This guide provides a C program that uses libcurl to send SMS and MMS messages using the Twilio REST API, allowing developers to easily integrate this functionality into their applications. The guide assumes a POSIX-compliant environment and requires the installation of libcurl and the use of a Twilio phone number with SMS capabilities. The example Makefile is provided for building the program on various platforms, including Mac OSX, Raspbian, and Windows 7. The `twilio_send_message` function is explained in detail, demonstrating how to prepare account credentials, HTTP parameters, and libcurl settings to execute a POST request to the Twilio API. The guide also includes an example of running the program from the command line, passing in account credentials, from and to numbers, and a message body.