Manpages - open.2

open, openat, creat - open and possibly create a file

  #include <fcntl.h>

  int open(const char *pathname, int flags);
  int open(const char *pathname, int flags, mode_t mode);

  int creat(const char *pathname, mode_t mode);

  int openat(int dirfd, const char *pathname, int flags);
  int openat(int dirfd, const char *pathname, int flags",mode_t"mode);

  /* Documented separately, in openat2(2): */
  int openat2(int dirfd, const char *pathname,
   const struct open_how *how, size_t size);

Feature Test Macro Requirements for glibc (see *feature_test_macros*(7)):

*openat*():

      Since glibc 2.10:
          _POSIX_C_SOURCE >= 200809L
      Before glibc 2.10:
          _ATFILE_SOURCE

The open*() system call opens the file specified by pathname. If the specified file does not exist, it may optionally (if *O_CREAT is specified in flags) be created by *open*().

The return value of *open*() is a file descriptor, a small, nonnegative integer that is an index to an entry in the process’s table of open file descriptors. The file descriptor is used in subsequent system calls (*read*(2), *write*(2), *lseek*(2), *fcntl*(2), etc.) to refer to the open file. The file descriptor returned by a successful call will be the lowest-numbered file descriptor not currently open for the process.

By default, the new file descriptor is set to remain open across an execve*(2) (i.e., the *FD_CLOEXEC file descriptor flag described in fcntl*(2) is initially disabled); the *O_CLOEXEC flag, described below, can be used to change this default. The file offset is set to the beginning of the file (see *lseek*(2)).

A call to *open*() creates a new open file description, an entry in the system-wide table of open files. The open file description records the file offset and the file status flags (see below). A file descriptor is a reference to an open file description; this reference is unaffected if pathname is subsequently removed or modified to refer to a different file. For further details on open file descriptions, see NOTES.

The argument flags must include one of the following access modes: O_RDONLY, O_WRONLY, or O_RDWR. These request opening the file read-only, write-only, or read/write, respectively.

In addition, zero or more file creation flags and file status flags can be bitwise-or’d in flags. The file creation flags are O_CLOEXEC, O_CREAT, O_DIRECTORY, O_EXCL, O_NOCTTY, O_NOFOLLOW, O_TMPFILE, and O_TRUNC. The file status flags are all of the remaining flags listed below. The distinction between these two groups of flags is that the file creation flags affect the semantics of the open operation itself, while the file status flags affect the semantics of subsequent I/O operations. The file status flags can be retrieved and (in some cases) modified; see *fcntl*(2) for details.

The full list of file creation flags and file status flags is as follows:

O_APPEND

The file is opened in append mode. Before each *write*(2), the file offset is positioned at the end of the file, as if with *lseek*(2). The modification of the file offset and the write operation are performed as a single atomic step.

O_APPEND may lead to corrupted files on NFS filesystems if more than one process appends data to a file at once. This is because NFS does not support appending to a file, so the client kernel has to simulate it, which can’t be done without a race condition.

O_ASYNC

Enable signal-driven I/O: generate a signal (SIGIO by default, but this can be changed via *fcntl*(2)) when input or output becomes possible on this file descriptor. This feature is available only for terminals, pseudoterminals, sockets, and (since Linux 2.6) pipes and FIFOs. See *fcntl*(2) for further details. See also BUGS, below.

O_CLOEXEC (since Linux 2.6.23)

Enable the close-on-exec flag for the new file descriptor. Specifying this flag permits a program to avoid additional fcntl*(2) *F_SETFD operations to set the FD_CLOEXEC flag.

Note that the use of this flag is essential in some multithreaded programs, because using a separate fcntl*(2) *F_SETFD operation to set the FD_CLOEXEC flag does not suffice to avoid race conditions where one thread opens a file descriptor and attempts to set its close-on-exec flag using fcntl*(2) at the same time as another thread does a *fork*(2) plus *execve*(2). Depending on the order of execution, the race may lead to the file descriptor returned by *open*() being unintentionally leaked to the program executed by the child process created by *fork*(2). (This kind of race is in principle possible for any system call that creates a file descriptor whose close-on-exec flag should be set, and various other Linux system calls provide an equivalent of the *O_CLOEXEC flag to deal with this problem.)

O_CREAT

If pathname does not exist, create it as a regular file.

