# open

## PROLOG



           This manual page is part of the POSIX Programmer's Manual.  The
           Linux implementation of this interface may differ (consult the
           corresponding Linux manual page for details of Linux behavior), or
           the interface may not be implemented on Linux.
    


## NAME



           open, openat — open file
    


## SYNOPSIS



           #include <sys/stat.h>
           #include <fcntl.h>
    
           int open(const char *_path_ , int _oflag_ , ...);
           int openat(int _fd_ , const char *_path_ , int _oflag_ , ...);
    


## 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
           pathname naming the file.
    
           The _open_() function shall return a file descriptor for the named
           file, allocated as described in _Section 2.14_ , _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 FIFO.
    
           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_CLOEXEC flag for the new file
                         descriptor shall be set.
    
           O_CREAT       If the file exists, this flag has no effect except
                         as noted under O_EXCL below. Otherwise, if
                         O_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 type **mode_t** modified as follows: a bitwise
                         AND 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
                         set _errno_ to **[ENOTDIR]**.
    
           O_DSYNC       Write I/O operations on the file descriptor shall
                         complete as defined by synchronized I/O data
                         integrity completion.
    
           O_EXCL        If O_CREAT and O_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
                         executing _open_() naming the same filename in the
                         same directory with O_EXCL and O_CREAT set. If
                         O_EXCL and O_CREAT are set, and _path_ names a
                         symbolic link, _open_() shall fail and set _errno_ to
                         **[EEXIST]** , regardless of the contents of the symbolic
                         link. If O_EXCL is set and O_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 set _errno_ to
                         **[ELOOP]**.
    
           O_NONBLOCK    When opening a FIFO with O_RDONLY or O_WRONLY set:
    
                          *  If O_NONBLOCK is 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.
    
                          *  If O_NONBLOCK is 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:
    
                          *  If O_NONBLOCK is set, the _open_() 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, the _open_() 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 the O_NONBLOCK flag.
    
           O_RSYNC       Read I/O operations on the file descriptor shall
                         complete at the same level of integrity as specified
                         by the O_DSYNC and O_SYNC flags. If both O_DSYNC and
                         O_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 both
                         O_SYNC and O_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.
    
                         The O_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 opened O_RDWR or O_WRONLY, its
                         length shall be truncated to 0, and the mode and
                         owner shall be unchanged. It shall have no effect on
                         FIFO special files or terminal device files. Its
                         effect on other file types is implementation-
                         defined. The result of using O_TRUNC without either
                         O_RDWR or O_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 either O_TTY_INIT is set in _oflag_
                         or O_TTY_INIT has the value zero, _open_() shall set
                         any non-standard **termios** structure terminal
                         parameters to a state that provides conforming
                         behavior; see the Base Definitions volume of
                         POSIX.1‐2017, _Section 11.2_ , _Parameters that Can be_
                         _Set_.  It is unspecified whether O_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 whether
                         O_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, or O_TRUNC is specified and write permission
                  is denied.
    
           **EEXIST** O_CREAT and O_EXCL are set, and the named file exists.
    
           **EINTR** A signal was caught during _open_().
    
           **EINVAL** The implementation does not support synchronized I/O for
                  this file.
    
           **EIO** The _path_ argument names a STREAMS file and a hangup or
                  error occurred during the _open_().
    
           **EISDIR** The named file is a directory and _oflag_ includes O_WRONLY
                  or O_RDWR, or includes O_CREAT without O_DIRECTORY.
    
           **ELOOP** A loop exists in symbolic links encountered during
                  resolution of the _path_ argument, or O_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 pathname is longer than
                  {NAME_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, or O_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** or **ENOTDIR**
                  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, an **[ENOENT]** error
                  shall not occur.
    
           **ENOSR** The _path_ argument names a STREAMS-based file and the system
                  is unable to allocate a STREAM.
    
           **ENOSPC** The directory or file system that would contain the new
                  file cannot be expanded, the file does not exist, and
                  O_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;
                  or O_CREAT and O_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
                  pathname component names an existing file that is neither a
                  directory nor a symbolic link to a directory; or
                  O_DIRECTORY was specified and the _path_ argument resolves to
                  a non-directory file.
    
