box-o-sand/gnu-c/temperatures.c

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


double c2f(double celsius) {
    return ((9.0 / 5.0) * celsius) + 32.0;
}


double f2c(double fahrenheit) {
    return (5.0 / 9.0) * (fahrenheit - 32.0);
}


void die_usage(const char *prog, const char *msg) {
    fprintf(stderr, "Usage: %s <temp>[FC]\n", prog);
    if (msg != NULL) {
        fprintf(stderr, "%s\n", msg);
    }
    exit(EXIT_FAILURE);
}


int main(int argc, char * argv[]) {
    const char * prog = basename(argv[0]);
    int ret;
    char msg[255];
    double temp;
    char scale = '\0';
    const char *in_fmt = "%lf%c";
    const char *out_fmt = "%.2lf%c\n";
    const char *temp_string;

    if (argc < 2) {
        die_usage(prog, NULL);
    }

    if (((int)strlen(argv[1]) == 1) && (argv[1][0] == '-')) {
        ret = fscanf(stdin, in_fmt, &temp, &scale);
    } else {
        temp_string = argv[1];
        ret = sscanf(temp_string, in_fmt, &temp, &scale);
    }

    if ((ret == 0) || (ret == EOF)) {
        die_usage(prog, "No temperature provided!");
    }

    if (ret == 1) {
        die_usage(prog, "No scale provided!");
    }

    switch (scale) {
        case 'C':
            printf(out_fmt, c2f(temp), 'F');
            break;
        case 'F':
            printf(out_fmt, f2c(temp), 'C');
            break;
        default:
            sprintf(msg, "Unknown scale: '%c'", scale);
            die_usage(prog, msg);
    }

    exit(EXIT_SUCCESS);
}