All functions return int (or some sort of signed value). If this value is negative it represents an error code. Codes up to SND_ERROR_BEGIN (500000) represents standard system errors. Codes equal or greather than this value represents sound library API errors. All error codes begin with the prefix SND_ERROR_.
This error is caused if the driver uses an incompatible kernel API for this interface and hence the library doesn't know how this API can be used.
This functions converts error code to a string. Its functionality is the same as the strerror function from the standard C library, but this function returns correct strings for sound error codes, too.