Quick demo using strtod.

sdtest.c

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

/*
    Example Usage:

        char * str = getString();
        double val;
        if (tryParseDouble(str, &val)) {
            doSomethingWith(val);
        }

*/
int tryParseDouble(char * str, double * rv)
{
    if (str == NULL || rv == NULL) {
        return 0;
    }

    char * end = NULL;
    double d = strtod(str, &end);

    if (end == str) {
        // No characters consumed.
        return 0;
    } 
    else {
        *rv = d;
        return 1;
    }
}

int main(int argc, char const *argv[])
{
    char * strs[] = {
        "-100.7",
        "+10e-4",
        "+",
        "notadouble",
    };

    double d;
    int i;
    for (i = 0; i < 4; ++i) {
        if (tryParseDouble(strs[i], &d)) {
            printf("%d: %s -> %f\n", i, strs[i], d);
        }
        else {
            printf("%d: %s is not a double.\n", i, strs[i]);
        }
    }

    return 0;
}

// $ make sdtest
// $ ./sdtest 
// 0: -100.7 -> -100.700000
// 1: +10e-4 -> 0.001000
// 2: + is not a double.
// 3: notadouble is not a double.