findtabs.c

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

int main (int argc, char* argv[]) {
    char buf[1024];
    int i, ln;
    char* c, *lineEnd;

    for (i = 1; i < argc; ++i) {
        FILE* fp = fopen (argv[i], "r");

        if (!fp) {
            printf ("%s: %s\n", argv[i], strerror (errno));
            continue;
        }

        for (ln = 1; fgets (buf, sizeof buf, fp); ++ln) {
            lineEnd = &buf[strlen (buf)]; // position of '\0'

            for (c = &buf[0]; *c == '\t'; ++c)
                ;

            if (c != &buf[0] && (*c == '\n' || *c == '\r'))
                printf ("%s:%d: line all tabs\n", argv[i], ln);
            else {
                // lineEnd - 2 to skip '\0' and '\n'
                if (lineEnd != &buf[0] && *(lineEnd - 2) == '\t')
                    printf ("%s:%d: trailing tabs\n", argv[i], ln);
            }
        }

        fclose (fp);
    }
}