man pages section 9: DDI and DKI Kernel Functions

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Updated: July 2014



putbq - place a message at the head of a queue


#include <sys/stream.h>

int putbq(queue_t *q, mblk_t *bp);

Interface Level

Architecture independent level 1 (DDI/DKI).



Pointer to the queue.


Pointer to the message block.


The putbq() function places a message at the beginning of the appropriate section of the message queue. There are always sections for high priority and ordinary messages. If other priority bands are used, each will have its own section of the queue, in priority band order, after high priority messages and before ordinary messages. putbq() can be used for ordinary, priority band, and high priority messages. However, unless precautions are taken, using putbq() with a high priority message is likely to lead to an infinite loop of putting the message back on the queue, being rescheduled, pulling it off, and putting it back on.

This function is usually called when bcanput(9F) or canput(9F) determines that the message cannot be passed on to the next stream component. The flow control parameters are updated to reflect the change in the queue's status. If QNOENB is not set, the service routine is enabled.

Return Values

The putbq() function returns 1 upon success and 0 upon failure.

Upon failure, the caller should call freemsg(9F) to free the pointer to the message block.


The putbq() function can be called from user, interrupt, or kernel context.


See the bufcall(9F) function page for an example of putbq().

See also

bcanput(9F), bufcall(9F), canput(9F), getq(9F), putq(9F)

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