#include <stdio.h>
char *print_msg(int a, int b, int c)
{
    int ns = 0;
    if (a==b) ns++;
    if (b==c) ns++;
    if (c==a) ns++;

    switch(ns)
    {
        case 3: return "正三角形";
        case 1: return "二等辺三角形";
        case 0: return "不等辺形";
        default:
        printf("エラー！ ns=%d\n", ns);
    }
    return "";
}

void main(int argc, char **argv)
{
    //int a = atoi(argv[1]);
    //int b = atoi(argv[2]);
    //int c = atoi(argv[3]);
    int a,b,c;
    a=-1;b=-1;c=-1;


    printf("%sです\n", print_msg(a,b,c));
}

正三角形です