The owner (user ID) of the new file is set to the effective user ID of the process.

The group ownership (group ID) of the new file is set either to the effective group ID of the process (System V semantics) or to the group ID of the parent directory (BSD semantics). On Linux, the behavior depends on whether the set-group-ID mode bit is set on the parent directory: if that bit is set, then BSD semantics apply; otherwise, System V semantics apply. For some filesystems, the behavior also depends on the bsdgroups and sysvgroups mount options described in *mount*(8).

The mode argument specifies the file mode bits to be applied when a new file is created. If neither O_CREAT nor O_TMPFILE is specified in flags, then mode is ignored (and can thus be specified as 0, or simply omitted). The mode argument must be supplied if O_CREAT or O_TMPFILE is specified in flags; if it is not supplied, some arbitrary bytes from the stack will be applied as the file mode.

The effective mode is modified by the process’s umask in the usual way: in the absence of a default ACL, the mode of the created file is (mode & ~umask).

Note that mode applies only to future accesses of the newly created file; the *open*() call that creates a read-only file may well return a read/write file descriptor.

The following symbolic constants are provided for mode:

S_IRWXU

00700 user (file owner) has read, write, and execute permission

S_IRUSR

00400 user has read permission

S_IWUSR

00200 user has write permission

S_IXUSR

00100 user has execute permission

S_IRWXG

00070 group has read, write, and execute permission

S_IRGRP

00040 group has read permission

S_IWGRP

00020 group has write permission

S_IXGRP

00010 group has execute permission

S_IRWXO

00007 others have read, write, and execute permission

S_IROTH

00004 others have read permission

S_IWOTH

00002 others have write permission

S_IXOTH

00001 others have execute permission

According to POSIX, the effect when other bits are set in mode is unspecified. On Linux, the following bits are also honored in mode:

S_ISUID

0004000 set-user-ID bit

S_ISGID

0002000 set-group-ID bit (see *inode*(7)).

S_ISVTX

0001000 sticky bit (see *inode*(7)).

O_DIRECT (since Linux 2.4.10)

Try to minimize cache effects of the I/O to and from this file. In general this will degrade performance, but it is useful in special situations, such as when applications do their own caching. File I/O is done directly to/from user-space buffers. The O_DIRECT flag on its own makes an effort to transfer data synchronously, but does not give the guarantees of the O_SYNC flag that data and necessary metadata are transferred. To guarantee synchronous I/O, O_SYNC must be used in addition to O_DIRECT. See NOTES below for further discussion.

A semantically similar (but deprecated) interface for block devices is described in *raw*(8).

O_DIRECTORY

If pathname is not a directory, cause the open to fail. This flag was added in kernel version 2.1.126, to avoid denial-of-service problems if *opendir*(3) is called on a FIFO or tape device.

O_DSYNC

Write operations on the file will complete according to the requirements of synchronized I/O data integrity completion.

By the time *write*(2) (and similar) return, the output data has been transferred to the underlying hardware, along with any file metadata that would be required to retrieve that data (i.e., as though each *write*(2) was followed by a call to *fdatasync*(2)). See NOTES below.

O_EXCL

Ensure that this call creates the file: if this flag is specified in conjunction with O_CREAT, and pathname already exists, then open*() fails with the error *EEXIST.

When these two flags are specified, symbolic links are not followed: if pathname is a symbolic link, then *open*() fails regardless of where the symbolic link points.

In general, the behavior of O_EXCL is undefined if it is used without O_CREAT. There is one exception: on Linux 2.6 and later, O_EXCL can be used without O_CREAT if pathname refers to a block device. If the block device is in use by the system (e.g., mounted), open*() fails with the error *EBUSY.

On NFS, O_EXCL is supported only when using NFSv3 or later on kernel 2.6 or later. In NFS environments where O_EXCL support is not provided, programs that rely on it for performing locking tasks will contain a race condition. Portable programs that want to perform atomic file locking using a lockfile, and need to avoid reliance on NFS support for O_EXCL, can create a unique file on the same filesystem (e.g., incorporating hostname and PID), and use *link*(2) to make a link to the lockfile. If *link*(2) returns 0, the lock is successful. Otherwise, use *stat*(2) on the unique file to check if its link count has increased to 2, in which case the lock is also successful.

