C library function - freopen()

Description

The C library function FILE *freopen(const char *filename, const char *mode, FILE *stream) associates a new filename with the given open stream and at the same time closes the old file in the stream.

Declaration

Following is the declaration for freopen() function.

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

Parameters

  • filename − This is the C string containing the name of the file to be opened.

  • mode − This is the C string containing a file access mode. It includes −

Sr.No. Mode & Description
1

"r"

Opens a file for reading. The file must exist.
2

"w"

Creates an empty file for writing. If a file with the same name already exists then its content is erased and the file is considered as a new empty file.
3

"a"

Appends to a file. Writing operations appends data at the end of the file. The file is created if it does not exist.
4

"r+"

Opens a file to update both reading and writing. The file must exist.
5

"w+"

Creates an empty file for both reading and writing.
6

"a+"

Opens a file for reading and appending.

  • stream − This is the pointer to a FILE object that identifies the stream to be re-opened.

Return Value

If the file was re-opened successfully, the function returns a pointer to an object identifying the stream or else, null pointer is returned.

Example

The following example shows the usage of freopen() function.

Live Demo 
#include <stdio.h>

int main () {
   FILE *fp;

   printf("This text is redirected to stdout\n");

   fp = freopen("file.txt", "w+", stdout);

   printf("This text is redirected to file.txt\n");

   fclose(fp);
   
   return(0);
}

Let us compile and run the above program that will send the following line at STDOUT because initially we did not open stdout −

This text is redirected to stdout

After a call to freopen(), it associates STDOUT to file file.txt, so whatever we write at STDOUT that goes inside file.txt. So, the file file.txt will have the following content.

This text is redirected to file.txt

Now let's see the content of the above file using the following program −

#include <stdio.h>

int main () {
   FILE *fp;
   int c;

   fp = fopen("file.txt","r");
   while(1) {
      c = fgetc(fp);
      if( feof(fp) ) {
         break ;
      }
      printf("%c", c);
   }
   fclose(fp);
   return(0);
}
stdio_h.htm
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