Read an Integer from the User, Part 1

by Dave Sinkula on May 20th, 2005

Obtaining user input can be done in many surprisingly different ways. This code is somewhere in the middle: safer than scanf("%d", &n), but not bulletproof. It is meant to be a simple but relatively safe demonstration.

The function mygeti reads user input from the stdin into a string using fgets. Then it attempts to convert this string to an integer using sscanf. If both functions succeed, a 1 is returned; if either fails, a 0 is returned.

Leading whitespace, and other trailing characters are some things not handled. Those issues are handled in Read an Integer from the User, Part 2. Reading a Floating-Point Value from the User can be done similarly.

C Code Snippet (Toggle Plain Text)

#include <stdio.h>
 
int mygeti(int *result)
{
        char buff [ 13 ]; /* signed 32-bit value, extra room for '\n' and '\0' */
        return fgets(buff, sizeof buff, stdin) && sscanf(buff, "%d", result) == 1;
}
 
int main(void)
{
        int value;
        do {
                fputs("Enter an integer: ", stdout);
                fflush(stdout);
        } while ( !mygeti(&value) );
        printf("value = %d\n", value);
        return 0;
}
 
/* my output
Enter an integer: one
Enter an integer:
Enter an integer: 42
value = 42
 
Enter an integer: 24f
value = 24
 
Enter an integer:    125 help
value = 125
*/

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