O_LARGEFILE

(LFS) Allow files whose sizes cannot be represented in an off_t (but can be represented in an off64_t) to be opened. The _LARGEFILE64_SOURCE macro must be defined (before including any header files) in order to obtain this definition. Setting the _FILE_OFFSET_BITS feature test macro to 64 (rather than using O_LARGEFILE) is the preferred method of accessing large files on 32-bit systems (see *feature_test_macros*(7)).

O_NOATIME (since Linux 2.6.8)

Do not update the file last access time (st_atime in the inode) when the file is *read*(2).

This flag can be employed only if one of the following conditions is true:

• The effective UID of the process matches the owner UID of the file.
• The calling process has the CAP_FOWNER capability in its user namespace and the owner UID of the file has a mapping in the namespace.

This flag is intended for use by indexing or backup programs, where its use can significantly reduce the amount of disk activity. This flag may not be effective on all filesystems. One example is NFS, where the server maintains the access time.

O_NOCTTY

If pathname refers to a terminal device—see *tty*(4)—it will not become the process’s controlling terminal even if the process does not have one.

O_NOFOLLOW

If the trailing component (i.e., basename) of pathname is a symbolic link, then the open fails, with the error ELOOP. Symbolic links in earlier components of the pathname will still be followed. (Note that the ELOOP error that can occur in this case is indistinguishable from the case where an open fails because there are too many symbolic links found while resolving components in the prefix part of the pathname.)

This flag is a FreeBSD extension, which was added to Linux in version 2.1.126, and has subsequently been standardized in POSIX.1-2008.

See also O_PATH below.

O_NONBLOCK or O_NDELAY

When possible, the file is opened in nonblocking mode. Neither the *open*() nor any subsequent I/O operations on the file descriptor which is returned will cause the calling process to wait.

Note that the setting of this flag has no effect on the operation of poll*(2), *select*(2), *epoll*(7), and similar, since those interfaces merely inform the caller about whether a file descriptor is “ready”, meaning that an I/O operation performed on the file descriptor with the *O_NONBLOCK flag clear would not block.

Note that this flag has no effect for regular files and block devices; that is, I/O operations will (briefly) block when device activity is required, regardless of whether O_NONBLOCK is set. Since O_NONBLOCK semantics might eventually be implemented, applications should not depend upon blocking behavior when specifying this flag for regular files and block devices.

For the handling of FIFOs (named pipes), see also fifo*(7). For a discussion of the effect of *O_NONBLOCK in conjunction with mandatory file locks and with file leases, see *fcntl*(2).

O_PATH (since Linux 2.6.39)

Obtain a file descriptor that can be used for two purposes: to indicate a location in the filesystem tree and to perform operations that act purely at the file descriptor level. The file itself is not opened, and other file operations (e.g., read*(2), *write*(2), *fchmod*(2), *fchown*(2), *fgetxattr*(2), *ioctl*(2), *mmap*(2)) fail with the error *EBADF.

The following operations can be performed on the resulting file descriptor:

• *close*(2).
• *fchdir*(2), if the file descriptor refers to a directory (since Linux 3.5).
• *fstat*(2) (since Linux 3.6).
• *fstatfs*(2) (since Linux 3.12).
• Duplicating the file descriptor (dup*(2), *fcntl*(2) *F_DUPFD, etc.).
• Getting and setting file descriptor flags (fcntl*(2) *F_GETFD and F_SETFD).
• Retrieving open file status flags using the fcntl*(2) *F_GETFL operation: the returned flags will include the bit O_PATH.
• Passing the file descriptor as the dirfd argument of openat*() and the other “*at()” system calls. This includes *linkat*(2) with *AT_EMPTY_PATH (or via procfs using AT_SYMLINK_FOLLOW) even if the file is not a directory.
• Passing the file descriptor to another process via a UNIX domain socket (see SCM_RIGHTS in *unix*(7)).

When O_PATH is specified in flags, flag bits other than O_CLOEXEC, O_DIRECTORY, and O_NOFOLLOW are ignored.

Opening a file or directory with the O_PATH flag requires no permissions on the object itself (but does require execute permission on the directories in the path prefix). Depending on the subsequent operation, a check for suitable file permissions may be performed (e.g., fchdir*(2) requires execute permission on the directory referred to by its file descriptor argument). By contrast, obtaining a reference to a filesystem object by opening it with the *O_RDONLY flag requires that the caller have read permission on the object, even when the subsequent operation (e.g., *fchdir*(2), *fstat*(2)) does not require read permission on the object.

