The logerror() function is used to output text to the standard error.log file (if the log option is enabled) and is also visible in a debugger error output window.

Declared in src/emu/mame.h as

void CLIB_DECL logerror(const char *format, ...) ATTR_PRINTF(1,2);

• *format – Pointer to a const char with the text to be printed. Supports printf-style parameters.
• ... – A list of variables that are referenced in the text.

Nothing.

logerror("port 0: pc = %4x\n", activecpu_get_pc());

Will output

port 0: pc = 023a

into the error.log file and the debugger error window.