Tcl_RegisterConfig manual page - Tcl Library Procedures

Tcl_Interp *interp (in): Refers to the interpreter the embedded configuration information is registered for. Must not be NULL.
const char *pkgName (in): Contains the name of the package registering the embedded configuration as ASCII string. This means that this information is in UTF-8 too. Must not be NULL.
const Tcl_Config *configuration (in): Refers to an array of Tcl_Config entries containing the information embedded in the library. Must not be NULL. The end of the array is signaled by either a key identical to NULL, or a key referring to the empty string.
const char *valEncoding (in): Contains the name of the encoding used to store the configuration values as ASCII string. This means that this information is in UTF-8 too. Must not be NULL.

The function described here has its base in TIP 59 and provides extensions with support for the embedding of configuration information into their library and the generation of a Tcl-level interface for querying this information.

To embed configuration information into their library an extension has to define a non-volatile array of Tcl_Config entries in one if its source files and then call Tcl_RegisterConfig to register that information.

Tcl_RegisterConfig takes four arguments; first, a reference to the interpreter we are registering the information with, second, the name of the package registering its configuration information, third, a pointer to an array of structures, and fourth a string declaring the encoding used by the configuration values.

The string valEncoding contains the name of an encoding known to Tcl. All these names are use only characters in the ASCII subset of UTF-8 and are thus implicitly in the UTF-8 encoding. It is expected that keys are legible English text and therefore using the ASCII subset of UTF-8. In other words, they are expected to be in UTF-8 too. The values associated with the keys can be any string however. For these the contents of valEncoding define which encoding was used to represent the characters of the strings.

Each element of the configuration array refers to two strings containing the key and the value associated with that key. The end of the array is signaled by either an empty key or a key identical to NULL. The function makes no copy of the configuration array. This means that the caller has to make sure that the memory holding this array is never released. This is the meaning behind the word non-volatile used earlier. The easiest way to accomplish this is to define a global static array of Tcl_Config entries. See the file “generic/tclPkgConfig.c” in the sources of the Tcl core for an example.

When called Tcl_RegisterConfig will

create a namespace having the provided pkgName, if not yet existing.
create the command pkgconfig in that namespace and link it to the provided information so that the keys from configuration and their associated values can be retrieved through calls to pkgconfig.

The command pkgconfig will provide two subcommands, list and get:

::pkgName::pkgconfig list: Returns a list containing the names of all defined keys.
::pkgName::pkgconfig get key: Returns the configuration value associated with the specified key.

TCL_CONFIG

The Tcl_Config structure contains the following fields:

typedef struct Tcl_Config {
    const char *key;
    const char *value;
} Tcl_Config;

KEYWORDS

embedding, configuration, library