13.2. freopen()

Reopen an existing FILE*, associating it with a new path

Prototypes

#include <stdio.h>

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

Description

Let's say you have an existing FILE* stream that's already open, but you want it to suddenly use a different file than the one it's using. You can use freopen() to "re-open" the stream with a new file.

Why on Earth would you ever want to do that? Well, the most common reason would be if you had a program that normally would read from stdin, but instead you wanted it to read from a file. Instead of changing all your scanf()s to fscanf()s, you could simply reopen stdin on the file you wanted to read from.

Another usage that is allowed on some systems is that you can pass NULL for filename, and specify a new mode for stream. So you could change a file from "r+" (read and write) to just "r" (read), for instance. It's implementation dependent which modes can be changed.

When you call freopen(), the old stream is closed. Otherwise, the function behaves just like the standard fopen().

Return Value

freopen() returns stream if all goes well.

If something goes wrong (e.g. you tried to open a file for read that didn't exist), fopen() will return NULL.

Example

#include <stdio.h>

int main(void)
{
    int i, i2;

    scanf("%d", &i); // read i from stdin

    // now change stdin to refer to a file instead of the keyboard
    freopen("someints.txt", "r", stdin);

    scanf("%d", &i2); // now this reads from the file "someints.txt"

    printf("Hello, world!\n"); // print to the screen

    // change stdout to go to a file instead of the terminal:
    freopen("output.txt", "w", stdout);

    printf("This goes to the file \"output.txt\"\n");

    // this is allowed on some systems--you can change the mode of a file:
    freopen(NULL, "wb", stdout); // change to "wb" instead of "w"

    return 0;
}

See Also

fclose()
fopen()