fopen

Opens a file by returning the address of a FILE structure.

Format

#include  <stdio.h>

FILE *fopen  (const char *file_spec, const char
             *a_mode); 
             (ANSI C)

FILE *fopen  (const char *file_spec, const char
             *a_mode, . . . ); (DEC C Extension)

Arguments

file_spec

A character string containing a valid file specification.

a_ mode

The access mode indicator. Use one of the following character strings: "r", "w", "a", "r+", "w+", "rb", "r+b", "rb+", "wb", "w+b", "wb+", "ab", "a+b", "ab+", or "a+".

These access modes have the following effects:

• "r" opens an existing file for reading.

• "w" creates a new file, if necessary, and opens the file for writing. If the file exists, it creates a new file with the same name and a higher version number.

• "a" opens the file for append access. An existing file is positioned at the end-of-file, and data is written there. If the file does not exist, the DEC C RTL creates it.

The update access modes allow a file to be opened for both reading and writing. When used with existing files, "r+" and "a+" differ only in the initial positioning within the file. The modes are as follows:

• "r+" opens an existing file for read update access. It is opened for reading, positioned first at the beginning-of-file, but writing is also allowed.

• "w+" opens a new file for write update access.

• "a+" opens a file for append update access. The file is first positioned at the end-of-file (writing). If the file does not exist, the DEC C RTL creates it.

• "b" means binary access mode. In this case, no conversion of carriage-control information is attempted.

. . .

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

If you specify a directory in the file name and it is a search list that contains an error, DEC C for OpenVMS Systems interprets it as a file open error.

The file control block can be freed with the fclose function, or by default on normal program termination.

Return Values

x	File pointer.
NULL	Indicates an error. The constant NULL is defined in the <stdio.h> header file to be the NULL pointer value. The function returns NULL to signal the following errors: File protection violations Attempts to open a nonexistent file for read access Failure to open the specified file

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