fgets that would skip when called the second time

Question

I think the problem of my code is with my fgets but I do not know how to fix it. So, when I call the function for the first time, everything works, but if for the second time, the function skips everything after the printf.

So the output would be:

Output:
Please enter x and y coordinates separated by a comma for the piece you wish to enter: 3,4
x: 3, y:4
Please enter x and y coordinates separated by a comma for the piece you wish to enter: x:
, y:

This is the code:

void functioncall()
{
    char coord[4];
    printf("Please enter x and y coordinates separated by a comma for the piece you wish to place: ");
    fgets(coord, 4, stdin);

    char *xcoord = strtok(coord, ",");
    char *ycoord = strtok(NULL, " ");
    if (!ycoord)
    {
        ycoord = "";
    }

    printf("x: %s, y: %s\n", xcoord, ycoord);
}

I cannot give an input when it is called for the second time.

Answer 1

The reason for the behaviour you see is the size of the coord array. When you specify the size as 4, it means you can get 1 digit, 1 comma, 1 digit, and 1 null byte stored — which doesn't leave room for the newline, so the second call to the function reads the newline (only), which doesn't parse well.

You should allow for much more space — users are endlessly inventive in what they type (leading blanks, trailing blanks, intermediate blanks, signs, leading zeros, etc.). I tend to use 4096 for single lines of input — partly for the shock value, and also because if someone's willing to write a 4-page essay on a single line, they deserve what they get.

This code works for me:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

static void functioncall(void)
{
    char coord[4096];
    printf("Please enter x and y coordinates separated by a comma for the piece you wish to place: ");
    if (fgets(coord, sizeof(coord), stdin) == 0)
    {
        fprintf(stderr, "Got EOF in %s\n", __func__);
        exit(EXIT_FAILURE);
    }
    const char delims[] = ", \n\t";

    char *xcoord = strtok(coord, delims);
    char *ycoord = strtok(NULL, delims);
    if (!ycoord)
    {
        ycoord = "";
    }

    printf("x: [%s] y: [%s]\n", xcoord, ycoord);
}

int main(void)
{
    functioncall();
    functioncall();
    return 0;
}

Example run (program name cd19, source file cd19.c):

$ ./cd19
Please enter x and y coordinates separated by a comma for the piece you wish to place: 234 , 495
x: [234] y: [495]
Please enter x and y coordinates separated by a comma for the piece you wish to place: 1,2
x: [1] y: [2]
$

The choice of delimiter characters ensures that the 234 , 495 example works OK (the tab is optional, but not necessarily a bad idea). It does, however, mean that the user is not obliged to enter a comma.