detab.c

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

#define DEFAULT_TAB_STOP 4
#define DEFAULT_REPLACE  ' ' // used as tab replacement

int main(int argc, char *argv[]) {
    char *arg_end;
    int c, tab_stop, curr_col, spaces;

    curr_col = 1; // denotes the current column

    if (argc < 2) {
        tab_stop = DEFAULT_TAB_STOP;
    } else {
        tab_stop = (int)strtol(argv[1], &arg_end, 10);

        if(*arg_end) {
            fprintf(stderr, "error: invalid argument: %s\n", arg_end);
            exit(1);
        } else if(tab_stop > 8) {
            fprintf(stderr, "error: tab stop size too big: %d\n", tab_stop);
            exit(1);
        }
    }

    while ((c = getchar()) != EOF) {
        if (c == '\n') {
            // reset tab stop
            printf("\n");
            curr_col = 1;
        } else if(c == '\t') {
            if(curr_col % tab_stop == 0) {
                // we're at the tab stop, so just print one space
                printf("%c", DEFAULT_REPLACE);
                curr_col++;
            } else {
                // get number of spaces until next tab stop
                spaces = (tab_stop - (curr_col % tab_stop)) + 1;
                curr_col += spaces;

                for (; spaces > 0; spaces--)
                    printf("%c", DEFAULT_REPLACE);
            }
        } else {
            printf("%c", c);
            curr_col++;
        }
    }

    return 0;
}