Clarify documentation for sp_read() and sp_write().

[libserialport.git] / README

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libserialport: cross-platform library for accessing serial ports
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libserialport is a minimal library written in C that is intended to take care
of the OS-specific details when writing software that uses serial ports.

By writing your serial code to use libserialport, you enable it to work
transparently on any platform supported by the library.

The operations that are supported are:

- Port enumeration (obtaining a list of serial ports on the system).
- Opening and closing ports.
- Setting port parameters (baud rate, parity, etc).
- Reading, writing and flushing data.
- Obtaining error information.

libserialport is an open source project released under the LGPL3+ license.

Status
======

The library should build and work on any Windows or Unix-based system. If it
does not, please submit a bug.

Enumeration is currently only implemented on Windows, Mac OS X and Linux. On
other systems enumeration will return no results, but ports can still be opened
by name and then used.

If you know how to enumerate available ports on another OS, please submit a bug
with this information, or better still a patch implementing it.

Future
======

Future versions will add additional API calls for obtaining metadata about a
port, e.g. for USB devices the USB VID and PID of the underlying device.

Dependencies
============

On Linux, libudev is required. On other systems no other libraries are required.

The libudev dependency could be eliminated in favour of direct sysfs queries at
the cost of some brevity. This is not currently a priority but if you feel like
doing this feel free to submit a patch.

Building
========

The package uses a GNU style build system and requires a Unix style shell.
On Windows it can be built with the MinGW toolchain and MSYS environment.

Run "./autogen.sh" to generate the build system, "./configure" to setup, then
"make" to build the library and "make install" to install it.

API
===

The API is simple, and designed to be a minimal wrapper around the serial port
support in each OS.

Most functions take a pointer to a struct sp_port, which represents an serial
port. This structure is obtained from the array returned by sp_list_ports().

All functions can return only three possible error values. SP_ERR_ARG indicates
the function was called with invalid arguments. SP_ERR_FAIL indicates that the
OS reported a failure. SP_ERR_MEM indicates that a memory allocation failed.
All of these error values are negative.

When SP_ERR_FAIL is returned, an error code or string description of the error
can be obtained by calling sp_last_error_code() or sp_last_error_message(). The
error code or message is that provided by the OS; libserialport does not define
any error codes or messages of its own.

Functions calls that succeed return SP_OK, which is equal to zero, or where
otherwise documented a positive value.

The available functions are as follows:

Enumeration
-----------

int sp_get_port_by_name(const char *portname, struct sp_port **port_ptr);

 Obtains a pointer to a new sp_port structure representing the named port. The
 user should allocate a variable of type "struct sp_port *" and pass a pointer
 to this to receive the result.
 
 Returns: SP_OK on success, SP_ERR_FAIL on failure, SP_ERR_MEM on allocation failure,
          or SP_ERR_ARG if an invalid pointer is passed. If any error is returned,
          the value pointed to by port_ptr will be set to NULL.

void sp_free_port(struct sp_port *port);

 Frees a port structure obtained from sp_get_port_by_name().

int sp_list_ports(struct sp_port ***list_ptr);

 Lists the serial ports available on the system. The result obtained is an
 array of pointers to sp_port structures, terminated by a NULL. The user should
 allocate a variable of type "struct sp_port **" and pass a pointer to this to
 receive the result.

 The result should be freed after use by calling sp_free_port_list().

 Returns: SP_OK on success, SP_ERR_FAIL on failure, SP_ERR_MEM on allocation failure,
          or SP_ERR_ARG if an invalid pointer is passed. If any error is returned,
          the value pointed to by list_ptr will be set to NULL.

void sp_free_port_list(struct sp_port **list);

 Frees a port list obtained from sp_list_ports().

Opening and closing ports
-------------------------

int sp_open(struct sp_port *port, int flags);

 Opens the specified serial port.

 Parameters:

  port:      Pointer to port structure.
  flags:     Flags to use when opening the serial port. Possible
             flags are: SP_MODE_RDWR, SP_MODE_RDONLY, and SP_MODE_NONBLOCK.

 Returns: SP_OK on success, SP_ERR_FAIL on failure, or SP_ERR_ARG
          if an invalid port is passed.

int sp_close(struct sp_port *port);

 Closes the specified serial port.

 Returns: SP_OK on success, SP_ERR_FAIL on failure, or SP_ERR_ARG
          if an invalid port is passed.

Setting port parameters
-----------------------

int sp_set_params(struct sp_port *port, int baudrate,
			      int bits, int parity, int stopbits,
			      int flowcontrol, int rts, int dtr);

 Sets serial parameters for the specified serial port.

 Parameters:

  port:        Pointer to port structure.
  baudrate:    Baud rate to set.
  bits:        Number of data bits to use.
  parity:      Parity setting to use
               (SP_PARITY_NONE, SP_PARITY_EVEN or SP_PARITY_ODD)
  stopbits:    Number of stop bits to use (1 or 2).
  flowcontrol: Flow control setting to use
               (SP_FLOW_NONE, SP_FLOW_HARDWARE or SP_FLOW_SOFTWARE)

 Returns: SP_OK on success, SP_ERR_FAIL on failure, or SP_ERR_ARG
          for invalid arguments.

Reading, writing and flushing data
----------------------------------

int sp_read(struct sp_port *port, const void *buf, size_t count)

 Reads bytes from the specified serial port. Note that this function may
 return after reading less than the specified number of bytes; it is the
 user's responsibility to iterate as necessary in this case.

 Parameters:

  port:  Pointer to port structure.
  buf:   Buffer in which to store the bytes read.
  count: Maximum number of bytes to read.

 Returns: The number of bytes read, SP_ERR_FAIL on failure,
          or SP_ERR_ARG for invalid arguments.

int sp_write(struct sp_port *port, const void *buf, size_t count)

 Write bytes to the specified serial port. Note that this function may
 return after writing less than the specified number of bytes; it is the
 user's responsibility to iterate as necessary in this case.

 Parameters:

  port:  Pointer to port structure.
  buf:   Buffer containing the bytes to write.
  count: Maximum number of bytes to write.

 Returns: The number of bytes written, SP_ERR_FAIL on failure,
          or SP_ERR_ARG for invalid arguments.

int sp_flush(struct sp_port *port);

 Flushes serial port buffers.

 Returns: SP_OK on success, SP_ERR_FAIL on failure, or SP_ERR_ARG
          if an invalid port is passed.

Error handling
--------------

int sp_last_error_code();

 Gets the error code for a failed operation.

 In order to obtain the correct result, this function should be called
 straight after the failure, before executing any other system operations.

 Returns: The system's numeric code for the error that caused the last
          operation to fail.

char *sp_last_error_message();

 Gets the error message for failed operation.

 In order to obtain the correct result, this function should be called
 straight after the failure, before executing other system operations.

 Returns: The system's message for the error that caused the last
          operation to fail. This string may be allocated by the function,
          and should be freed after use by calling sp_free_error_message.

void sp_free_error_message(char *message);

 Frees the error message returned by sp_last_error_message().