open#
Synopsis#
#include <sys/stat.h>
#include <fcntl.h>
int open(const char *path, int oflag, ...);
int openat(int fd, const char *path, int oflag, ...);
Status#
Partially implemented
Conformance#
IEEE Std 1003.1-2017
Description#
The open()
function shall establish the connection between a file and a file descriptor. It shall create an open file
description that refers to a file and a file descriptor that refers to that open file description. The file descriptor
is used by other I/O functions to refer to that file. The path argument points to a path name naming the file.
The open()
function shall return a file descriptor for the named file, allocated as described in File Descriptor
Allocation. The open file description is new, and therefore the file descriptor shall not share it with any other
process in the system. The FD_CLOEXEC
file descriptor flag associated with the new file descriptor shall be
cleared unless the O_CLOEXEC
flag is set in oflag.
The file offset used to mark the current position within the file shall be set to the beginning of the file.
The file status flags and file access modes of the open file description shall be set according to the value of oflag.
Values for oflag are constructed by a bitwise-inclusive OR of flags from the following list, defined in <fcntl.h>
.
Applications shall specify exactly one of the first five values (file access modes) below in the value of oflag:
O_EXEC
- Open for execute only (non-directory files). The result is unspecified if this flag is applied to a directory.O_RDONLY
- Open for reading only.O_RDWR
- Open for reading and writing. The result is undefined if this flag is applied to aFIFO
.O_SEARCH
- Open directory for search only. The result is unspecified if this flag is applied to a non-directory file.O_WRONLY
- Open for writing only.
Any combination of the following may be used:
O_APPEND
- If set, the file offset shall be set to the end of the file prior to each write.O_CLOEXEC
- If set, theFD_CLOEXEC
flag for the new file descriptor shall be set.O_CREAT
- If the file exists, this flag has no effect except as noted underO_EXCL
below. Otherwise, ifO_DIRECTORY
is not set the file shall be created as a regular file; the user ID of the file shall be set to the effective user ID of the process; the group ID of the file shall be set to the group ID of the file’s parent directory or to the effective group ID of the process; and the access permission bits (see<sys/stat.h>
) of the file mode shall be set to the value of the argument following the oflag argument taken as typemode_t
modified as follows: a bitwiseAND
is performed on the file-mode bits and the corresponding bits in the complement of the process’ file mode creation mask. Thus, all bits in the file mode whose corresponding bit in the file mode creation mask is set are cleared. When bits other than the file permission bits are set, the effect is unspecified. The argument following the oflag argument does not affect whether the file is open for reading, writing, or for both. Implementations shall provide a way to initialize the file’s group ID to the group ID of the parent directory. Implementations may, but need not, provide an implementation-defined way to initialize the file’s group ID to the effective group ID of the calling process.O_DIRECTORY
- If path resolves to a non-directory file, fail and seterrno
toENOTDIR
.O_DSYNC
- Write I/O operations on the file descriptor shall complete as defined by synchronized I/O data integrity completion.O_EXCL
- IfO_CREAT
andO_EXCL
are set,open()
shall fail if the file exists. The check for the existence of the file and the creation of the file if it does not exist shall be atomic with respect to other threads executingopen()
naming the same filename in the same directory withO_EXCL
andO_CREAT
set. IfO_EXCL
andO_CREAT
are set, and path names a symbolic link,open()
shall fail and seterrno
toEEXIST
, regardless of the contents of the symbolic link. IfO_EXCL
is set andO_CREAT
is not set, the result is undefined.O_NOCTTY
- If set and path identifies a terminal device,open()
shall not cause the terminal device to become the controlling terminal for the process. If path does not identify a terminal device,O_NOCTTY
shall be ignored.O_NOFOLLOW
- If path names a symbolic link, fail and seterrno
toELOOP
.O_NONBLOCK
When opening a
FIFO
withO_RDONLY
orO_WRONLY
set:If
O_NONBLOCK
is set, aopen()
for reading-only shall return without delay. Aopen()
for writing-only shall return an error if no process currently has the file open for reading.If
O_NONBLOCK
is clear, aopen()
for reading-only shall block the calling thread until a thread opens the file for writing. Aopen()
for writing-only shall block the calling thread until a thread opens the file for reading.
