fgets(); how does this work inside this part of code? C programming

Question

I wrote a Tic Tac Toe game, using tutorials. Now I'm just going trough the code to see what i don't understand and I came up with this part, witch is confusing me.

 char userInput[4];

int moveOk = 0;
int move = -1;

while(moveOk == 0)
{
    printf("Enter a number from 1 - 9: ");
    fgets(userInput, 3, stdin);
    fflush(stdin);
    printf("\n\n");
*The code continues, but the rest of it is not important*

How does this part work? I don't even know how to formulate the question. Sorry. So what are the three values in fgets(); and how to they interact with each other?

fgets(userInput, 3, stdin);
    fflush(stdin);

You should remove `fflush(stdin)` since it's undefined behavior, `fflush()` is for output streams. — Iharob Al Asimi, Jan 07 '15 at 23:46
Have you read documentation for `fgets`? As for `fflush(stdin)`, that's wrong; it's undefined behavior. — jamesdlin, Jan 07 '15 at 23:46
Undefined Behavior doesn't mean there would necessarily be wrong behavior. — Iharob Al Asimi, Jan 07 '15 at 23:48
Try taking out `fflush(stdin)`. If your program breaks, there is a problem in the code somewhere else. — yellowantphil, Jan 07 '15 at 23:51
See [this](http://stackoverflow.com/a/2979217/1983495) answer for more information. — Iharob Al Asimi, Jan 07 '15 at 23:51
@iharob Undefined behavior means anything could happen. and it could be different tommorrow than it was today. — Jasen, Jan 08 '15 at 00:10
Note: A better use of `char userInput[4]; ... fgets(userInput, 3, stdin);` --> `char userInput[4]; ... if (fgets(userInput, sizeof userInput, stdin) == NULL) Handle_EOForIOerror(); else GoodToGo();` — chux - Reinstate Monica, Jan 08 '15 at 00:16

Iharob Al Asimi · Answer 1 · 2015-01-08T00:08:16.293

From fgets manual

char *fgets(char *s, int size, FILE *stream);

fgets() reads in at most one less than size characters from stream and stores them into the buffer pointed to by s. Reading stops after an EOF or a newline. If a newline is read, it is stored into the buffer. A terminating null byte ('\0') is stored after the last character in the buffer.

so the fgets(), is taking at most 2 characters from the input stream, or until return key is pressed (sending a '\n' character) or EOF is sent, and stores the result in userInput.

You can then try to convert the two character string to a number using strtol.

fgets(); how does this work inside this part of code? C programming

1 Answers1