           **ENXIO** O_NONBLOCK is set, the named file is a FIFO, 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
                  either O_WRONLY, O_RDWR, O_CREAT (if the file does not
                  exist), or O_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 not O_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 neither AT_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 than {SYMLOOP_MAX} symbolic links were encountered
                  during resolution of the _path_ argument.
    
           **ENAMETOOLONG**
                  The length of a pathname exceeds {PATH_MAX}, or pathname
                  resolution of a symbolic link produced an intermediate
                  result with a length that exceeds {PATH_MAX}.
    
           **ENOMEM** The _path_ argument names a STREAMS 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_ is O_WRONLY or O_RDWR.
    
           _The following sections are informative._
    


## EXAMPLES



       **Opening a File for Writing by the Owner**
           The following example opens the file **/tmp/file** , either by creating
           it (if it does not already exist), or by truncating its length to
           0 (if it does exist). In the former case, if the call creates a
           new file, the access permission bits in the file mode of the file
           are set to permit reading and writing by the owner, and to permit
           reading only by group members and others.
    
           If the call to _open_() is successful, the file is opened for
           writing.
    
               #include <fcntl.h>
               ...
               int fd;
               mode_t mode = S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH;
               char *pathname = "/tmp/file";
               ...
               fd = open(pathname, O_WRONLY | O_CREAT | O_TRUNC, mode);
               ...
    
       **Opening a File Using an Existence Check**
           The following example uses the _open_() function to try to create
           the **LOCKFILE** file and open it for writing. Since the _open_()
           function specifies the O_EXCL flag, the call fails if the file
           already exists. In that case, the program assumes that someone
           else is updating the password file and exits.
    
               #include <fcntl.h>
               #include <stdio.h>
               #include <stdlib.h>
    
               #define LOCKFILE "/etc/ptmp"
               ...
               int pfd; /* Integer for file descriptor returned by open() call. */
               ...
               if ((pfd = open(LOCKFILE, O_WRONLY | O_CREAT | O_EXCL,
                   S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH)) == -1)
               {
                   fprintf(stderr, "Cannot open /etc/ptmp. Try again later.\n");
                   exit(1);
               }
               ...
    
       **Opening a File for Writing**
           The following example opens a file for writing, creating the file
           if it does not already exist. If the file does exist, the system
           truncates the file to zero bytes.
    
               #include <fcntl.h>
               #include <stdio.h>
               #include <stdlib.h>
    
               #define LOCKFILE "/etc/ptmp"
               ...
               int pfd;
               char pathname[PATH_MAX+1];
               ...
               if ((pfd = open(pathname, O_WRONLY | O_CREAT | O_TRUNC,
                   S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH)) == -1)
               {
                   perror("Cannot open output file\n"); exit(1);
               }
               ...
    


## APPLICATION USAGE



           POSIX.1‐2008 does not require that terminal parameters be
           automatically set to any state on first open, nor that they be
           reset after the last close. It is possible for a non-conforming
           application to leave a terminal device in a state where the next
           process to use that device finds it in a non-conforming state, but
           has no way of determining this. To ensure that the device is set
           to a conforming initial state, applications which perform a first
           open of a terminal (other than a pseudo-terminal) should do so
           using the O_TTY_INIT flag to set the parameters associated with
           the terminal to a conforming state.
    
           Except as specified in this volume of POSIX.1‐2017, the flags
           allowed in _oflag_ are not mutually-exclusive and any number of them
           may be used simultaneously. Not all combinations of flags make
           sense. For example, using O_SEARCH | O_CREAT will successfully
           open a pre-existing directory for searching, but if there is no
           existing file by that name, then it is unspecified whether a
           regular file will be created. Likewise, if a non-directory file
           descriptor is successfully returned, it is unspecified whether
           that descriptor will have execute permissions as if by O_EXEC
           (note that it is unspecified whether O_EXEC and O_SEARCH have the
           same value).
    


## RATIONALE



           Some implementations permit opening FIFOs with O_RDWR. Since FIFOs
           could be implemented in other ways, and since two file descriptors
           can be used to the same effect, this possibility is left as
           undefined.
    
           See getgroups(3p) about the group of a newly created file.
    
           The use of _open_() to create a regular file is preferable to the
           use of _creat_(), because the latter is redundant and included only
           for historical reasons.
    
