A parse_float function with test

parse_float_with_test.c

enum parseFloatErrorCodes {
    SUCCESS, DOUBLE_DOT,PARSE_ERROR, INTEGER_OVERFLOW,
    TOO_MANY_DIGITS_AFTER_DECIMAL_POINT
};
int parse_float(char* inp, float *x)
{ 
    char *p;
    int ival, dot;
    for (ival = 0, dot = 0, p = inp; *p; ++p) {
        int overflow_check = ival;
        if (('0' <= *p) && (*p <= '9')) {
            ival = ival * 10 + (int)(*p - '0');
            if (dot > 0) ++dot;
        } else if (*p == '.') {
            if (dot > 0) { return DOUBLE_DOT; }
            dot = 1;
        } else { return PARSE_ERROR; }
        if (overflow_check > ival) { return INTEGER_OVERFLOW; }
    }
    *x = ival;
    if (dot == 0) *x *= 100.0;
    else if (dot == 1) *x *= 100.0;
    else if (dot == 2) *x *= 10.0;
    else if (dot > 3) { return TOO_MANY_DIGITS_AFTER_DECIMAL_POINT; }
    *x /= 100.0;
    return SUCCESS;
}

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

void print_error(const char* buf, int code)
{
	const char* str[] = {
		"SUCCESS", "DOUBLE_DOT",
		"PARSE_ERROR", "INTEGER_OVERFLOW", 
		"TOO_MANY_DIGITS_AFTER_DECIMAL_POINT"
	};
	if((SUCCESS <= code) && (code <= TOO_MANY_DIGITS_AFTER_DECIMAL_POINT)) {
		printf("parse of \"%s\" failed with %s\n", buf, str[code]);
	} else {
		printf("unkown error code %d\n", code);
	}
}

int main(int argc, char** argv)
{
	const char *inp[] = {
		"123",
		"934.23",
		"579346.24",
		"1.5",
		".2",
		"3205986024967406709846.35",
		"4535.23.13",
		"SOMETHING ELSE",
		"38575.23Numbers",
		"123,54",
		"0.123",
		""
	};
	const char** I = inp;
	char buf[40];
	int len;
	for (; strlen(*I) > 0; ++I) {
		float x = .0f;
		int errcode = SUCCESS;
		len=strlen(*I);
		if(len > 39) { printf("buffer overflow.\n"); return -1; }
		memcpy(buf, *I, len);
		buf[len] = '\0';
		errcode = parse_float(buf, &x);
		if (errcode == SUCCESS) {
			printf("read float %5.2f\n", x);
		} else {
			print_error(buf, errcode);
		}
	}
	return 0;
}

gcc -g -o parse_float parse_float.c
./parse_float
read float 123.00
read float 934.23
read float 579346.25
read float  1.50
read float  0.20
parse of "3205986024967406709846.35" failed with INTEGER_OVERFLOW
parse of "0.123" failed with TOO_MANY_DIGITS_AFTER_DECIMAL_POINT

More error codes triggered:

 ./parse_float
read float 123.00
read float 934.23
read float 579346.25
read float  1.50
read float  0.20
parse of "3205986024967406709846.35" failed with INTEGER_OVERFLOW
parse of "4535.23.13" failed with DOUBLE_DOT
parse of "SOMETHING ELSE" failed with PARSE_ERROR
parse of "38575.23Numbers" failed with PARSE_ERROR
parse of "123,54" failed with PARSE_ERROR
parse of "0.123" failed with TOO_MANY_DIGITS_AFTER_DECIMAL_POINT