When opening a block special or character special file that supports non-blocking opens:
If
O_NONBLOCK
is set, theopen()
function shall return without blocking for the device to be ready or available. Subsequent behavior of the device is device-specific.If
O_NONBLOCK
is clear, theopen()
function shall block the calling thread until the device is ready or available before returning.
Otherwise, the
O_NONBLOCK
flag shall not cause an error, but it is unspecified whether the file status flags will include theO_NONBLOCK
flag.
O_RSYNC
- Read I/O operations on the file descriptor shall complete at the same level of integrity as specified by theO_DSYNC
andO_SYNC
flags. If bothO_DSYNC
andO_RSYNC
are set in oflag, all I/O operations on the file descriptor shall complete as defined by synchronized I/O data integrity completion. If bothO_SYNC
andO_RSYNC
are set in flags, all I/O operations on the file descriptor shall complete as defined by synchronized I/O file integrity completion.O_SYNC
- Write I/O operations on the file descriptor shall complete as defined by synchronized I/O file integrity completion. TheO_SYNC
flag shall be supported for regular files, even if the Synchronized Input and Output option is not supported.O_TRUNC
- If the file exists and is a regular file, and the file is successfully openedO_RDWR
orO_WRONLY,
its length shall be truncated to0
, and the mode and owner shall be unchanged. It shall have no effect onFIFO
special files or terminal device files. Its effect on other file types is implementation-defined. The result of usingO_TRUNC
without eitherO_RDWR
orO_WRONLY
is undefined.O_TTY_INIT
- If path identifies a terminal device other than a pseudo-terminal, the device is not already open in any process, and eitherO_TTY_INIT
is set in oflag orO_TTY_INIT
has the value zero,open()
shall set any non-standard termios structure terminal parameters to a state that provides conforming behavior; see XBD Parameters that Can be Set. It is unspecified whetherO_TTY_INIT
has any effect if the device is already open in any process. If path identifies the slave side of a pseudo-terminal that is not already open in any process,open()
shall set any non-standard termios structure terminal parameters to a state that provides conforming behavior, regardless of whetherO_TTY_INIT
is set. If path does not identify a terminal device,O_TTY_INIT
shall be ignored.
If O_CREAT
and O_DIRECTORY
are set, and the requested access mode is neither O_WRONLY
nor O_RDWR,
the result is
unspecified.
If O_CREAT
is set and the file did not previously exist, upon successful completion, open()
shall mark for update
the last data access, last data modification, and last file status change timestamps of the file and the last data
modification and last file status change timestamps of the parent directory.
If O_TRUNC
is set and the file did previously exist, upon successful completion, open()
shall mark for update the
last data modification and last file status change timestamps of the file.
If both the O_SYNC
and O_DSYNC
flags are set, the effect is as if only the O_SYNC
flag was set.
If path refers to a STREAMS
file, oflag may be constructed from O_NONBLOCK
OR’ed with either O_RDONLY,
O_WRONLY,
or O_RDWR
. Other flag values are not applicable to STREAMS
devices and shall have no effect on them.
The value O_NONBLOCK
affects the operation of STREAMS
drivers and certain functions applied to file descriptors
associated with STREAMS
files. For STREAMS
drivers, the implementation of O_NONBLOCK
is device-specific.
The application shall ensure that it specifies the O_TTY_INIT
flag on the first open of a terminal device since
system boot or since the device was closed by the process that last had it open. The application need not specify the
O_TTY_INIT
flag when opening pseudo-terminals. If path names the master side of a pseudo-terminal device, then it
is unspecified whether open()
locks the slave side so that it cannot be opened. Conforming applications shall call
unlockpt()
before opening the slave side.
The largest value that can be represented correctly in an object of type off_t
shall be established as the offset
maximum in the open file description.
The openat()
function shall be equivalent to the open()
function except in the case where 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. If the access mode of the open file description associated
with the file descriptor is not O_SEARCH,
the function shall check whether directory searches are permitted using
the current permissions of the directory underlying the file descriptor. If the access mode is O_SEARCH,
the function
shall not perform the check.
