filefind.c search one file into other (use mmap)

// avp 2016 find one file (av[2]) into other file (av[1])
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mman.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <err.h>
#include <unistd.h>
#include <fcntl.h>
#include <sysexits.h>

struct fmap {
  char *data;
  size_t len;
};

struct fmap
mapfile (const char *fn)
{
  struct fmap fm = {0};

  if (fn) {
    int fd = open(fn, O_RDONLY);
    struct stat st;

    if (fd > -1)
      if ((fstat(fd, &st), fm.len = st.st_size) &&
          (fm.data = (char *)mmap(NULL, st.st_size,
                                  PROT_READ, MAP_PRIVATE, fd, 0))) {
        madvise(fm.data, fm.len, MADV_SEQUENTIAL);
        close(fd);
      }
  }

  return fm;
}

static inline size_t
skipnl (struct fmap *p, size_t from)
{
  while (from < p->len && p->data[from++] != '\n');

  return from;
}

int
main (int ac, char *av[])
{
  if (ac != 3)
    err(EX_USAGE,
    "search file2 in file1 (by lines) Usage: %s file1 file2", av[0]);
  struct fmap f1 = mapfile(av[1]),
    f2 = mapfile(av[2]);

  if (!f1.data)
    err(EX_DATAERR, "file %s", av[1]);
  if (!f2.data)
    err(EX_DATAERR, "file %s", av[2]);

  size_t i = 0,
    lineno = 0, // current line number in f1
    n2l = 0;    // number of '\n' in f2 (for adjust lineno when found f2 in f1
  for (i = 0; i < f2.len; i++)
    if (f2.data[i] == '\n')
      n2l++;
  int need_skipnl = f2.data[f2.len - 1] != '\n';

  i = 0;
  while (i < f1.len) {
    if (memcmp(f1.data + i, f2.data, f2.len) == 0) {
      printf("%ld\n", (long)lineno + 1);
      lineno += n2l;
      i += f2.len;
      if (need_skipnl)
        i = skipnl(&f1, i), lineno++;
    } else
      i = skipnl(&f1, i), lineno++;
  }

  return munmap(f1.data, f1.len) + munmap(f2.data, f2.len);
}