open

Opens a file for reading, writing, or editing. It positions the file at its beginning (byte 0).

Format

#include  <fcntl.h>

int open  (const char *file_spec, int flags,
          mode_t mode); 
          (ANSI C)

int open  (const char *file_spec, int
          flags, . . . ); (DEC C
          Extension)

Arguments

file_spec

A null-terminated character string containing a valid file specification. If you specify a directory in the file_ spec and it is a search list that contains an error, DEC C interprets it as a file open error.

flags

The following values are defined in the <file.h> header file:

O_RDONLY	Open for reading only
O_WRONLY	Open for writing only
O_RDWR	Open for reading and writing
O_NDELAY	Open for asynchronous input
O_ APPEND	Append on each write
O_CREAT	Create a file if it does not exist
O_TRUNC	Create a new version of this file
O_EXCL	Error if attempting to create existing file

These flags are set using the bitwise OR operator (|) to separate specified flags. Opening a file with O_APPEND causes each write on the file to be appended to the end. If O_TRUNC is specified and the file exists, open creates a new file by incrementing the version number by 1, leaving the old version in existence.

If O_CREAT is set and the named file does not exist, the DEC C RTL creates it with any attributes specified in the fourth and subsequent arguments ( . . . ). If O_ EXCL is set with O_CREAT and the named file exists, the attempted open returns an error.

mode

An unsigned value that specifies the file-protection mode. The compiler performs a bitwise AND operation on the mode and the complement of the current protection mode.

You can construct modes by using the bitwise OR operator (|) to separate specified modes. The modes are:

0400	OWNER:READ
0200	OWNER:WRITE
0100	OWNER:EXECUTE
0040	GROUP:READ
0020	GROUP:WRITE
0010	GROUP:EXECUTE
0004	WORLD:READ
0002	WORLD:WRITE
0001	WORLD:EXECUTE

The system is given the same access privileges as the owner. A WRITE privilege also implies a DELETE privilege.

. . .

Optional file attribute arguments. The file attribute arguments are the same as those used in the creat function. For more information, see the creat function.

Description

Notes

• If you intend to do random writing to a file, the file must be opened for update by specifying a flags value of O_RDWR.

• To create files with OpenVMS RMS default protections by using the UNIX system-call functions umask, mkdir, creat, and open, call mkdir, creat, and open with a file-protection mode argument of 0777 in a program that never specifically calls umask. These default protections include correctly establishing protections based on ACLs, previous versions of files, and so on. In programs that do vfork/exec calls, the new process image inherits whether umask has ever been called or not from the calling process image. The umask setting and whether the umask function has ever been called are both inherited attributes.

See also creat, read, write, close, dup, dup2, and lseek in this section.

Return Values

x	A nonnegative file descriptor number.
-1	Indicates that the file does not exist, that it is protected against reading or writing, or that it cannot be opened for another reason.

Example

#include <unixio.h>
#include <file.h>

main()
{
int file,stat;
int flags;

 flags = O_RDWR ;    /*  Open for read and write,
                      *  with user default file protection,
                      *  with a maximum fixed record size of 2048 bytes,
                      *  and a block size 2048 bytes.
                      */
 file = open("file.dat",flags,0,"rfm=fix","mrs=2048","bls=2048");
 if (file == -1)
  perror ("OPEN error"), exit(1);

 close (file);
}