open(3p) — Linux manual page

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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

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, 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_    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         top

       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         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, or includes O_CREAT without O_DIRECTORY.

       ELOOP  A loop exists in symbolic links encountered during
              resolution of the path argument, or O_ 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         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.1‐2017, 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         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.1‐2017 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_ 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.1‐2017, 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 1003.1-2017, 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:https://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(3P)

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