PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT

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

PROLOG         top

       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         top

       open, openat — open file relative to directory file descriptor

SYNOPSIS         top

       #include <sys/stat.h>
       #include <fcntl.h>

       int open(const char *path, int oflag, ...);
       int openat(int fd, const char *path, int oflag, ...);

DESCRIPTION         top

       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
       that is the lowest file descriptor not currently open for that
       process. 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, the file shall be
                     created; 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.1‐2008, 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 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 file descriptor was opened without O_SEARCH, the
       function shall check whether directory searches are permitted using
       the current permissions of the directory underlying the file
       descriptor. If the file descriptor was opened with 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         top

       Upon successful completion, these functions shall open the file and
       return a non-negative integer representing the lowest numbered unused
       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         top

       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.

       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 names 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 fd was not opened with 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.

       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         top

   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         top

       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.1‐2008, 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         top

       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.1‐2008 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.1‐2008, 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         top

       None.

SEE ALSO         top

       chmod(3p), close(3p), creat(3p), dirfd(3p), dup(3p), exec(1p),
       fcntl(3p), fdopendir(3p), link(3p), lseek(3p), mkdtemp(3p),
       mknod(3p), read(3p), symlink(3p), umask(3p), unlockpt(3p), write(3p)

       The Base Definitions volume of POSIX.1‐2008, Chapter 11, General
       Terminal Interface, fcntl.h(0p), sys_stat.h(0p), sys_types.h(0p)

COPYRIGHT         top

       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The Open
       Group Base Specifications Issue 7, Copyright (C) 2013 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
       applied.) 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.unix.org/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                 2013                            OPEN(3P)

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