fopen - Open Files

Syntax

#include <stdio.h>
FILE *fopen(const char *filename, const char *mode);

Description

fopen opens the file specified by filename. mode is a character string specifying the type of access requested for the file. The mode variable contains one positional parameter followed by optional keyword parameters.

The possible values for the positional parameters are:

Mode

Description
r
Open a text file for reading. The file must exist.
w
Create a text file for writing. If the given file exists, its contents are destroyed.
a
Open a text file in append mode for writing at the end of the file. fopen creates the file if it does not exist.
r+
Open a text file for both reading and writing. The file must exist.
w+
Create a text file for both reading and writing. If the given file exists, its contents are destroyed.
a+
Open a text file in append mode for reading or updating at the end of the file. fopen creates the file if it does not exist.
rb
Open a binary file for reading. The file must exist.
wb
Create an empty binary file for writing. If the file exists, its contents are destroyed.
ab
Open a binary file in append mode for writing at the end of the file. fopen creates the file if it does not exist.
r+b or rb+
Open a binary file for both reading and writing. The file must exist.
w+b or wb+
Create an empty binary file for both reading and writing. If the file exists, its contents will be destroyed.
a+b or ab+
Open a binary file in append mode for writing at the end of the file. fopen creates the file if it does not exist.

Warning: Use the w, w+, wb, w+b, and wb+ parameters with care; data in existing files of the same name will be lost.

Text files contain printable characters and control characters organized into lines. Each line ends with a new-line character, except for the last line, which does not require one. The system may insert or convert control characters in an output text stream.

Note: Data output to a text stream may not compare as equal to the same data on input.

Binary files contain a series of characters. For binary files, the system does not translate control characters on input or output.

When you open a file with a, a+, ab, a+b or ab+ mode, all write operations take place at the end of the file. Although you can reposition the file pointer using fseek or rewind, the write functions move the file pointer back to the end of the file before they carry out any operation. This action prevents you from overwriting existing data.

When you specify the update mode (using + in the second or third position), you can both read from and write to the file. However, when switching between reading and writing, you must include an intervening positioning function such as fseek, fsetpos, rewind, or fflush. Output may immediately follow input if the end-of-file was detected.

The keyword parameters are:

blksize=value

Specifies the maximum length, in bytes, of a physical block of records. For fixed-length records, the maximum size is 32760 bytes. For variable-length records, the maximum is 32756. The default buffer size is 4096 bytes.
lrecl=value
Specifies the length, in bytes, for fixed-length records and the maximum length for variable-length records. For fixed-length records, the maximum length is 32760 bytes. For variable-length records, the maximum is 32756. If the value of LRECL is larger than the value of BLKSIZE, the LRECL value is ignored.
recfm=value
value can be:

F

fixed-length, unblocked records
V
variable-length, unblocked records The default for The Developer's Toolkit compiler is fixed-length record format.
type=value
value can be:

memory This parameter identifies this file as a memory file that is accessible only from C programs. This is the default.

The Developer's Toolkit compiler does not support record I/O.

fopen generally fails if parameters are mismatched.

The file attributes can be altered only if the open mode specified with the fopen function is one of the write modes (w, w+, wb, or wb+). The system deletes the existing file and creates a new file with the attributes specified in fopen.

Returns

fopen returns a pointer to a file structure that can be used to access the open file. A NULL pointer return value indicates an error.

Example Code

This example attempts to open a file for reading.

#include <stdio.h>
int main(void)
{
   FILE *stream;

   if (NULL == (stream = fopen("myfile.dat", "r")))
      printf("Could not open data file\n");
   else
      fclose(stream);

    /* The following call opens a fixed record length file                    */
    /* for reading and writing in record mode.                                */

   if (NULL == (stream = fopen("myfile.dat",
                        "rb+, lrecl=80, blksize=240, recfm=f, type=record")))
      printf("Could not open data file\n");
   else
      fclose(stream);
   return 0;

   /****************************************************************************
      The output should be:

      Could not open data file
   ****************************************************************************/
}

Related Information

• creat
  
• fclose
  
• fflush
  
• fread
  
• freopen
  
• open
  
• sopen
  
• fseek
  
• fsetpos
  
• fwrite
  
• rewind

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