OPEN(3)                    POSIX Programmer's Manual                   OPEN(3)



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

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 aa 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 aa 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 aa 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() 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.

SEE ALSO
       chmod(), close(), creat(), dirfd(), dup(), exec, fcntl(), fdopendir(),
       link(), lseek(), mkdtemp(), mknod(), read(), symlink(), umask(),
       unlockpt(), write()

       The Base Definitions volume of POSIX.1-2017, Chapter 11, General
       Terminal Interface, <fcntl.h>, <sys_stat.h>, <sys_types.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1-2017, Standard for Information Technology --
       Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  In the event of any discrepancy between this version and the
       original IEEE and The Open Group Standard, the original IEEE and The
       Open Group Standard is the referee document. The original Standard can
       be obtained online at http://www.opengroup.org/unix/online.html .

       Any typographical or formatting errors that appear in this page are
       most likely to have been introduced during the conversion of the source
       files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .



IEEE/The Open Group                  2017                              OPEN(3)
