table of contents
|OPEN(2)||System Calls Manual||OPEN(2)|
open or create a file for reading, writing or executing
LIBRARY¶Standard C Library (libc, -lc)
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_CREATflag). In this case
openat() require an additional argument mode_t mode, and the file is created with mode mode as described in chmod(2) and modified by the process' umask value (see umask(2)). 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
openat() is passed the special value
AT_FDCWDin the fd parameter, the current working directory is used and the behavior is identical to a call to
open(). 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_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 don't assign controlling terminal O_TTY_INIT restore default terminal attributes O_DIRECTORY error if file is not a directory O_CLOEXEC set FD_CLOEXEC upon open
O_APPENDset causes each write on the file to be appended to the end. If
O_TRUNCis specified and the file exists, the file is truncated to zero length. If
O_EXCLis set with
O_CREATand the file already exists,
open() returns an error. This may be used to implement a simple exclusive access locking mechanism. If
O_EXCLis 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_NONBLOCKflag 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_FSYNCis used in the mask, all writes will immediately be written to disk, the kernel will not cache written data and all writes on the descriptor will not return until the data to be written completes.
O_SYNCis a synonym for
O_FSYNCrequired by POSIX. If
O_NOFOLLOWis 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) semantics can be obtained by setting
O_SHLOCKfor a shared lock, or
O_EXLOCKfor 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_DIRECTmay 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_NOCTTYmay 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_INITmay 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_DIRECTORYmay 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_CLOEXECmay be used to set
FD_CLOEXECflag for the newly returned file descriptor. 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_RESTARTflag is set for the signal. A sleeping open of a fifo (see mkfifo(2)) is restarted as normal. When a new file is created it is given the group of the directory which contains it. Unless
O_CLOEXECflag was specified, the new descriptor is set to remain open across execve(2) system calls; see close(2), fcntl(2) and
O_CLOEXECdescription. The system imposes a limit on the number of file descriptors open simultaneously by one process. The getdtablesize(2) system call returns the current system limit.
RETURN VALUES¶If successful,
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:
- A component of the path prefix is not a directory.
- A component of a pathname exceeded 255 characters, or an entire path name exceeded 1023 characters.
O_CREATis not set and the named file does not exist.
- A component of the path name that must exist does not exist.
- Search permission is denied for a component of the path prefix.
- The required permissions (for reading and/or writing) are denied for the given flags.
O_TRUNCis specified and write permission is denied.
O_CREATis specified, the file does not exist, and the directory in which it is to be created does not permit writing.
O_CREATis 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) manual page for more information.
- The named file has its immutable flag set and the file is to be modified.
- The named file has its append-only flag set, the file is to be modified,
O_TRUNCis specified or
O_APPENDis not specified.
- Too many symbolic links were encountered in translating the pathname.
- The named file is a directory, and the arguments specify it is to be modified.
- The named file resides on a read-only file system, and the file is to be modified.
O_CREATis specified and the named file would reside on a read-only file system.
- The process has already reached its limit for open file descriptors.
- The system file table is full.
O_NOFOLLOWwas specified and the target is a symbolic link.
- The named file is a character special or block special file, and the device associated with this special file does not exist.
O_NONBLOCKis set, the named file is a fifo,
O_WRONLYis set, and no process has the file open for reading.
open() operation was interrupted by a signal.
O_EXLOCKis specified but the underlying file system does not support locking.
- The named file is a special file mounted through a file system that does not support access to it (e.g. NFS).
O_NONBLOCKand one of
O_EXLOCKis specified and the file is locked.
O_CREATis 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.
O_CREATis specified, the file does not exist, and there are no free inodes on the file system on which the file is being created.
O_CREATis 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.
O_CREATis 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.
- An I/O error occurred while making the directory entry or allocating the
- The file is a pure procedure (shared text) file that is being executed and
open() system call requests write access.
- The path argument points outside the process's allocated address space.
O_EXCLwere specified and the file exists.
- An attempt was made to open a socket (not currently implemented).
- An attempt was made to open a descriptor with an illegal combination of
- [Eq EBADF]
- The path argument does not specify an
absolute path and the fd argument is
AT_FDCWDnor a valid file descriptor open for searching.
- [Eq ENOTDIR]
- The path argument is not an absolute path
and fd is neither
AT_FDCWDnor a file descriptor associated with a directory.
- [Eq ENOTDIR]
O_DIRECTORYis specified and the file is not a directory.
SEE ALSO¶chmod(2), close(2), dup(2), fexecve(2), fhopen(2), getdtablesize(2), getfh(2), lgetfh(2), lseek(2), read(2), umask(2), write(2), fopen(3)
open() function appeared in Version 6 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.
|February 7, 2013||Debian|