Next: , Up: Generic Charset Conversion

6.5.1 Generic Character Set Conversion Interface

This set of functions follows the traditional cycle of using a resource: open–use–close. The interface consists of three functions, each of which implements one step.

Before the interfaces are described it is necessary to introduce a data type. Just like other open–use–close interfaces the functions introduced here work using handles and the iconv.h header defines a special type for the handles used.

— Data Type: iconv_t

This data type is an abstract type defined in iconv.h. The user must not assume anything about the definition of this type; it must be completely opaque.

Objects of this type can get assigned handles for the conversions using the iconv functions. The objects themselves need not be freed, but the conversions for which the handles stand for have to.

The first step is the function to create a handle.

— Function: iconv_t iconv_open (const char *tocode, const char *fromcode)

The iconv_open function has to be used before starting a conversion. The two parameters this function takes determine the source and destination character set for the conversion, and if the implementation has the possibility to perform such a conversion, the function returns a handle.

If the wanted conversion is not available, the iconv_open function returns (iconv_t) -1. In this case the global variable errno can have the following values:

The process already has OPEN_MAX file descriptors open.
The system limit of open file is reached.
Not enough memory to carry out the operation.
The conversion from fromcode to tocode is not supported.

It is not possible to use the same descriptor in different threads to perform independent conversions. The data structures associated with the descriptor include information about the conversion state. This must not be messed up by using it in different conversions.

An iconv descriptor is like a file descriptor as for every use a new descriptor must be created. The descriptor does not stand for all of the conversions from fromset to toset.

The GNU C library implementation of iconv_open has one significant extension to other implementations. To ease the extension of the set of available conversions, the implementation allows storing the necessary files with data and code in an arbitrary number of directories. How this extension must be written will be explained below (see glibc iconv Implementation). Here it is only important to say that all directories mentioned in the GCONV_PATH environment variable are considered only if they contain a file gconv-modules. These directories need not necessarily be created by the system administrator. In fact, this extension is introduced to help users writing and using their own, new conversions. Of course, this does not work for security reasons in SUID binaries; in this case only the system directory is considered and this normally is prefix/lib/gconv. The GCONV_PATH environment variable is examined exactly once at the first call of the iconv_open function. Later modifications of the variable have no effect.

The iconv_open function was introduced early in the X/Open Portability Guide, version 2. It is supported by all commercial Unices as it is required for the Unix branding. However, the quality and completeness of the implementation varies widely. The iconv_open function is declared in iconv.h.

The iconv implementation can associate large data structure with the handle returned by iconv_open. Therefore, it is crucial to free all the resources once all conversions are carried out and the conversion is not needed anymore.

— Function: int iconv_close (iconv_t cd)

The iconv_close function frees all resources associated with the handle cd, which must have been returned by a successful call to the iconv_open function.

If the function call was successful the return value is 0. Otherwise it is -1 and errno is set appropriately. Defined error are:

The conversion descriptor is invalid.

The iconv_close function was introduced together with the rest of the iconv functions in XPG2 and is declared in iconv.h.

The standard defines only one actual conversion function. This has, therefore, the most general interface: it allows conversion from one buffer to another. Conversion from a file to a buffer, vice versa, or even file to file can be implemented on top of it.

— Function: size_t iconv (iconv_t cd, char **inbuf, size_t *inbytesleft, char **outbuf, size_t *outbytesleft)

The iconv function converts the text in the input buffer according to the rules associated with the descriptor cd and stores the result in the output buffer. It is possible to call the function for the same text several times in a row since for stateful character sets the necessary state information is kept in the data structures associated with the descriptor.

The input buffer is specified by *inbuf and it contains *inbytesleft bytes. The extra indirection is necessary for communicating the used input back to the caller (see below). It is important to note that the buffer pointer is of type char and the length is measured in bytes even if the input text is encoded in wide characters.

The output buffer is specified in a similar way. *outbuf points to the beginning of the buffer with at least *outbytesleft bytes room for the result. The buffer pointer again is of type char and the length is measured in bytes. If outbuf or *outbuf is a null pointer, the conversion is performed but no output is available.

If inbuf is a null pointer, the iconv function performs the necessary action to put the state of the conversion into the initial state. This is obviously a no-op for non-stateful encodings, but if the encoding has a state, such a function call might put some byte sequences in the output buffer, which perform the necessary state changes. The next call with inbuf not being a null pointer then simply goes on from the initial state. It is important that the programmer never makes any assumption as to whether the conversion has to deal with states. Even if the input and output character sets are not stateful, the implementation might still have to keep states. This is due to the implementation chosen for the GNU C library as it is described below. Therefore an iconv call to reset the state should always be performed if some protocol requires this for the output text.

The conversion stops for one of three reasons. The first is that all characters from the input buffer are converted. This actually can mean two things: either all bytes from the input buffer are consumed or there are some bytes at the end of the buffer that possibly can form a complete character but the input is incomplete. The second reason for a stop is that the output buffer is full. And the third reason is that the input contains invalid characters.

In all of these cases the buffer pointers after the last successful conversion, for input and output buffer, are stored in inbuf and outbuf, and the available room in each buffer is stored in inbytesleft and outbytesleft.

Since the character sets selected in the iconv_open call can be almost arbitrary, there can be situations where the input buffer contains valid characters, which have no identical representation in the output character set. The behavior in this situation is undefined. The current behavior of the GNU C library in this situation is to return with an error immediately. This certainly is not the most desirable solution; therefore, future versions will provide better ones, but they are not yet finished.

If all input from the input buffer is successfully converted and stored in the output buffer, the function returns the number of non-reversible conversions performed. In all other cases the return value is (size_t) -1 and errno is set appropriately. In such cases the value pointed to by inbytesleft is nonzero.

The conversion stopped because of an invalid byte sequence in the input. After the call, *inbuf points at the first byte of the invalid byte sequence.
The conversion stopped because it ran out of space in the output buffer.
The conversion stopped because of an incomplete byte sequence at the end of the input buffer.
The cd argument is invalid.

The iconv function was introduced in the XPG2 standard and is declared in the iconv.h header.

The definition of the iconv function is quite good overall. It provides quite flexible functionality. The only problems lie in the boundary cases, which are incomplete byte sequences at the end of the input buffer and invalid input. A third problem, which is not really a design problem, is the way conversions are selected. The standard does not say anything about the legitimate names, a minimal set of available conversions. We will see how this negatively impacts other implementations, as demonstrated below.