C89: Read number from stdin and reformat

readnum.c

/*
 *  Compile:
 *  gcc -Wall -Werror -pedantic -ansi -std=c89 readnum.c
 */

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

#define MAX_INPUT_LEN 1000
#define PRECISION 2

int is_digit(char c) {
    return c >= '0' && c <= '9';
}

int string_is_float(char s[]) {
    unsigned int i;

    if (!s[0]) /* Length */
        return (0);
    for (i = 0; s[i] && (s[i] == '-' || s[i] == '+'); ++i) ;
    for (; s[i] && is_digit(s[i]); ++i) ;
    if (!s[i])
        return (i);
    if (s[i++] != '.')
        return (0);
    for (; s[i] && is_digit(s[i]); ++i) ;
    return !s[i] ? i : 0;
}

char *format_float_input(char s[], unsigned int precision)
{
    unsigned int i;
    unsigned int j;
    int sign;
    int len;
    char *fmtd;
    char *res;

    if (!(len = string_is_float(s)))
        return (NULL);
    res = fmtd = malloc(sizeof(char) * len);
    sign = 1;
    for (i = 0; i < len && (s[i] == '-' || s[i] == '+'); ++i)
        sign *= s[i] == '-' ? -1 : 1;
    if (sign == -1)
        *fmtd++ = '-';
    for (; i < len && is_digit(s[i]); ++i)
        *fmtd++ = s[i];
    if (i++ == 0)
        *fmtd++ = '0';
    *fmtd++ = '.';
    for (j = 0; j < 2; ++i, ++j)
        *fmtd++ = (i < len ? s[i] : '0');
    *fmtd = '\0';
    return res;
}

int main() {
    char input[MAX_INPUT_LEN];
    char *fmtd;

    printf("Input a number (\"q\" to quit):\n");
    while (1) 
    {
        printf("> ");
        fgets(input, MAX_INPUT_LEN, stdin);
        input[strcspn(input, "\n")] = '\0';
        if (!*input || !strcmp(input, "q"))
            break;
        fmtd = format_float_input(input, PRECISION);
        printf("%s\n", fmtd ? fmtd : "Invalid number!");
        free(fmtd);
    }
    return fmtd ? 0 : 1;
}