test.c

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

/* counts case-insensitive entries for ch */
static int numchars(const char *s, int ch)
{
    int r, cv;
    const char *sv;

    r = 0;

    cv = tolower(ch);
    sv = s;
    while(*sv && (sv = strchr(sv, cv))) {
        r++;
        sv++;
    }

    cv = toupper(ch);
    sv = s;
    while(*sv && (sv = strchr(sv, cv))) {
        r++;
        sv++;
    }

    return r;
}

int main(void)
{
    char line[512] = { 0 };
    int na, nb, nc;

    na = 0;
    nb = 0;
    nc = 0;

    while(fgets(line, sizeof(line), stdin)) {
        na += numchars(line, 'a');
        nb += numchars(line, 'b');
        nc += numchars(line, 'c');
    }

    printf("na = %d\n", na);
    printf("nb = %d\n", nb);
    printf("nc = %d\n", nc);

    return 0;
}