           The use of the O_TRUNC flag on FIFOs and directories (pipes cannot
           be _open_()-ed) must be permissible without unexpected side-effects
           (for example, _creat_() on a FIFO must not remove data). Since
           terminal special files might have type-ahead data stored in the
           buffer, O_TRUNC should not affect their content, particularly if a
           program that normally opens a regular file should open the current
           controlling terminal instead. Other file types, particularly
           implementation-defined ones, are left implementation-defined.
    
           POSIX.1‐2008 permits **[EACCES]** to be returned for conditions other
           than those explicitly listed.
    
           The O_NOCTTY flag was added to allow applications to avoid
           unintentionally acquiring a controlling terminal as a side-effect
           of opening a terminal file. This volume of POSIX.1‐2017 does not
           specify how a controlling terminal is acquired, but it allows an
           implementation to provide this on _open_() if the O_NOCTTY flag is
           not set and other conditions specified in the Base Definitions
           volume of POSIX.1‐2017, _Chapter 11_ , _General Terminal Interface_ are
           met.
    
           In historical implementations the value of O_RDONLY is zero.
           Because of that, it is not possible to detect the presence of
           O_RDONLY and another option. Future implementations should encode
           O_RDONLY and O_WRONLY as bit flags so that:
    
               O_RDONLY | O_WRONLY == O_RDWR
    
           O_EXEC and O_SEARCH are specified as two of the five file access
           modes.  Since O_EXEC does not apply to directories, and O_SEARCH
           only applies to directories, their values need not be distinct.
           Since O_RDONLY has historically had the value zero,
           implementations are not able to distinguish between O_SEARCH and
           O_SEARCH | O_RDONLY, and similarly for O_EXEC.
    
           In general, the _open_() function follows the symbolic link if _path_
           names a symbolic link. However, the _open_() function, when called
           with O_CREAT and O_EXCL, is required to fail with **[EEXIST]** if _path_
           names an existing symbolic link, even if the symbolic link refers
           to a nonexistent file. This behavior is required so that
           privileged applications can create a new file in a known location
           without the possibility that a symbolic link might cause the file
           to be created in a different location.
    
           For example, a privileged application that must create a file with
           a predictable name in a user-writable directory, such as the
           user's home directory, could be compromised if the user creates a
           symbolic link with that name that refers to a nonexistent file in
           a system directory. If the user can influence the contents of a
           file, the user could compromise the system by creating a new
           system configuration or spool file that would then be interpreted
           by the system. The test for a symbolic link which refers to a
           nonexisting file must be atomic with the creation of a new file.
    
           In addition, the _open_() function refuses to open non-directories
           if the O_DIRECTORY flag is set. This avoids race conditions
           whereby a user might compromise the system by substituting a hard
           link to a sensitive file (e.g., a device or a FIFO) while a
           privileged application is running, where opening a file even for
           read access might have undesirable side-effects.
    
           In addition, the _open_() function does not follow symbolic links if
           the O_NOFOLLOW flag is set.  This avoids race conditions whereby a
           user might compromise the system by substituting a symbolic link
           to a sensitive file (e.g., a device) while a privileged
           application is running, where opening a file even for read access
           might have undesirable side-effects.
    
           The POSIX.1‐1990 standard required that the group ID of a newly
           created file be set to the group ID of its parent directory or to
           the effective group ID of the creating process. FIPS 151‐2
           required that implementations provide a way to have the group ID
           be set to the group ID of the containing directory, but did not
           prohibit implementations also supporting a way to set the group ID
           to the effective group ID of the creating process.  Conforming
           applications should not assume which group ID will be used. If it
           matters, an application can use _chown_() to set the group ID after
           the file is created, or determine under what conditions the
           implementation will set the desired group ID.
    
           The purpose of the _openat_() function is to enable opening files in
           directories other than the current working directory without
           exposure to race conditions. Any part of the path of a file could
           be changed in parallel to a call to _open_(), resulting in
           unspecified behavior. By opening a file descriptor for the target
           directory and using the _openat_() function it can be guaranteed
           that the opened file is located relative to the desired directory.
           Some implementations use the _openat_() function for other purposes
           as well. In some cases, if the _oflag_ parameter has the O_XATTR bit
           set, the returned file descriptor provides access to extended
           attributes. This functionality is not standardized here.
    


## FUTURE DIRECTIONS



           None.
    


* * *