Thread: Life without fflush(stdin)

Originally Posted by vart

one way to solve it - to use fgets for all your inputs

I am using fgets, but for integer I have to use scanf.

Have you considered just designing an protocol for your program, then just go through and read the data you need? It's far more straightforward than writing interactive programs that try to synchronise stdin and stdout. The plus side of this is you can use stdout for useful information instead of annoying prompts, and your protocol will work not only with terminals, but files and pipes too.

vart and grumpy have the right of it. I actually prefer strtol/strtod instead of atoi for parsing out long or double data from a string. They're like atoi, but they have more features and better error handling.

It should be noted that it might still be necessary to flush the input when using "fgets()" with stdin.

Consider these two examples:

Code:

/* example 1 - User input less than array size - newline gets stored */
 
char userString[10];
 
fgets(userString,10,stdin);
 
/* user enters "Hello\n" */
 
userString[0] = 'H'
userString[1] = 'e'
userString[2] = 'l'
userString[3] = 'l'
userString[4] = 'o'
userString[5] = '\n'
userString[6] = '\0'

/* it's usually desired to have additional code that removes the newline from the array */

Code:

/* example 2 - User input greater than array size */
/* Protected from overflow, but newline does not get stored */
 
char userString[5];
 
fgets(userString,5,stdin);
 
/* user enters "Goodbye\n" */
 
userString[0] = 'G'
userString[1] = 'o'
userString[2] = 'o'
userString[3] = 'd'
userString[4] = '\0'
 
/* no newline in array - this means 'b', 'y', 'e', and '\n' are still */
/* waiting on the buffer and need to be removed */

I usually approach user input like this:

- Read from stdin with "fgets()"
- If there's a newline in the string, replace it with a null character
- Else, flush the input buffer (such as with the example provided in the link by vart in post #2)
- Process/validate data
- etc...

It's obviously best to have this sort of code in its own function. As an exercise, you can create separate functions for reading strings, reading integers, etc, using the advice given by grumpy and anduril462.

No, you don't have to use it after every input; in fact it might be easier to design input routines that don't skip input. If you can read a big enough string something like flushing is not a problem. You can even write up something that dynamically allocates a string big enough.

using fflush(stdin) is not wrong...
it actually clears the input stream of any unintended characters, like when you use a loop to take a string input character by character, first time you press a key and that is saved into the input stream and thereby at the variable, next when you press enter '\n' is inputted into the input stream thus automatically saving it into the variable location...thereby actually skipping to ask for user input.
IDE does not matter