NAME

open − open for reading or writing

SYNOPSIS

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

int open (const char ∗path, int oflag, ... /∗ mode_t mode ∗/);

DESCRIPTION

path points to a path name naming a file.  open opens a file descriptor for the named file and sets the file status flags according to the value of oflag. oflag values are constructed by OR-ing Flags from the following list (only one of the first three flags below may be used):

O_RDONLY Open for reading only. 

O_WRONLY Open for writing only. 

O_RDWR Open for reading and writing. 

O_NDELAY or O_NONBLOCK
These flags may affect subsequent reads and writes [see read(2) and write(2)].  If both O_NDELAY and O_NONBLOCK are set, O_NONBLOCK will take precedence. 

When opening a FIFO with O_RDONLY or O_WRONLY set:

If O_NDELAY or O_NONBLOCK is set: An open for reading-only will return without delay; an open for writing-only will return an error if no process currently has the file open for reading. 

If O_NDELAY and O_NONBLOCK are clear: An open for reading-only will block until a process opens the file for writing; an open for writing-only will block until a process opens the file for reading. 

When opening a block-special or character-special file:

If O_NDELAY or O_NONBLOCK is set: The open will return without waiting for the device to be ready or available; subsequent behavior of the device is device specific. 

If O_NDELAY and O_NONBLOCK are clear: The open will block until the device is ready or available. 

O_APPEND If set, the file pointer will be set to the end of the file prior to each write. 

O_SYNC When opening a regular file, this flag affects subsequent writes.  If set, each write(2) will wait for both the file data and file status to be physically updated. 

O_DSYNC When opening a regular file, this flag affects subsequent writes.  If set, each write(2) will wait for the file data to be physically updated.  This flag is only valid with the POSIX 1003.4 extensions library, libposix34.a. 

O_NOCTTY If set and the file is a terminal, the terminal will not be allocated as the calling process’s controlling terminal. 

O_CREAT If the file exists, this flag has no effect, except as noted under O_EXCL below.  Otherwise, the file is created and the owner ID of the file is set to the effective user ID of the process, the group ID of the file is set to the effective group ID of the process, or if the S_ISGID bit is set in the directory in which the file is being created, the file’s group ID is set to the group ID of its parent directory.  If the group ID of the new file does not match the effective group ID or one of the supplementary groups IDs, the S_ISGID bit is cleared.  The access permission bits of the file mode are set to the value of mode, modified as follows [see creat(2)]:

All bits set in the file mode creation mask of the process are cleared [see umask(2)]. 

The “save text image after execution bit” of the mode is cleared [see chmod(2)]. 

O_TRUNC If the file exists, its length is truncated to 0 and the mode and owner are unchanged.  O_TRUNC has no effect on FIFO special files or directories. 

O_EXCL If O_EXCL and O_CREAT are set, open will fail if the file exists.  The check for the existence of the file and the creation of the file if it does not exist is atomic with respect to other processes executing open naming the same filename in the same directory with O_EXCL and O_CREAT set. 

When opening a STREAMS file, oflag may be constructed from O_NDELAY or O_NONBLOCK OR-ed with either O_RDONLY, O_WRONLY , or O_RDWR.  Other flag values are not applicable to STREAMS devices and have no effect on them.  The values of O_NDELAY and O_NONBLOCK affect the operation of STREAMS drivers and certain system calls [see read(2), getmsg(2), putmsg(2), and write(2)].  For drivers, the implementation of O_NDELAY and O_NONBLOCK is device specific.  Each STREAMS device driver may treat these options differently. 

When open is invoked to open a named stream, and the connld module [see connld(7)] has been pushed on the pipe, open blocks until the server process has issued an I_RECVFD ioctl [see streamio(7)] to receive the file descriptor. 

If path is a symbolic link and O_CREAT and O_EXCL are set, the link is not followed. 

The file pointer used to mark the current position within the file is set to the beginning of the file. 

The new file descriptor is the lowest numbered file descriptor available and is set to remain open across exec system calls [see fcntl(2)]. 

Certain flag values can be set following open as described in fcntl(2). 

If O_CREAT is set and the file did not previously exist, upon successful completion open marks for update the st_atime, st_ctime and st_mtime fields of the file and the st_ctime and st_mtime fields of the parent directory. 

If O_TRUNC is set and the file did previously exist, upon successful completion open marks for update the st_ctime and st_mtime fields of the file. 

The named file is opened unless one or more of the following are true:

EACCES The file does not exist and write permission is denied by the parent directory of the file to be created. 

EACCES O_TRUNC is specified and write permission is denied

EACCES A component of the path prefix denies search permission. 

EACCES oflag permission is denied for an existing file. 

EAGAIN The file exists, mandatory file/record locking is set, and there are outstanding record locks on the file [see chmod(2)]. 

EAGAIN O_NDELAY or O_NONBLOCK is set, the named file is a STREAMS device and there is another process trying to open it at the same time. 

EEXIST O_CREAT and O_EXCL are set, and the named file exists. 

EFAULT path points outside the allocated address space of the process. 

EINTR A signal was caught during the open system call. 

EIO A hangup or error occurred during the open of the STREAMS-based device. 

EISDIR The named file is a directory and oflag is write or read/write. 

ELOOP Too many symbolic links were encountered in translating path. 

EMFILE The process has too many open files [see getrlimit(2)]. 

EMULTIHOP Components of path require hopping to multiple remote machines and the file system does not allow it. 

ENAMETOOLONG The length of the path argument exceeds {PATH_MAX}, or the length of a path component exceeds {NAME_MAX} while {_POSIX_NO_TRUNC} is in effect. 

ENFILE The system file table is full. 

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

ENOENT O_CREAT is set and a component of the path prefix does not exist or is the null pathname. 

ENOLINK path points to a remote machine, and the link to that machine is no longer active. 

ENOMEM The system is unable to allocate a send descriptor. 

ENOSPC O_CREAT and O_EXCL are set, and the file system is out of inodes. 

ENOSPC O_CREAT is set and the directory that would contain the file cannot be extended. 

ENOSR Unable to allocate a stream. 

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. 

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

ENXIO A STREAMS module or driver open routine failed. 

EROFS The named file resides on a read-only file system and either O_WRONLY, O_RDWR, O_CREAT, or O_TRUNC is set in oflag (if the file does not exist). 

NOTE

On some previous versions of the UNIX operating system an application could not open a file for writing which was the executable for a running process.  System V Release 4 does not enforce that restriction. 

SEE ALSO

intro(2), chmod(2), close(2), creat(2), dup(2), exec(2), fcntl(2), getrlimit(2), lseek(2), read(2), getmsg(2), putmsg(2), stat(2), umask(2), write(2), stat(5)

DIAGNOSTICS

Upon successful completion, the file descriptor is returned.  Otherwise, a value of −1 is returned and errno is set to indicate the error.