/* file: pbs-and.c		G. Moody		25 February 2012

Read N sorted files, write lines common to all of them (logical "and")
*/

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define LMAX 128

char **line;
FILE **ifile;
int n;

/* cleanup: close input files, free allocated memory, and quit. */
void cleanup()
{
    while (n-- > 0) {
	fclose(ifile[n]);
	free(line[n]);
    }
    free(line);
    free(ifile);
    exit(0);
}

/* init: open input files and read the first line of each.  If any input
   is unreadable or empty, the output will be empty, so exit immediately
   in this case. */
void init(int argc, char **argv)
{
    if (argc < 3)
	exit(1);

    argc--; argv++;	/* skip argv[0] */
    ifile = (FILE **)calloc(argc, sizeof(FILE *));
    line = (char **)calloc(argc, sizeof(char *));
    for (n = 0; n < argc; n++) {
	if ((ifile[n] = fopen(argv[n], "r")) == NULL)
	    cleanup();
	line[n] = (char *)calloc(LMAX, sizeof(char));
	if (fgets(line[n], LMAX, ifile[n]) == NULL)
	    cleanup();
    }
}

main(int argc, char **argv)
{
    init(argc, argv);

    do {
	int i, j;

	for (i = 1; i < n; i++) {
	    j = strcmp(line[0], line[i]);
	    if (j < 0) {
		if (fgets(line[0], LMAX, ifile[0])) { i = 0; continue; }
		else cleanup();
	    }
	    if (j > 0) {
		if (fgets(line[i], LMAX, ifile[i])) { i--; continue; }
		else cleanup();
	    }
	}
	fputs(line[0], stdout);
    } while (fgets(line[0], LMAX, ifile[0]));

    cleanup();
}