NAME

open − open for reading or writing

SYNOPSIS

#include <fcntl.h>
int open (path, oflag [, mode] )
char ∗path;
int oflag, 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. For non-STREAMS [see intro(2)] files, 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 This flag may affect subsequent reads and writes [see read(2) and write(2)].

When opening a FIFO with O_RDONLY or O_WRONLY set:

If O_NDELAY is set:

An open for reading-only returns without delay.  An open for writing-only returns an error if no process currently opens the file for reading. 

If O_NDELAY is clear:

An open for reading-only blocks until a process opens the file for writing.  An open for writing-only blocks until a process opens the file for reading. 

When opening a file associated with a communication line:

If O_NDELAY is set:

The open returns without waiting for carrier. 

If O_NDELAY is clear:

The open blocks until carrier is present. 

O_APPEND If set, the file pointer is 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) waits for both the file data and file status to be physically updated.

O_CREAT If the file exists, this flag has no effect.  Otherwise, 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, and the low-order 12 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_EXCL If O_EXCL and O_CREAT are set, open fails if the file exists. 

When opening a STREAMS file, oflag may be constructed from O_NDELAY 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 value of O_NDELAY affects 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 is device-specific.  Each STREAMS device driver may treat this option differently. 

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

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 set to remain open across exec system calls [see fcntl(2)].

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

­[EACCES] A component of the path prefix denies search permission. 

­[EACCES] oflag permission is denied for the named file. 

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

­[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 a STREAMS open.

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

­[EMFILE] NOFILES file descriptors are currently open. 

­[EMULTIHOP] Components of path require hopping to multiple remote machines. 

­[ENFILE] The system file table is full. 

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

­[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. 

­[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 is set, the named file is a FIFO, O_WRONLY is set, and no process has opened the file for reading. 

­[ENXIO] A STREAMS module or driver open routine failed. 

­[EROFS] The named file resides on a read-only file system and oflag is write or read/write. 

­[ETXTBSY] The file is a pure procedure (shared text) file that is being executed and oflag is write or read/write. 

SEE ALSO

chmod(2), close(2), creat(2), dup(2), fcntl(2), intro(2), lseek(2), read(2), getmsg(2), putmsg(2), umask(2), write(2). 

DIAGNOSTICS

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

September 02, 1992