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OPEN(2) BSD System Calls Manual OPEN(2) NAME open, openat -- open or create a file for reading, writing or executing LIBRARY Standard C Library (libc, -lc) SYNOPSIS #include <fcntl.h> 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) 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. For openat() and relative path, the file to be opened is determined relative to the directory asso- ciated with the file descriptor fd instead of the current working direc- tory. 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(). When openat() is called with an absolute path, it ignores the fd argu- ment. In capsicum(4) capability mode, open() is not permitted. The path argu- ment to openat() must be strictly relative to a file descriptor fd. path must not be an absolute path and must not contain ".." components which cause the path resolution to escape the directory hierarchy starting at fd. Additionally, no symbolic link in path may target absolute path or contain escaping ".." components. fd must not be AT_FDCWD. If the vfs.lookup_cap_dotdot sysctl(3) MIB is set to zero, ".." compo- nents in the paths, used in capability mode, are completely disabled. If the vfs.lookup_cap_dotdot_nonlocal MIB is set to zero, ".." is not al- lowed if found on non-local filesystem. 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 (historical synonym for O_SYNC) O_SYNC synchronous writes O_DSYNC synchronous data 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 O_RESOLVE_BENEATH path resolution must not cross the fd directory Opening a file with O_APPEND set causes each write on the file to be ap- pended 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_SYNC is used in the mask, all writes will immediately and syn- chronously be written to disk. O_FSYNC is an historical synonym for O_SYNC. If O_DSYNC is used in the mask, all data and metadata required to read the data will be synchronously written to disk, but changes to metadata such as file access and modification timestamps may be written later. 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) 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 read- ing 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 re- duce 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. O_RESOLVE_BENEATH returns ENOTCAPABLE if any intermediate component of the specified relative path does not reside in the directory hierarchy beneath the starting directory. Absolute paths or even the temporal es- cape from beneath of the starting directory is not allowed. 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). 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 de- scriptor. 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)) 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) system calls; see close(2), fcntl(2) and O_CLOEXEC description. The system imposes a limit on the number of file descriptors open simul- taneously by one process. The getdtablesize(2) 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 im- mutable flag set, see the chflags(2) 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 translat- ing the pathname. [EISDIR] The named file is a directory, and the arguments spec- ify 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 speci- fied 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 direc- tory 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 allo- cated 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 ille- gal combination of O_RDONLY, O_WRONLY, or O_RDWR, and O_EXEC or O_SEARCH. [EINVAL] The O_RESOLVE_BENEATH flag is specified and path is absolute. [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 direc- tory. [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 ".." compo- nent leading to a directory outside of the directory hierarchy specified by fd, and the process is in capa- bility mode. [ENOTCAPABLE] The O_RESOLVE_BENEATH flag was provided, and the rela- tive path escapes the fd 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), capsicum(4) 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 sym- bolic link to distinguish it from the case of too many symbolic link tra- versals in one of its non-final components. HISTORY The open() function appeared in Version 1 AT&T UNIX. The openat() func- tion was introduced in FreeBSD 8.0. O_DSYNC appeared in 13.0. 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 search- ing, not whether the underlying directory currently permits searches. The present implementation of the openat checks the current permissions of directory instead. The mode argument is variadic and may result in different calling conven- tions than might otherwise be expected. BSD February 23, 2021 BSD
NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUES | ERRORS | SEE ALSO | STANDARDS | HISTORY | BUGS
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