# Passenger::MessageChannel Class Reference
[Apache-independent support classes and function]

`#include <MessageChannel.h>`

List of all members.

## Detailed Description

Convenience class for I/O operations on file descriptors.

This class provides convenience methods for:

- sending and receiving raw data over a file descriptor.
- sending and receiving messages over a file descriptor.
- file descriptor passing over a Unix socket. All of these methods use exceptions for error reporting.

There are two kinds of messages:

- Array messages. These are just a list of strings, and the message itself has a specific length. The contained strings may not contain NUL characters (`'\0'`). Note that an array message must have at least one element.
- Scalar messages. These are byte strings which may contain arbitrary binary data. Scalar messages also have a specific length. The protocol is designed to be low overhead, easy to implement and easy to parse.

MessageChannel is to be wrapped around a file descriptor. For example:

        int p[2];
        pipe(p);
        MessageChannel channel1(p[0]);
        MessageChannel channel2(p[1]);

        // Send an array message.
        channel2.write("hello", "world !!", NULL);
        list<string> args;
        channel1.read(args);    // args now contains { "hello", "world !!" }

        // Send a scalar message.
        channel2.writeScalar("some long string which can contain arbitrary binary data");
        string str;
        channel1.readScalar(str);

The life time of a MessageChannel is independent from that of the wrapped file descriptor. If a MessageChannel object is destroyed, the file descriptor is not automatically closed. Call close() if you want to close the file descriptor.

Note:
  I/O operations are not buffered.

  Be careful with mixing the sending/receiving of array messages, scalar messages and file descriptors. If you send a collection of any of these in a specific order, then the receiving side must receive them in the exact some order. So suppose you first send a message, then a file descriptor, then a scalar, then the receiving side must first receive a message, then a file descriptor, then a scalar. If the receiving side does things in the wrong order then bad things will happen.

  MessageChannel is not thread-safe, but is reentrant.

## Public Member Functions

| MessageChannel ()
| Construct a new MessageChannel with no underlying file descriptor.
| MessageChannel (int fd)
| Construct a new MessageChannel with the given file descriptor.
void | close ()
| Close the underlying file descriptor.
template<typename StringArrayType, typename StringArrayConstIteratorType>
void | write (const StringArrayType &args)
| Send an array message, which consists of the given elements, over the underlying file descriptor.
void | write (const list< string > &args)
| Send an array message, which consists of the given elements, over the underlying file descriptor.
void | write (const vector< string > &args)
| Send an array message, which consists of the given elements, over the underlying file descriptor.
void | write (const char *name,...)
| Send an array message, which consists of the given strings, over the underlying file descriptor.
void | writeScalar (const string &str)
| Send a scalar message over the underlying file descriptor.
void | writeScalar (const char *data, unsigned int size)
| Send a scalar message over the underlying file descriptor.
void | writeRaw (const char *data, unsigned int size)
| Send a block of data over the underlying file descriptor.
void | writeRaw (const string &data)
| Send a block of data over the underlying file descriptor.
void | writeFileDescriptor (int fileDescriptor)
| Pass a file descriptor.
bool | read (vector< string > &args)
| Read an array message from the underlying file descriptor.
bool | readScalar (string &output)
| Read a scalar message from the underlying file descriptor.
bool | readRaw (void *buf, unsigned int size)
| Read exactly `size` bytes of data from the underlying file descriptor, and put the result in `buf`.
int | readFileDescriptor ()
| Receive a file descriptor, which had been passed over the underlying file descriptor.
void | setReadTimeout (unsigned int msec)
| Set the timeout value for reading data from this channel.
void | setWriteTimeout (unsigned int msec)
| Set the timeout value for writing data to this channel.

## Constructor & Destructor Documentation

Passenger::MessageChannel::MessageChannel | ( |  | ) | `[inline]`

Construct a new MessageChannel with no underlying file descriptor.

Thus the resulting MessageChannel object will not be usable. This constructor exists to allow one to declare an "empty" MessageChannel variable which is to be initialized later.

## Member Function Documentation

void Passenger::MessageChannel::close | ( |  | ) | `[inline]`

Close the underlying file descriptor.

If this method is called multiple times, the file descriptor will only be closed the first time.

Exceptions:

  | SystemException |
  | boost::thread_interrupted |

template<typename StringArrayType, typename StringArrayConstIteratorType>

void Passenger::MessageChannel::write | ( | const StringArrayType & | args | ) | `[inline]`

Send an array message, which consists of the given elements, over the underlying file descriptor.

Parameters:

  | args | An object which contains the message elements. This object must support STL-style iteration, and each iterator must have an std::string as value. Use the StringArrayType and StringArrayConstIteratorType template parameters to specify the exact type names.

Exceptions:

  | SystemException | An error occured while writing the data to the file descriptor.
  | boost::thread_interrupted |

Precondition:
  None of the message elements may contain a NUL character (`'\0'`).

See also:
  read(), write(const char *, ...)

void Passenger::MessageChannel::write | ( | const list< string > & | args | ) | `[inline]`

Send an array message, which consists of the given elements, over the underlying file descriptor.

Parameters:

  | args | The message elements.

Exceptions:

  | SystemException | An error occured while writing the data to the file descriptor.
  | boost::thread_interrupted |

Precondition:
  None of the message elements may contain a NUL character (`'\0'`).

See also:
  read(), write(const char *, ...)

void Passenger::MessageChannel::write | ( | const vector< string > & | args | ) | `[inline]`

Send an array message, which consists of the given elements, over the underlying file descriptor.

Parameters:

  | args | The message elements.

Exceptions:

  | SystemException | An error occured while writing the data to the file descriptor.
  | boost::thread_interrupted |

Precondition:
  None of the message elements may contain a NUL character (`'\0'`).

