Architecture independent level 1 (DDI/DKI).
msgdsize() counts the number of bytes in a data message. Only bytes included in the data blocks of type M_DATA are included in the count.
The number of data bytes in a message, expressed as an integer.
msgdsize() can be called from user or interrupt context.
See bufcall(9F) for an example that uses msgdsize().
Writing Device Drivers
STREAMS Programming Guide