Using the freopen function you can read/write from a file, like you are reading from the standard input/output. freeopen first tries to close any file already associated with the stream given as third parameter and disassociates it. Then, whether that stream was successfuly closed or not, freopen opens the file whose name is passed in the first parameter, filename, and associates it with the specified stream just as fopen would do using the mode value specified as the second parameter.
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.
The prototype of freopen is:
FILE *freopen(const char *filename, const char *mode, FILE *stream);
Example:
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;
}
