Examples for reading text files in FreeBSD kernel

Question

Could anyone give some simple examples (function names are good) for reading text files line by line (binary is OK if text is really hard) in a FreeBSD kernel module, from a given directory?

Really appreciate your kind help.

Answer 1

Here's a sample kernel module that'll cat your /etc/motd on load:

// kernel module motd catter.
// Doug Luce [e-mail]

#include <sys/param.h>
#include <sys/vnode.h>
#include <sys/fcntl.h>
#include <sys/module.h>
#include <sys/kernel.h>
#include <sys/namei.h>
#include <sys/proc.h>
#include <sys/sbuf.h>

static int catfile(const char *filename) {
  struct sbuf *sb;
  static char buf[128];
  struct nameidata nd;
  off_t ofs;
  ssize_t resid;
  int error, flags, len;

  NDINIT(&nd, LOOKUP, FOLLOW, UIO_SYSSPACE, filename, curthread);
  flags = FREAD;
  error = vn_open(&nd, &flags, 0, NULL);
  if (error)
    return (error);

  NDFREE(&nd, NDF_ONLY_PNBUF);

  ofs = 0;
  len = sizeof(buf) - 1;
  sb = sbuf_new_auto();
  while (1) {
    error = vn_rdwr(UIO_READ, nd.ni_vp, buf, len, ofs,
                    UIO_SYSSPACE, IO_NODELOCKED, curthread->td_ucred,
                    NOCRED, &resid, curthread);
    if (error)
      break;
    if (resid == len)
      break;
    buf[len - resid] = 0;
    sbuf_printf(sb, "%s", buf);
    ofs += len - resid;
  }

  VOP_UNLOCK(nd.ni_vp, 0);
  vn_close(nd.ni_vp, FREAD, curthread->td_ucred, curthread);
  uprintf("%s", sbuf_data(sb));
  return 0;
}

static int EventHandler(struct module *inModule, int inEvent, void *inArg) {
  switch (inEvent) {
  case MOD_LOAD:
    uprintf("MOTD module loading.\n");
    if (catfile("/etc/motd") != 0)
      uprintf("Error reading MOTD.\n");
    return 0;
  case MOD_UNLOAD:
    uprintf("MOTD module unloading.\n");
    return 0;
  default:
    return EOPNOTSUPP;
  }
}

static moduledata_t  moduleData = {
  "motd_kmod",
  EventHandler,
  NULL
};

DECLARE_MODULE(motd_kmod, moduleData, SI_SUB_DRIVERS, SI_ORDER_MIDDLE);

This was cobbled together mostly from bits of https://svnweb.freebsd.org/base/release/10.1.0/sys/kern/vfs_mountroot.c?revision=274417&view=markup

There's no nice scanning/parsing facilities native kernel-side, so that's usually done the hard way.