ALSA project - the C library reference
The ALSA library can accept runtime arguments for some configuration blocks. This extension is built on top of the basic configuration file syntax.
Arguments are defined using the id (key)
@args and array values containing the string names of the arguments:
An argument's type is specified with the id (key)
@args and the argument name. The type and the default value are specified in the compound block:
Arguments are referred to with a dollar-sign ($) and the name of the argument:
To use a block with arguments, write the argument values after the key, separated with a colon (:). For example, all these names for PCM interfaces give the same result:
As you see, arguments can be specified in their proper order or by name. Note that arguments enclosed in braces are parsed in the same way as in configuration files, but using the override method by default.