open()

NAME

open() - open or create a file for reading or writing

SYNOPSIS 

#include <fcntl.h>


int open (const char *path, int o_flag, ...)

DESCRIPTION

The file name specified by path is opened for reading and/or writing as specified by the argument o_flag and the file descriptor returned to the calling process. The o_flag argument may indicate the file is to be created if it does not exist (by specifying the O_CREAT flag), and modified by the process' umask value (see umask(2)).

The o_flag is a bitwise inclusive OR of the values from the following two tables.

You must include one of the three values (file access modes) from the following table in the value of o_flag.

Symbol Meaning
O_RDONLY Open for reading only
O_RDWR Open for reading and writing
O_WRONLY Open for writing only

You can specify any combination of the remaining flags.

Symbol Meaning
O_APPEND Append on each write
O_CREAT Create file if it does not exist
O_EXCL Error if create and file exists
O_NOCTTY Do not acquire as a controlling terminal
O_NONBLOCK Do not block on open
O_TRUNC Truncate size to 0
O_SYNC Perform synchronous I/O operations

When the file is opened with the O_CREAT flag set and it doesn't already exist, open(2) requires a third argument, which it treats as a mode of type mode_t. This argument specifies the mode of the newly-created file. (For a description of a mode see the reference page for chmod(2).)

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, open(2) 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, open(2) will fail even if the symbolic link points to a non-existent name. If the O_NONBLOCK flag is specified and the open(2) call would result in the process being blocked for some reason (e.g., waiting for carrier on a dialup line), open(2) returns immediately. The first time the process attempts to perform I/O on the open file it will block (not currently implemented).

If successful, open(2) 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(1) system calls; see close(2) and fcntl(2).

The system imposes a limit on the number of file descriptors open simultaneously by one process.

RETURN VALUES

On success, the open(2) call returns an integer file descriptor which refers to the file. On failure, the call returns -1 and sets errno

ERRORS

The named file is opened unless:

[EACCES]
Search permission is denied for a component of the path prefix.
[EACCES]
The required permissions (for reading and/or writing) are denied for the given flags.
[EACCES]
O_CREAT is specified, the file does not exist, and the directory in which it is to be created does not permit writing.
[EEXIST]
O_CREAT and O_EXCL were specified and the file exists.
[EFAULT]
Path points outside the process's allocated address space.
[EINTR]
The open(2) operation was interrupted by a signal.
[EINVAL]
The value of the oflag argument is not valid.
[EIO]
An I/O error occurred while making the directory entry or allocating the file serial number for O_CREAT.
[EISDIR]
The named file is a directory, and the arguments specify it is to be opened for writing.
[ELOOP]
Too many symbolic links were encountered in translating the pathname.
[EMFILE]
The process has already reached its limit for open file descriptors.
[ENAMETOOLONG]
A component of a pathname exceeded {NAME_MAX} characters, or an entire pathname exceeded {PATH_MAX} characters.
[ENFILE]
The system has already reached its limit for open file descriptors.
[ENOENT]
O_CREAT is not set and the named file does not exist.
[ENOENT]
A component of the pathname that must exist does not exist.
[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.
[ENOSPC]
O_CREAT is specified, the file does not exist, and there are no free file serial numbers on the file system on which the file is being created.
[ENOTDIR]
A component of the path prefix is not a directory.
[ENXIO]
The named file is a character special or block special file, and the device associated with this special file does not exist or the file is located on a FAT partition in a directory other than /dev.
[EROFS]
The named file resides on a read-only file system, and the file is to be modified.

SEE ALSO

chmod(2)

close(2)

creat(2)

dup(2)

fcntl(2)

lseek(2)

read(2)

write(2)

umask(2)