Synopsis

libfo is a tiny library that allows user to simulate errors of POSIX and libc functions.

Description

When testing software it's often that ones find it very hard or even impossible to test some error conditions, especially when it comes to POSIX or libc functions. Now how would you test if socket() fails? You would have to first create tons of file descriptors so kernel wouldn't allocate you anymore. It also might be hard to simulate error of write() function in some scenarios.

Now thanks to libfo your problem can be solved really easy. libfo consists only of 2 (yes, two) functions, and one of them is init function. Library does not required you to make any changed to production code, all magic is done in test suite.

  • fo_init(3) - initializes everything, need to be called as early as possible, best to call it right after main().
  • fo_fail(3) - configures how, when and what function should fail.

Small example:

/* production-file.c
 */

int store_to_file(const char *f, const char *s)
{
    int fd;
    int w;

    if ((fd = open(f, O_CREAT | O_TRUNC | O_RDWR, 0666)) < 0)
    {
        perror("open()");
        return -1;
    }

    w = write(fd, s, strlen(s));
    if (w == -1)
    {
        /* write() error
         */

         run_diagnostics();
         return -1;
    }

    if (w != strlen(s))
    {
        /* incomplete write
         */

         close(fd);
         unlink(f);
         return -1;
    }

    if (write(fd, "\n", 1) != 1)
    {
        /* failed to store newline
         */

         close(fd);
         unlink(f);
         return -1;
    }

    return 0;
}

/* test-suite.c
 */
void foo(void)
{
    /* open() is very easy to check, just pass file that doesn't
     * exit and expect function to return -1 and ENOENT errno
     */
    mt_ferr(store_to_file("/file/that/doesnt/exist", "message"), ENOENT);

    /* install point of failure. After fo_fail(), next write() will
     * fail returning -1 and errno will be set to EIO.
     */
    fo_fail(fo_write, 1, -1, EIO);
    mt_ferr(store_to_file("/tmp/file", "message"), EIO);

    /* now let's emulate partial write. write() will return with
     * value 4, and errno set to ENOSPC, emulating that there is
     * not enough space on disk.
     */
    fo_fail(fo_write, 1, 4, ENOSPC);
    mt_fail(store_to_file("/tmp/file", "message") == 4);
    mt_fail(errno == ENOSPC);

     /* by setting countdown (second argument) we can configure
      * when write() shall fail. Setting it to 1, will cause first
      * (and only first) call to write fail. We can sent it to 2,
      * causing first write() to succed and second to return error.
      * So now we can simulate correct data write, but fail to
      * store last new line character
      */
    fo_fail(fo_write, 2, -1, ENOSPC);
    mt_ferr(store_to_file("/tmp/file", "message"), ENOSPC);
}

Now testing such code could be very, if not impossible to test. It could be really difficult to make write() to return -1 once open() succeed. And not testing such scenario might be painful - notice there is little bug there, when file descriptor is not closed when write() returns -1.

Of course, one needs to also realize its limitations. Notice that when partial write error is simulated, POSIX write() will not be called, and even though write() returns 4 (which leads program to belive some data were stored) no data were really stored, because no real write() has been called at all that time. This however could be solved by modifying write() functions in fo.c file to call write() with count 4, and then return.

Usage

Convinced? I'm glad. I don't like repeating myself, so to know more about library itself and how to use it, please check libfo(7) manual page.

Installing

Package can be installed to system with classic (which accepts DESTDIR envvar)

make install

This will install only fogen program and manual pages for quick access.

Downloads

Released prebuild libraries and source tarballs can be downloaded from https://libfo.kurwinet.pl/downloads.html

Dependencies

Generated code is c99 compliant but requires RTLD_NEXT special handle for dlsym which may not be supported on all operating systems.

License

Library is licensed under BSD 2-clause license. See LICENSE file for details. So yeah, feel free to use it anyway you see fit.

Contact

Michał Łyszczek michal.lyszczek@bofc.pl

See also