int open(const char *path, int oflag, ...);
int openat(int fd, const char *path, int oflag, ...);
IEEE Std 1003.1-2017
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 pathname naming the file.
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 a
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, the
FD_CLOEXECflag for the new file descriptor shall be set.
O_CREAT- If the file exists, this flag has no effect except as noted under
O_EXCLbelow. Otherwise, if
O_DIRECTORYis 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 type
mode_tmodified as follows: a bitwise
ANDis 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 set
O_DSYNC- Write I/O operations on the file descriptor shall complete as defined by synchronized I/O data integrity completion.
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 executing
open()naming the same filename in the same directory with
O_CREATare set, and path names a symbolic link,
open()shall fail and set
EEXIST, regardless of the contents of the symbolic link. If
O_EXCLis set and
O_CREATis 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_NOCTTYshall be ignored.
O_NOFOLLOW- If path names a symbolic link, fail and set
When opening a
O_NONBLOCKis set, an
open()for reading-only shall return without delay. An
open()for writing-only shall return an error if no process currently has the file open for reading.
O_NONBLOCKis clear, an
open()for reading-only shall block the calling thread until a thread opens the file for writing. An
open()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:
O_NONBLOCKis set, the
open()function shall return without blocking for the device to be ready or available. Subsequent behavior of the device is device-specific.
O_NONBLOCKis clear, the
open()function shall block the calling thread until the device is ready or available before returning.
O_NONBLOCKflag shall not cause an error, but it is unspecified whether the file status flags will include the
O_RSYNC- Read I/O operations on the file descriptor shall complete at the same level of integrity as specified by the
O_SYNCflags. If both
O_RSYNCare set in oflag, all I/O operations on the file descriptor shall complete as defined by synchronized I/O data integrity completion. If both
O_RSYNCare 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. The
O_SYNCflag 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 opened
O_WRONLY,its length shall be truncated to
0, and the mode and owner shall be unchanged. It shall have no effect on
FIFOspecial files or terminal device files. Its effect on other file types is implementation-defined. The result of using
O_TTY_INIT- If path identifies a terminal device other than a pseudo-terminal, the device is not already open in any process, and either
O_TTY_INITis set in oflag or
O_TTY_INIThas 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 whether
O_TTY_INIThas 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 whether
O_TTY_INITis set. If path does not identify a terminal device,
O_TTY_INITshall be ignored.
O_DIRECTORY are set and the requested access mode is neither
O_RDWR, the result is unspecified.
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.
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_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_RDWR. Other flag values are not applicable to
STREAMS devices and shall have no effect on them. The value
the operation of
STREAMS drivers and certain functions applied to file descriptors associated with
STREAMS files. For
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.
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
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
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.
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, or
O_TRUNCis specified and write permission is denied.
O_EXCLare set, and the named file exists.
EINTR- A signal was caught during
EINVAL- The implementation does not support synchronized I/O for this file.
EIO- The path argument names a
STREAMSfile and a hangup or error occurred during the
EISDIR- The named file is a directory and oflag includes O_WRONLY or
O_RDWR, or includes
ELOOP- A loop exists in symbolic links encountered during resolution of the path argument, or
O_NOFOLLOWwas specified and the path argument names a symbolic link.
EMFILE- All file descriptors available to the process are currently open.
The length of a component of a pathname is longer than
ENFILE- The maximum allowable number of files is currently open in the system.
O_CREATis not set and a component of path does not name an existing file, or
O_CREATis set and a component of the path prefix of path does not name an existing file, or path points to an empty string.
O_CREATis 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, an
ENOENTerror shall not occur.
ENOSR- The path argument names a
STREAMS-based file and the system is unable to allocate a
ENOSPC- The directory or file system that would contain the new file cannot be expanded, the file does not exist, and
ENOTDIR- A component of the path prefix names an existing file that is neither a directory nor a symbolic link to a directory; or
O_EXCLare not specified, the path argument contains at least one non-
<slash>character and ends with one or more trailing
<slash>characters, and the last pathname component names an existing file that is neither a directory nor a symbolic link to a directory; or
O_DIRECTORYwas specified and the path argument resolves to a non-directory file.
O_NONBLOCKis set, the named file is a
O_WRONLYis 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 either
O_CREAT(if the file does not exist), or
O_TRUNCis set in the oflag argument.
openat() function shall fail if:
EACCES- The access mode of the open file description associated with fd is not
O_SEARCHand 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 neither
AT_FDCWDnor 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 than
SYMLOOP_MAXsymbolic links were encountered during resolution of the path argument.
The length of a pathname exceeds
PATH_MAX, or pathname resolution of a symbolic link produced an intermediate result with a
length that exceeds
ENOMEM- The path argument names a
STREAMSfile 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 is