beginners guide away from scanf

read.c

/* https://web.archive.org/web/20250417094758/https://sekrit.de/webdocs/c/beginners-guide-away-from-scanf.html */

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

int main(void)
{
    long a;
    char buf[1024]; // use 1KiB just to be sure
    int success; // flag for successful conversion

    do
    {
        printf("enter a number: ");
        if (!fgets(buf, 1024, stdin))
        {
            // reading input failed:
            return 1;
        }

        // have some input, convert it to integer:
        char *endptr;

        errno = 0; // reset error number
        a = strtol(buf, &endptr, 10);
        if (errno == ERANGE)
        {
            printf("Sorry, this number is too small or too large.\n");
            success = 0;
        }
        else if (endptr == buf)
        {
            // no character was read
            success = 0;
        }
        else if (*endptr && *endptr != '\n')
        {
            // *endptr is neither end of string nor newline,
            // so we didn't convert the *whole* input
            success = 0;
        }
        else
        {
            success = 1;
        }
    } while (!success); // repeat until we got a valid number

    printf("You entered %ld.\n", a);
}