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open.2freebsd

BSD mandoc

NAME

open - open or create a file for reading or writing

LIBRARY

Lb libc

SYNOPSIS

In fcntl.h Ft int Fn open const char *path int flags ...

DESCRIPTION

The file name specified by Fa path is opened for reading and/or writing as specified by the argument Fa flags and the file descriptor returned to the calling process. The Fa flags argument may indicate the file is to be created if it does not exist (by specifying the O_CREAT flag). In this case Fn open requires a third argument Fa mode_t mode , and the file is created with mode Fa mode as described in chmod(2) and modified by the process' umask value (see umask(2)).

The flags specified are formed by or 'ing the following values

 O_RDONLY        open for reading only
 O_WRONLY        open for writing only
 O_RDWR          open for reading and writing
 O_NONBLOCK      do not block on open
 O_APPEND        append on each write
 O_CREAT         create file if it does not exist
 O_TRUNC         truncate size to 0
 O_EXCL          error if create and file exists
 O_SHLOCK        atomically obtain a shared lock
 O_EXLOCK        atomically obtain an exclusive lock
 O_DIRECT        eliminate or reduce cache effects
 O_FSYNC         synchronous writes
 O_SYNC          synchronous writes
 O_NOFOLLOW      do not follow symlinks
 O_NOCTTY        don't assign controlling terminal
 

Opening a file with O_APPEND set causes each write on the file to be appended to the end. If O_TRUNC is specified and the file exists, the file is truncated to zero length. If O_EXCL is set with O_CREAT and the file already exists, Fn open returns an error. This may be used to implement a simple exclusive access locking mechanism. If O_EXCL is set and the last component of the pathname is a symbolic link, Fn open will fail even if the symbolic link points to a non-existent name. If the O_NONBLOCK flag is specified and the Fn open system call would result in the process being blocked for some reason (e.g., waiting for carrier on a dialup line), Fn open returns immediately. The descriptor remains in non-blocking mode for subsequent operations.

If O_FSYNC is used in the mask, all writes will immediately be written to disk, the kernel will not cache written data and all writes on the descriptor will not return until the data to be written completes.

O_SYNC is a synonym for O_FSYNC required by POSIX

If O_NOFOLLOW is used in the mask and the target file passed to Fn open is a symbolic link then the Fn open will fail.

When opening a file, a lock with flock(2) semantics can be obtained by setting O_SHLOCK for a shared lock, or O_EXLOCK for an exclusive lock. If creating a file with O_CREAT the request for the lock will never fail (provided that the underlying file system supports locking).

O_DIRECT may be used to minimize or eliminate the cache effects of reading and writing. The system will attempt to avoid caching the data you read or write. If it cannot avoid caching the data, it will minimize the impact the data has on the cache. Use of this flag can drastically reduce performance if not used with care.

O_NOCTTY may be used to ensure the OS does not assign this file as the controlling terminal when it opens a tty device. This is the default on Fx , but is present for POSIX compatibility. The Fn open system call will not assign controlling terminals on Fx .

If successful, Fn open returns a non-negative integer, termed a file descriptor. It returns -1 on failure. The file pointer used to mark the current position within the file is set to the beginning of the file.

When a new file is created it is given the group of the directory which contains it.

The new descriptor is set to remain open across execve(2) system calls; see close(2) and fcntl(2).

The system imposes a limit on the number of file descriptors open simultaneously by one process. The getdtablesize(2) system call returns the current system limit.

RETURN VALUES

If successful, Fn open returns a non-negative integer, termed a file descriptor. It returns -1 on failure, and sets errno to indicate the error.

ERRORS

The named file is opened unless:

Bq Er ENOTDIR

A component of the path prefix is not a directory.

Bq Er ENAMETOOLONG

A component of a pathname exceeded 255 characters, or an entire path name exceeded 1023 characters.

Bq Er ENOENT

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

Bq Er ENOENT

A component of the path name that must exist does not exist.

Bq Er EACCES

Search permission is denied for a component of the path prefix.

Bq Er EACCES

The required permissions (for reading and/or writing) are denied for the given flags.

Bq Er EACCES

O_TRUNC is specified and write permission is denied.

Bq Er EACCES

O_CREAT is specified, the file does not exist, and the directory in which it is to be created does not permit writing.

Bq Er EPERM

O_CREAT is specified, the file does not exist, and the directory in which it is to be created has its immutable flag set, see the chflags(2) manual page for more information.

Bq Er EPERM

The named file has its immutable flag set and the file is to be modified.

Bq Er EPERM

The named file has its append-only flag set, the file is to be modified, and O_TRUNC is specified or O_APPEND is not specified.

Bq Er ELOOP

Too many symbolic links were encountered in translating the pathname.

Bq Er EISDIR

The named file is a directory, and the arguments specify it is to be modified.

Bq Er EROFS

The named file resides on a read-only file system, and the file is to be modified.

Bq Er EROFS

O_CREAT is specified and the named file would reside on a read-only file system.

Bq Er EMFILE

The process has already reached its limit for open file descriptors.

Bq Er ENFILE

The system file table is full.

Bq Er EMLINK

O_NOFOLLOW was specified and the target is a symbolic link.

Bq Er ENXIO

The named file is a character special or block special file, and the device associated with this special file does not exist.

Bq Er ENXIO

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

Bq Er EINTR

The Fn open operation was interrupted by a signal.

Bq Er EOPNOTSUPP

O_SHLOCK or O_EXLOCK is specified but the underlying file system does not support locking.

Bq Er EOPNOTSUPP

The named file is a special file mounted through a file system that does not support access to it (e.g. NFS).

Bq Er EWOULDBLOCK

O_NONBLOCK and one of O_SHLOCK or O_EXLOCK is specified and the file is locked.

Bq Er ENOSPC

O_CREAT is specified, the file does not exist, and the directory in which the entry for the new file is being placed cannot be extended because there is no space left on the file system containing the directory.

Bq Er ENOSPC

O_CREAT is specified, the file does not exist, and there are no free inodes on the file system on which the file is being created.

Bq Er EDQUOT

O_CREAT is specified, the file does not exist, and the directory in which the entry for the new file is being placed cannot be extended because the user's quota of disk blocks on the file system containing the directory has been exhausted.

Bq Er EDQUOT

O_CREAT is specified, the file does not exist, and the user's quota of inodes on the file system on which the file is being created has been exhausted.

Bq Er EIO

An I/O error occurred while making the directory entry or allocating the inode for O_CREAT

Bq Er ETXTBSY

The file is a pure procedure (shared text) file that is being executed and the Fn open system call requests write access.

Bq Er EFAULT

The Fa path argument points outside the process's allocated address space.

Bq Er EEXIST

O_CREAT and O_EXCL were specified and the file exists.

Bq Er EOPNOTSUPP

An attempt was made to open a socket (not currently implemented).

Bq Er EINVAL

An attempt was made to open a descriptor with an illegal combination of O_RDONLY O_WRONLY and O_RDWR

SEE ALSO

chmod(2), close(2), dup(2), fhopen(2), getdtablesize(2), getfh(2), lgetfh(2), lseek(2), read(2), umask(2), write(2), fopen(3)

HISTORY

The Fn open function appeared in AT&T System v6 .