lw.c

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

int main(int argc, char **argv)
{
    FILE *ifp = NULL;
    FILE *ofp = NULL;
    char wrd[40] = { 0 };
    char line[256] = { 0 };

    if(argc > 1 && !strcmp(argv[1], "stdio")) {
        /* Use standard input/output streams
         * to read/write information. This is
         * useful when we want to use redirections
         * that are commonly used in UNIX environments. */
        ifp = stdin;
        ofp = stdout;
    }
    else {
        /* Open files for reading and writing. If one
         * of them fails to open, we return 1 indicating
         * an unexpected end of the program's execution. */
        ifp = fopen("in.txt", "r");
        ofp = fopen("out.txt", "w");
        if(!ifp || !ofp)
            return 1;
    }

    /* Go through the input file line by line */
    while(fgets(line, sizeof(line), ifp)) {
        /* This sscanf call is just to ensure that a string
         * has less than five words. The sscanf function returns
         * the amount of items it has filled; we can abuse this
         * mechanic and test whether there are five words or not. */
        if(sscanf(line, "%39s %39s %39s %39s %39s", wrd, wrd, wrd, wrd, wrd) < 5) {
            /* Print the line to the output file
             * and go on with the next one. */
            fputs(line, ofp);
            continue;
        }
    }

    /* Close files. */
    fclose(ofp);
    fclose(ifp);
    
    return 0;
}