to open a descriptor with an illegal combination of O\_RDONLY, O\_WRONLY, or O\_RDWR, and O\_EXEC or O\_SEARCH. - \[EBADF\] The *path* argument does not specify an absolute path and the *fd* argument is neither AT\_FDCWD nor a valid file descriptor open for searching. - \[ENOTDIR\] The *path* argument is not an absolute path and *fd* is neither AT\_FDCWD nor a file descriptor associated with a directory. - \[ENOTDIR\] O\_DIRECTORY is specified and the file is not a directory. - \[ECAPMODE\] AT\_FDCWD is specified and the process is in capability mode. - \[ECAPMODE\] **open**() was called and the process is in capability mode. - \[ENOTCAPABLE\] *path* is an absolute path or contained a ".." component leading to a directory outside of the directory hierarchy specified by *fd*. ## SEE ALSO [chmod(2)](/chmod/2.md), [close(2)](/close/2.md), [dup(2)](/dup/2.md), [fexecve(2)](/fexecve/2.md), [fhopen(2)](/fhopen/2.md), [getdtablesize(2)](/getdtablesize/2.md), [getfh(2)](/getfh/2.md), [lgetfh(2)](/lgetfh/2.md), [lseek(2)](/lseek/2.md), [read(2)](/read/2.md), [umask(2)](/umask/2.md), [write(2)](/write/2.md), [fopen(3)](/fopen/3.md), [capsicum(4)](/capsicum/4.md) ## STANDARDS These functions are specified by IEEE Std 1003.1-2008 (“POSIX.1”). FreeBSD sets *errno* to EMLINK instead of ELOOP as specified by POSIX when O\_NOFOLLOW is set in flags and the final component of pathname is a symbolic link to distinguish it from the case of too many symbolic link traversals in one of its non-final components. ## HISTORY The **open**() function appeared in Version 1 AT&T UNIX. The **openat**() function was introduced in FreeBSD 8.0. ## BUGS The Open Group Extended API Set 2 specification requires that the test for whether *fd* is searchable is based on whether *fd* is open for searching, not whether the underlying directory currently permits searches. The present implementation of the *openat* checks the current permissions of directory instead.