The freopen Functions in C & C++

The freopen Function.

The freopen function in the C Standard I/O library <cstdio> is a useful to redirect output destined for the stdout to a text file. I discussed how to use the fprintf function to send output to either a text file or the stdout. See here. The same result is achieved here using the printf and freopen functions.

Method 1.

freopen(Filename, "w", stdout);

There is no return value to indicate if the file was opened or if there was an error.

Method 2.

iReturn = freopen(Filename, "w", stdout);

On failure, a null pointer is returned otherwise the return value is a pointer to the file. Beyond testing for an error, this return value is used to close the file.

fclose(iReturn);

While my example uses the stdout, the freopen function can be used with the stdin and stderr objects.

 

Test Code.

I tested the functions in Visual C++ 2010 as an console application.

// The standard library includes the system function.

#include <cstdlib>

// C++ standard I/O library.

#include <cstdio>

int main()

{

     //**************************************************************

     // Redirect stdout to a file.

     // Declare pointer to the file.

     FILE* pFileHandle;

     char Filename[] = "MyFile.txt";

     // Returns null is the open failed.

     pFileHandle = freopen(Filename, "w", stdout);

     printf("pFileHandle is %X.\n\n", pFileHandle);

     //**************************************************************

     // Header.

     printf("Using freopen in C & C++\n\n");

     //**************************************************************

     // Some use of printf.

     printf("Sending the printf output to a file instead of the stdout.\n");

     printf("The freopen function allows for this redirection.\n\n");

     // Use variable for a string.

     char * Text = "A text string only from a variable.\n\n";

     printf("%s", Text);

     // Int type.

     int Salary = 375000;

     int Bonus = -35000;

     printf("Salary is: %8d.\n", Salary);

     printf("Bonus  is: %8d.\n", Bonus);

     printf("\n");

     //**************************************************************

     // Close the file.

     fclose(pFileHandle);

     // Keep console window open.

     system("pause");

     // Return some value.

     return 0;

} // end main

 Output.