fopen, _wfopen

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Header File

stdio.h

Category

Input/output Routines

Prototype

FILE *fopen(const char *filename, const char *mode);
FILE *_wfopen(const wchar_t *filename, const wchar_t *mode);

Description

Opens a stream.

fopen opens the file named by filename and associates a stream with it. fopen returns a pointer to be used to identify the stream in subsequent operations.

The mode string used in calls to fopen is one of the following values:

Value	Description
r	Open for reading only.
w	Create for writing. If a file by that name already exists, it will be overwritten.
a	Append; open for writing at end-of-file or create for writing if the file does not exist.
r+	Open an existing file for update (reading and writing).
w+	Create a new file for update (reading and writing). If a file by that name already exists, it will be overwritten.
a+	Open for append; open (or create if the file does not exist) for update at the end of the file.

To specify that a given file is being opened or created in text mode append a t to the mode string (rt, w+t, and so on). Similarly to specify binary mode append a b to the mode string (wb, a+b, and so on). fopen also allows the t or b to be inserted between the letter and the + character in the mode string; for example rt+ is equivalent to r+t.

If a t or b is not given in the mode string the mode is governed by the global variable _fmode. If _fmode is set to O_BINARY files are opened in binary mode. If _fmode is set to O_TEXT they are opened in text mode. These O_... constants are defined in fcntl.h.

When a file is opened for update, both input and output can be done on the resulting stream. However,

• output cannot be directly followed by input without an intervening fseek or rewind.
• input cannot be directly followed by output without an intervening fseek, rewind, or an input that encounters end-of-file.

Return Value

On successful completion fopen returns a pointer to the newly opened stream. In the event of error it returns NULL.

Example

/* Program to create backup of a file */

#include <stdio.h>

int main(void)
{
  FILE *in, *out;

  if ((in = fopen("TESTFILE.DAT", "rt"))
      == NULL)
  {
     fprintf(stderr, "Cannot open input file.\n");
     return 1;
  }

  if ((out = fopen("TESTFILE.BAK", "wt"))
      == NULL)
  {
     fprintf(stderr, "Cannot open output file.\n");
     return 1;
  }

  while (!feof(in))
     fputc(fgetc(in), out);

  fclose(in);
  fclose(out);

  return 0;
}

Portability

	POSIX	Win32	ANSI C	ANSI C++
fopen	+	+	+	+
_wfopen		NT only