The oflag 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 shall be used
and the behavior shall be identical to a call to open()
.
Return value#
Upon successful completion, these functions shall open the file and return a non-negative integer representing the file
descriptor. Otherwise, these functions shall return -1
and set errno
to indicate the error. If -1
is returned, no
files shall be created or modified.
Errors#
These functions shall fail if:
EACCES
- Search permission is denied on a component of the path prefix, or the file exists, and the permissions specified by oflag are denied, or the file does not exist and write permission is denied for the parent directory of the file to be created, orO_TRUNC
is specified and write permission is denied.EEXIST
-O_CREAT
andO_EXCL
are set, and the named file exists.EINTR
- A signal was caught duringopen()
.EINVAL
- The implementation does not support synchronized I/O for this file.EIO
- The path argument names aSTREAMS
file and a hangup or error occurred during theopen()
.EISDIR
- The named file is a directory and oflag includes O_WRONLY orO_RDWR
, or includesO_CREAT
withoutO_DIRECTORY
.ELOOP
- A loop exists in symbolic links encountered during resolution of the path argument, orO_NOFOLLOW
was specified and the path argument names a symbolic link.EMFILE
- All file descriptors available to the process are currently open.ENAMETOOLONG
- The length of a component of a path name is longer thanNAME_MAX
.ENFILE
- The maximum allowable number of files is currently open in the system.ENOENT
-O_CREAT
is not set and a component of path does not name an existing file, orO_CREAT
is set and a component of the path prefix of path does not name an existing file, or path points to an empty string.ENOENT
orENOTDIR
-O_CREAT
is set, and the path argument contains at least one non-<slash>
character and ends with one or more trailing<slash>
characters. If path without the trailing<slash>
characters would name an existing file, aENOENT
error shall not occur.ENOSR
- The path argument names aSTREAMS
-based file and the system is unable to allocate aSTREAM
.ENOSPC
- The directory or file system that would contain the new file cannot be expanded, the file does not exist, andO_CREAT
is specified.ENOTDIR
- A component of the path prefix names an existing file that is neither a directory nor a symbolic link to a directory; orO_CREAT
andO_EXCL
are not specified, the path argument contains at least one non-<slash>
character and ends with one or more trailing<slash>
characters, and the last path name component names an existing file that is neither a directory nor a symbolic link to a directory; orO_DIRECTORY
was specified, and the path argument resolves to a non-directory file.ENXIO
-O_NONBLOCK
is set, the named file is aFIFO
,O_WRONLY
is set, and no process has the file open for reading.ENXIO
- The named file is a character special or block special file, and the device associated with this special file does not exist.EOVERFLOW
- The named file is a regular file and the size of the file cannot be represented correctly in an object of type off_t.EROFS
- The named file resides on a read-only file system and eitherO_WRONLY
,O_RDWR
,O_CREAT
(if the file does not exist), orO_TRUNC
is set in the oflag argument.
The openat()
function shall fail if:
EACCES
- The access mode of the open file description associated with fd is notO_SEARCH
and the permissions of the directory underlying fd do not permit directory searches.EBADF
- The path argument does not specify an absolute path and the fd argument is neitherAT_FDCWD
nor a valid file descriptor open for reading or searching.ENOTDIR
- The path argument is not an absolute path and fd is a file descriptor associated with a non-directory file.
These functions may fail if:
EAGAIN
- The path argument names the slave side of a pseudo-terminal device that is locked.EINVAL
- The value of the oflag argument is not valid.ELOOP
- More thanSYMLOOP_MAX
symbolic links were encountered during resolution of the path argument.ENAMETOOLONG
- The length of a path name exceedsPATH_MAX
, or path name resolution of a symbolic link produced an intermediate result with a length that exceedsPATH_MAX
.ENOMEM
- The path argument names aSTREAMS
file and the system is unable to allocate resources.EOPNOTSUPP
- The path argument names a socket.ETXTBSY
- The file is a pure procedure (shared text) file that is being executed and oflag isO_WRONLY
orO_RDWR
.
Tests#
Untested
Known bugs#
None