If pathname is a symbolic link and the O_NOFOLLOW flag is also specified, then the call returns a file descriptor referring to the symbolic link. This file descriptor can be used as the dirfd argument in calls to *fchownat*(2), *fstatat*(2), *linkat*(2), and *readlinkat*(2) with an empty pathname to have the calls operate on the symbolic link.

If pathname refers to an automount point that has not yet been triggered, so no other filesystem is mounted on it, then the call returns a file descriptor referring to the automount directory without triggering a mount. fstatfs*(2) can then be used to determine if it is, in fact, an untriggered automount point (.f_type == AUTOFS_SUPER_MAGIC*).

One use of O_PATH for regular files is to provide the equivalent of POSIX.1’s O_EXEC functionality. This permits us to open a file for which we have execute permission but not read permission, and then execute that file, with steps something like the following:

    char buf[PATH_MAX];
    fd = open("some_prog", O_PATH);
    snprintf(buf, PATH_MAX, "/proc/self/fd/%d", fd);
    execl(buf, "some_prog", (char *) NULL);

An O_PATH file descriptor can also be passed as the argument of *fexecve*(3).

O_SYNC

Write operations on the file will complete according to the requirements of synchronized I/O file integrity completion (by contrast with the synchronized I/O data integrity completion provided by O_DSYNC.)

By the time *write*(2) (or similar) returns, the output data and associated file metadata have been transferred to the underlying hardware (i.e., as though each *write*(2) was followed by a call to *fsync*(2)). See NOTES below.

O_TMPFILE (since Linux 3.11)

Create an unnamed temporary regular file. The pathname argument specifies a directory; an unnamed inode will be created in that directory’s filesystem. Anything written to the resulting file will be lost when the last file descriptor is closed, unless the file is given a name.

O_TMPFILE must be specified with one of O_RDWR or O_WRONLY and, optionally, O_EXCL. If O_EXCL is not specified, then *linkat*(2) can be used to link the temporary file into the filesystem, making it permanent, using code like the following:

    char path[PATH_MAX];
    fd = open("/path/to/dir", O_TMPFILE | O_RDWR,
                            S_IRUSR | S_IWUSR);

    /* File I/O on 'fd'... */

    linkat(fd, "", AT_FDCWD, "/path/for/file", AT_EMPTY_PATH);

    /* If the caller doesn't have the CAP_DAC_READ_SEARCH
       capability (needed to use AT_EMPTY_PATH with linkat(2)),
       and there is a proc(5) filesystem mounted, then the
       linkat(2) call above can be replaced with:

    snprintf(path, PATH_MAX,  "/proc/self/fd/%d", fd);
    linkat(AT_FDCWD, path, AT_FDCWD, "/path/for/file",
                            AT_SYMLINK_FOLLOW);
    */

In this case, the open*() mode argument determines the file permission mode, as with *O_CREAT.

Specifying O_EXCL in conjunction with O_TMPFILE prevents a temporary file from being linked into the filesystem in the above manner. (Note that the meaning of O_EXCL in this case is different from the meaning of O_EXCL otherwise.)

There are two main use cases for O_TMPFILE:

• Improved *tmpfile*(3) functionality: race-free creation of temporary files that (1) are automatically deleted when closed; (2) can never be reached via any pathname; (3) are not subject to symlink attacks; and (4) do not require the caller to devise unique names.
• Creating a file that is initially invisible, which is then populated with data and adjusted to have appropriate filesystem attributes (*fchown*(2), *fchmod*(2), *fsetxattr*(2), etc.) before being atomically linked into the filesystem in a fully formed state (using *linkat*(2) as described above).

O_TMPFILE requires support by the underlying filesystem; only a subset of Linux filesystems provide that support. In the initial implementation, support was provided in the ext2, ext3, ext4, UDF, Minix, and tmpfs filesystems. Support for other filesystems has subsequently been added as follows: XFS (Linux 3.15); Btrfs (Linux 3.16); F2FS (Linux 3.16); and ubifs (Linux 4.9)

O_TRUNC

If the file already exists and is a regular file and the access mode allows writing (i.e., is O_RDWR or O_WRONLY) it will be truncated to length 0. If the file is a FIFO or terminal device file, the O_TRUNC flag is ignored. Otherwise, the effect of O_TRUNC is unspecified.

