Ports are represented as Scheme values with the types scheme_input_port_type and scheme_output_port_type. The function scheme_read takes an input port value and returns the next S-expression from the port. The function scheme_write takes an output port and a value and writes the value to the port. Other standard low-level port functions are also provided, such as scheme_getc.
File ports can be created with scheme_make_file_input_port and scheme_make_file_output_port; these functions take a FILE * file pointer and return a Scheme port. Strings can be read or written with scheme_make_string_input_port, which takes a NULL-terminated string, and scheme_make_string_output_port, which takes no arguments. The contents of a string output port can be obtained with scheme_get_string_output.
Custom ports, with arbitrary read/write handlers, can be created with scheme_make_input_port and scheme_make_output_port.