6.18.23 freopen Function

Reassigns an existing stream to a new file.

Include

<stdio.h>

Prototype

FILE *freopen(const char * restrict filename, const char * restrict mode, FILE * restrict stream);

Arguments

filename

name of the new file

mode

type of access permitted

stream

pointer to the currently open stream

Return Value

Returns a pointer to the new open file. If the function fails a null pointer is returned.

Remarks

The function closes the file associated with the stream as though fclose was called. Then it opens the new file as though fopen was called. freopen will fail if the specified stream is not open. See fopen for the possible types of file access.

This function requires a heap.

Example

#include <stdio.h>

int main(void)
{
  FILE *myfile1, *myfile2;
  int y;

  if ((myfile1 = fopen("afile1", "w+")) == NULL)
    printf("Cannot open afile1\n");
  else
  {
    printf("afile1 was opened\n");

    if ((myfile2 = freopen("afile2", "w+",
         myfile1)) == NULL)
    {
      printf("Cannot open afile2\n");
      fclose(myfile1);
    }
    else
    {
      printf("afile2 was opened\n");
      fclose(myfile2);
    }
  }
}

Example Output

afile1 was opened
afile2 was opened

Example Explanation

This program uses myfile2 to point to the stream when freopen is called, so if an error occurs, myfile1 will still point to the stream and can be closed properly. If the freopen call is successful, myfile2 can be used to close the stream properly.