On success, *open*(), *openat*(), and *creat*() return the new file descriptor (a nonnegative integer). On error, -1 is returned and errno is set to indicate the error.

*open*(), *openat*(), and *creat*() can fail with the following errors:

EACCES

The requested access to the file is not allowed, or search permission is denied for one of the directories in the path prefix of pathname, or the file did not exist yet and write access to the parent directory is not allowed. (See also *path_resolution*(7).)

EACCES

Where O_CREAT is specified, the protected_fifos or protected_regular sysctl is enabled, the file already exists and is a FIFO or regular file, the owner of the file is neither the current user nor the owner of the containing directory, and the containing directory is both world- or group-writable and sticky. For details, see the descriptions of /proc/sys/fs/protected_fifos and /proc/sys/fs/protected_regular in *proc*(5).

EBADF

(openat*()) pathname is relative but dirfd is neither *AT_FDCWD nor a valid file descriptor.

EBUSY

O_EXCL was specified in flags and pathname refers to a block device that is in use by the system (e.g., it is mounted).

EDQUOT

Where O_CREAT is specified, the file does not exist, and the user’s quota of disk blocks or inodes on the filesystem has been exhausted.

EEXIST

pathname already exists and O_CREAT and O_EXCL were used.

EFAULT

pathname points outside your accessible address space.

EFBIG

See EOVERFLOW.

EINTR

While blocked waiting to complete an open of a slow device (e.g., a FIFO; see *fifo*(7)), the call was interrupted by a signal handler; see *signal*(7).

EINVAL

The filesystem does not support the O_DIRECT flag. See NOTES for more information.

EINVAL

Invalid value in flags.

EINVAL

O_TMPFILE was specified in flags, but neither O_WRONLY nor O_RDWR was specified.

EINVAL

O_CREAT was specified in flags and the final component (“basename”) of the new file’s pathname is invalid (e.g., it contains characters not permitted by the underlying filesystem).

EINVAL

The final component (“basename”) of pathname is invalid (e.g., it contains characters not permitted by the underlying filesystem).

EISDIR

pathname refers to a directory and the access requested involved writing (that is, O_WRONLY or O_RDWR is set).

EISDIR

pathname refers to an existing directory, O_TMPFILE and one of O_WRONLY or O_RDWR were specified in flags, but this kernel version does not provide the O_TMPFILE functionality.

ELOOP

Too many symbolic links were encountered in resolving pathname.

ELOOP

pathname was a symbolic link, and flags specified O_NOFOLLOW but not O_PATH.

EMFILE

The per-process limit on the number of open file descriptors has been reached (see the description of RLIMIT_NOFILE in *getrlimit*(2)).

ENAMETOOLONG

pathname was too long.

ENFILE

The system-wide limit on the total number of open files has been reached.

ENODEV

pathname refers to a device special file and no corresponding device exists. (This is a Linux kernel bug; in this situation ENXIO must be returned.)

ENOENT

O_CREAT is not set and the named file does not exist.

ENOENT

A directory component in pathname does not exist or is a dangling symbolic link.

ENOENT

pathname refers to a nonexistent directory, O_TMPFILE and one of O_WRONLY or O_RDWR were specified in flags, but this kernel version does not provide the O_TMPFILE functionality.

ENOMEM

The named file is a FIFO, but memory for the FIFO buffer can’t be allocated because the per-user hard limit on memory allocation for pipes has been reached and the caller is not privileged; see *pipe*(7).

ENOMEM

Insufficient kernel memory was available.

ENOSPC

pathname was to be created but the device containing pathname has no room for the new file.

ENOTDIR

A component used as a directory in pathname is not, in fact, a directory, or O_DIRECTORY was specified and pathname was not a directory.

ENOTDIR

(*openat*()) pathname is a relative pathname and dirfd is a file descriptor referring to a file other than a directory.

ENXIO

O_NONBLOCK | O_WRONLY is set, the named file is a FIFO, and no process has the FIFO open for reading.

ENXIO

The file is a device special file and no corresponding device exists.

ENXIO

The file is a UNIX domain socket.