See also:
  read(), write(const char *, ...)

void Passenger::MessageChannel::write | ( | const char * | name,
|  |  | ... |
| ) |  |  | `[inline]`

Send an array message, which consists of the given strings, over the underlying file descriptor.

Parameters:

  | name | The first element of the message to send.
  | ... | Other elements of the message. These *must* be strings, i.e. of type char*. It is also required to terminate this list with a NULL.

Exceptions:

  | SystemException | An error occured while writing the data to the file descriptor.
  | boost::thread_interrupted |

Precondition:
  None of the message elements may contain a NUL character (`'\0'`).

See also:
  read(), write(const list<string> &)

void Passenger::MessageChannel::writeScalar | ( | const string & | str | ) | `[inline]`

Send a scalar message over the underlying file descriptor.

Parameters:

  | str | The scalar message's content.

Exceptions:

  | SystemException | An error occured while writing the data to the file descriptor.
  | boost::thread_interrupted |

See also:
  readScalar(), writeScalar(const char *, unsigned int)

void Passenger::MessageChannel::writeScalar | ( | const char * | data,
|  | unsigned int | size |
| ) |  |  | `[inline]`

Send a scalar message over the underlying file descriptor.

Parameters:

  | data | The scalar message's content.
  | size | The number of bytes in `data`.

Precondition:
  `data != NULL`

Exceptions:

  | SystemException | An error occured while writing the data to the file descriptor.
  | boost::thread_interrupted |

See also:
  readScalar(), writeScalar(const string &)

void Passenger::MessageChannel::writeRaw | ( | const char * | data,
|  | unsigned int | size |
| ) |  |  | `[inline]`

Send a block of data over the underlying file descriptor.

This method blocks until everything is sent.

Parameters:

  | data | The data to send.
  | size | The number of bytes in `data`.

Precondition:
  `data != NULL`

Exceptions:

  | SystemException | An error occured while writing the data to the file descriptor.
  | boost::thread_interrupted |

See also:
  readRaw()

void Passenger::MessageChannel::writeRaw | ( | const string & | data | ) | `[inline]`

Send a block of data over the underlying file descriptor.

This method blocks until everything is sent.

Parameters:

  | data | The data to send.

Precondition:
  `data != NULL`

Exceptions:

  | SystemException | An error occured while writing the data to the file descriptor.
  | boost::thread_interrupted |

void Passenger::MessageChannel::writeFileDescriptor | ( | int | fileDescriptor | ) | `[inline]`

Pass a file descriptor.

This only works if the underlying file descriptor is a Unix socket.

Parameters:

  | fileDescriptor | The file descriptor to pass.

Exceptions:

  | SystemException | Something went wrong during file descriptor passing.
  | boost::thread_interrupted |

Precondition:
  `fileDescriptor >= 0`

See also:
  readFileDescriptor()

bool Passenger::MessageChannel::read | ( | vector< string > & | args | ) | `[inline]`

Read an array message from the underlying file descriptor.

Parameters:

  | args | The message will be put in this variable.

Returns:
  Whether end-of-file has been reached. If so, then the contents of `args` will be undefined.

Exceptions:

  | SystemException | If an error occured while receiving the message.
  | boost::thread_interrupted |

See also:
  write()

bool Passenger::MessageChannel::readScalar | ( | string & | output | ) | `[inline]`

Read a scalar message from the underlying file descriptor.

Parameters:

  | output | The message will be put in here.

Returns:
  Whether end-of-file was reached during reading.

Exceptions:

  | SystemException | An error occured while writing the data to the file descriptor.
  | boost::thread_interrupted |

See also:
  writeScalar()

bool Passenger::MessageChannel::readRaw | ( | void * | buf,
|  | unsigned int | size |
| ) |  |  | `[inline]`

Read exactly `size` bytes of data from the underlying file descriptor, and put the result in `buf`.

If end-of-file has been reached, or if end-of-file was encountered before `size` bytes have been read, then `false` will be returned. Otherwise (i.e. if the read was successful), `true` will be returned.

Parameters:

  | buf | The buffer to place the read data in. This buffer must be at least `size` bytes long.
  | size | The number of bytes to read.

Returns:
  Whether reading was successful or whether EOF was reached.

Precondition:
  buf != NULL

Exceptions:

  | SystemException | Something went wrong during reading.
  | boost::thread_interrupted |

See also:
  writeRaw()

int Passenger::MessageChannel::readFileDescriptor | ( |  | ) | `[inline]`

Receive a file descriptor, which had been passed over the underlying file descriptor.

Returns:
  The passed file descriptor.

Exceptions:

  | SystemException | If something went wrong during the receiving of a file descriptor. Perhaps the underlying file descriptor isn't a Unix socket.
  | IOException | Whatever was received doesn't seem to be a file descriptor.
  | boost::thread_interrupted |

void Passenger::MessageChannel::setReadTimeout | ( | unsigned int | msec | ) | `[inline]`

Set the timeout value for reading data from this channel.

If no data can be read within the timeout period, then a SystemException will be thrown by one of the read methods, with error code EAGAIN or EWOULDBLOCK.

Parameters:

  | msec | The timeout, in milliseconds. If 0 is given, there will be no timeout.

Exceptions:

  | SystemException | Cannot set the timeout.

void Passenger::MessageChannel::setWriteTimeout | ( | unsigned int | msec | ) | `[inline]`

Set the timeout value for writing data to this channel.

If no data can be written within the timeout period, then a SystemException will be thrown, with error code EAGAIN or EWOULDBLOCK.

Parameters:

  | msec | The timeout, in milliseconds. If 0 is given, there will be no timeout.

Exceptions:

  | SystemException | Cannot set the timeout.

The documentation for this class was generated from the following file:

- MessageChannel.h