Skip to content

The uptime mock example

This is a very simple example to explain the mocking feature of cmocka. It implement the 'uptime' unix command in a very simple way to demonstrate how to test the time calculation.

The problem with testing the uptime command is that /proc/uptime constantly ticks. The result is random whenever you call the test. To actually test it we need to make sure that we work with fixed values. The mocking features of cmocka allows us to test it anyway!

Source files

  • proc_uptime.c: This implements the uptime() function reading and parsing the /proc/uptime file.
  • uptime.c: This is the actual uptime implementation, it calls calc_uptime() to get a human readable string representation of the uptime. This function calls uptime() from proc_uptime.c.
  • test_uptime.c: This is the test with the mocking function for uptime().

Linking magic

The test is linked using:

ld --wrap=uptime

This replaces the orginal uptime() function which reads from /proc/uptime with the mock function we implemented for testing calc_uptime().

The mock function we implemented has a special name. It is called __wrap_uptime(). All the symbols you want to mock (or replace) need to start with the prefix __wrap_. So ld --wrap=uptime will rename the orignal uptime() function to __real_uptime(). This means you can still reach the original function using that name and call it e.g. from the wrap function. The symbol uptime will be bound to __wrap_uptime.

You can find more details in the manpage: man ld

The uptime test

The code should be easy to understand. If you have a hard time following, there are two ways to understand how things work.

You can find out details about symbol binding using:

LD_DEBUG=symbols ./example/uptime/uptime
LD_DEBUG=symbols ./example/uptime/test_uptime

You can also use a debugger to step through the code!

Have fun!