oSIP is an implementation of SIP.

The GNU oSIP library is written in C and get no dependencies except the
standard C library. oSIP is thread safe and will generally be used in a
multi-threaded application. Nevertheless, this is optional.

oSIP is little in size and code and thus could be use to implement IP
soft-phone as well as embedded SIP software. oSIP is not limited to
endpoint agents, and can also be used to implement "SIP proxy".

oSIP does not intend to provide a high layer API for controlling
"SIP Session" at this step. Instead, it currently provides an API for the
SIP message parser, SDP message parser, and library to handle
"SIP transactions" as defined by the SIP document.

Homepage:
https://www.gnu.org/software/osip/
