Setting errno is one of the ways the c standard library can report errors to the application. Setting errno as is done in the TI-POSIX function mq_send() below will result in the aeabi function __aeabi_errno_addr() being called. This uses the sys/bios reent support function ReentSupport_getReent() to find the thread local storage reent structure. The TLS does not seem to be valid for SWI and HWI context, causing an assert.