EOPNOTSUPP

The filesystem containing pathname does not support O_TMPFILE.

EOVERFLOW

pathname refers to a regular file that is too large to be opened. The usual scenario here is that an application compiled on a 32-bit platform without -D_FILE_OFFSET_BITS=64 tried to open a file whose size exceeds (1<<31)-1 bytes; see also O_LARGEFILE above. This is the error specified by POSIX.1; in kernels before 2.6.24, Linux gave the error EFBIG for this case.

EPERM

The O_NOATIME flag was specified, but the effective user ID of the caller did not match the owner of the file and the caller was not privileged.

EPERM

The operation was prevented by a file seal; see *fcntl*(2).

EROFS

pathname refers to a file on a read-only filesystem and write access was requested.

ETXTBSY

pathname refers to an executable image which is currently being executed and write access was requested.

ETXTBSY

pathname refers to a file that is currently in use as a swap file, and the O_TRUNC flag was specified.

ETXTBSY

pathname refers to a file that is currently being read by the kernel (e.g., for module/firmware loading), and write access was requested.

EWOULDBLOCK

The O_NONBLOCK flag was specified, and an incompatible lease was held on the file (see *fcntl*(2)).

*openat*() was added to Linux in kernel 2.6.16; library support was added to glibc in version 2.4.

*open*(), *creat*() SVr4, 4.3BSD, POSIX.1-2001, POSIX.1-2008.

*openat*(): POSIX.1-2008.

*openat2*(2) is Linux-specific.

The O_DIRECT, O_NOATIME, O_PATH, and O_TMPFILE flags are Linux-specific. One must define _GNU_SOURCE to obtain their definitions.

The O_CLOEXEC, O_DIRECTORY, and O_NOFOLLOW flags are not specified in POSIX.1-2001, but are specified in POSIX.1-2008. Since glibc 2.12, one can obtain their definitions by defining either _POSIX_C_SOURCE with a value greater than or equal to 200809L or _XOPEN_SOURCE with a value greater than or equal to 700. In glibc 2.11 and earlier, one obtains the definitions by defining _GNU_SOURCE.

As noted in feature_test_macros*(7), feature test macros such as *_POSIX_C_SOURCE, _XOPEN_SOURCE, and _GNU_SOURCE must be defined before including any header files.

Under Linux, the O_NONBLOCK flag is sometimes used in cases where one wants to open but does not necessarily have the intention to read or write. For example, this may be used to open a device in order to get a file descriptor for use with *ioctl*(2).

The (undefined) effect of O_RDONLY | O_TRUNC varies among implementations. On many systems the file is actually truncated.

Note that *open*() can open device special files, but *creat*() cannot create them; use *mknod*(2) instead.

If the file is newly created, its st_atime, st_ctime, st_mtime fields (respectively, time of last access, time of last status change, and time of last modification; see stat*(2)) are set to the current time, and so are the st_ctime and st_mtime fields of the parent directory. Otherwise, if the file is modified because of the *O_TRUNC flag, its st_ctime and st_mtime fields are set to the current time.

The files in the /proc[pid]/fd/ directory show the open file descriptors of the process with the PID pid. The files in the /proc[pid]/fdinfo/ directory show even more information about these file descriptors. See *proc*(5) for further details of both of these directories.

The Linux header file <asm/fcntl.h> doesn’t define O_ASYNC; the (BSD-derived) FASYNC synonym is defined instead.

Currently, it is not possible to enable signal-driven I/O by specifying O_ASYNC when calling *open*(); use *fcntl*(2) to enable this flag.

One must check for two different error codes, EISDIR and ENOENT, when trying to determine whether the kernel supports O_TMPFILE functionality.

When both O_CREAT and O_DIRECTORY are specified in flags and the file specified by pathname does not exist, open*() will create a regular file (i.e., *O_DIRECTORY is ignored).

*chmod*(2), *chown*(2), *close*(2), *dup*(2), *fcntl*(2), *link*(2), *lseek*(2), *mknod*(2), *mmap*(2), *mount*(2), *open_by_handle_at*(2), *openat2*(2), *read*(2), *socket*(2), *stat*(2), *umask*(2), *unlink*(2), *write*(2), *fopen*(3), *acl*(5), *fifo*(7), *inode*(7), *path_resolution*(7), *symlink*(7)

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