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

Convenience class for I/O operations on file descriptors. More...

`#include <MessageChannel.h>`

List of all members.

## 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.
int | filenum () const
| Returns the underlying file descriptor.
bool | connected () const
| Returns whether close() has been called.
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, va_list &ap)
| Send an array message, which consists of the given strings, 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 | writeUint32 (unsigned int value)
| Write a 32-bit big-endian unsigned integer to the underlying file descriptor.
void | writeScalar (const string &str)
| Write a scalar message to the underlying file descriptor.
void | writeScalar (const char *data, unsigned int size)
| Write a scalar message to 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, bool negotiate=true)
| Pass a file descriptor.
bool | read (vector< string > &args)
| Read an array message from the underlying file descriptor.
bool | readUint32 (unsigned int &value, unsigned long long *timeout=NULL)
| Read a 32-bit big-endian unsigned integer from the underlying file descriptor.
bool | readScalar (string &output, unsigned int maxSize=0, unsigned long long *timeout=NULL)
| Read a scalar message from the underlying file descriptor.
bool | readRaw (void *buf, unsigned int size, unsigned long long *timeout=NULL)
| Read exactly `size` bytes of data from the underlying file descriptor, and put the result in `buf`.
bool | waitUntilReadable (unsigned long long *timeout)
| Waits at most `*timeout` milliseconds for the file descriptor to become readable.
int | readFileDescriptor (bool negotiate=true)
| 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.

## 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.
- data size limit enforcement and time constraint enforcement. 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.
  Some methods throw SecurityException and TimeoutException. When these exceptions are thrown, the channel will be left in an inconsistent state because only parts of the data have been read. You should close the channel after having caught these exceptions.

## 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 |

Postcondition:
  filenum() == -1
  !connected()

int Passenger::MessageChannel::filenum | ( |  | ) | const`[inline]`

Returns the underlying file descriptor.

-1 if it has already been closed.

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()

int Passenger::MessageChannel::readFileDescriptor | ( | bool | negotiate = `true` | ) | `[inline]`

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

Parameters:

  | negotiate | See Ruby's MessageChannel::send_io method's comments.

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 |

bool Passenger::MessageChannel::readRaw | ( | void * | buf,
|  | unsigned int | size,
|  | unsigned long long * | timeout = `NULL` |
| ) |  |  | `[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.
  | timeout | A pointer to an integer, which specifies the maximum number of milliseconds that may be spent on reading the `size` bytes of data. If the timeout expired then TimeoutException will be thrown. If this function returns without throwing an exception, then the total number of milliseconds spent on reading will be deducted from `timeout`. Pass NULL if you do not want to enforce a timeout.

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

Precondition:
  buf != NULL

Exceptions:

  | SystemException | Something went wrong during reading.
  | TimeoutException | Unable to read `size` bytes of data within `timeout` milliseconds.
  | boost::thread_interrupted |

See also:
  writeRaw()

bool Passenger::MessageChannel::readScalar | ( | string & | output,
|  | unsigned int | maxSize = `0`,
|  | unsigned long long * | timeout = `NULL` |
| ) |  |  | `[inline]`

Read a scalar message from the underlying file descriptor.

Parameters:

  | output | The message will be put in here.
  | maxSize | The maximum number of bytes that may be read. If the scalar to read is larger than this, then a SecurityException will be thrown. Set to 0 for no size limit.
  | timeout | A pointer to an integer, representing the maximum number of milliseconds to spend on reading the entire scalar. A TimeoutException will be thrown if unable to read the entire scalar within this time period. If no exception is thrown, the the amount of time spent on waiting will be deducted from `*timeout`. Pass NULL if you do not want to enforce any time limits.

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

Exceptions:

  | SystemException | An error occured while reading data from the file descriptor.
  | SecurityException | There is more data to read than allowed by maxSize.
  | TimeoutException | Unable to read the entire scalar within `timeout` milliseconds.
  | boost::thread_interrupted |

See also:
  writeScalar()

bool Passenger::MessageChannel::readUint32 | ( | unsigned int & | value,
|  | unsigned long long * | timeout = `NULL` |
| ) |  |  | `[inline]`

Read a 32-bit big-endian unsigned integer from the underlying file descriptor.

Parameters:

  | value | Upon success, the read value will be stored in here.
  | timeout | A pointer to an integer, representing the maximum number of milliseconds to spend on reading the entire integer. A TimeoutException will be thrown if the timeout expires. If no exception is thrown, the the amount of time spent on waiting will be deducted from `*timeout`. Pass NULL if you do not want to enforce any time limits.

Returns:
  True if a value was read, false if EOF was reached before all data can be read.

Exceptions:

  | SystemException | An error occurred while reading data from the 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.

bool Passenger::MessageChannel::waitUntilReadable | ( | unsigned long long * | timeout | ) | `[inline]`

Waits at most `*timeout` milliseconds for the file descriptor to become readable.

Returns true if it become readable within the timeout, false if the timeout expired.

`*timeout` may be 0, in which case this method will check whether the file descriptor is readable, and immediately returns without waiting.

If no exception is thrown, this method deducts the number of milliseconds that has passed from `*timeout`.

Exceptions:

  | SystemException |
  | boost::thread_interrupted |

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::write | ( | const char * | name,
|  | va_list & | ap |
| ) |  |  | `[inline]`

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

Like `write(const char *name, ...)` but takes a va_list instead.

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'`).

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 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 *, ...)

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::writeFileDescriptor | ( | int | fileDescriptor,
|  | bool | negotiate = `true` |
| ) |  |  | `[inline]`

Pass a file descriptor.

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

Parameters:

  | fileDescriptor | The file descriptor to pass.
  | negotiate | See Ruby's MessageChannel::send_io method's comments.

Exceptions:

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

Precondition:
  `fileDescriptor >= 0`

See also:
  readFileDescriptor()

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.

Note:
  Security guarantee: this method will not copy the data in memory, so it's safe to use this method to write passwords to the underlying file descriptor.

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::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.

Note:
  Security guarantee: this method will not copy the data in memory, so it's safe to use this method to write passwords to the underlying file descriptor.

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

Write a scalar message to the underlying file descriptor.

Note:
  Security guarantee: this method will not copy the data in memory, so it's safe to use this method to write passwords to 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::writeScalar | ( | const string & | str | ) | `[inline]`

Write a scalar message to the underlying file descriptor.

Note:
  Security guarantee: this method will not copy the data in memory, so it's safe to use this method to write passwords to 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::writeUint32 | ( | unsigned int | value | ) | `[inline]`

Write a 32-bit big-endian unsigned integer to the underlying file descriptor.

Exceptions:

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

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

- MessageChannel.h