why do we test fgets for return of null

[Kinasz]

This may sound silly, but why do we check for NULL when calling fgets.

if ( fgets( string_in, BUFSIZ, stdin ) == NULL ) {
//what goes here
}

I couldnt find an answer in any of my text books or on google.

Also if more characters than BUFSIZ allows is entered into the console, do they remain in the buffer until next time we read from it?

Thanks in advance.

[Sebastiani]

well, fgets probably won't return NULL on stdin - only with files when the EOF condition has been met. the usual file reading loop would be:

while(fgets(buff, max, fp)) {
 // process a line
 }

for stdin, you might just test that the first character in the buffer is not a newline (meaning the user entered nothing - just depressed 'ENTER'), ie:

 while(*fgets(buff, max, stdin) != '\n') {
  // process
 }

[anonytmouse]

You can input EOF to stdin, in which case fgets() will return NULL when there is nothing left to return.

In Windows/DOS the key combination to input EOF is Ctrl+Z.

Someone will be along shortly with the nix equivalent.

[Thantos]

nix equivalent is ctrl d

Also if more characters than BUFSIZ allows is entered into the console, do they remain in the buffer until next time we read from it?

yes

[Micko]

I think you should read:

http://cboard.cprogramming.com/showt...ghlight=string

[Kinasz]

Thanks for all your feedback guys, I will just check if I have it straight...

We should always check return values from interactive input, so with fgets we are checking that the return value does not point to a '\n'?

NB: Thanks also Thantos

[Dave_Sinkula]

>so with fgets we are checking that the return value does not point to a '\n'?

No, you are checking the return value of fgets to see whether the call was successful; that is, whether characters have been read into the array without error.

[Emmanuel Delaha]

This may sound silly, but why do we check for NULL when calling fgets.

if ( fgets( string_in, BUFSIZ, stdin ) == NULL ) {
//what goes here
}

I couldnt find an answer in any of my text books or on google.

What does your C-book say about the value returned by fgets()? Note that stdin is redirectable from any stream including a file. Note also that on most systems, you can simulate an EOF on stdin via a peculiar combinaion of keys (ctrl-Z, ctrl-D etc.)

Also if more characters than BUFSIZ allows is entered into the console, do they remain in the buffer until next time we read from it?

Assuming the size of the array of char pointed by string_in is >= BUFSIZ, yes, the unread characters are stored somewhere and ready do be read. You are aware of this situation because there is no final '\n' on the currently read string.