Conditional Breakpoints

Breakpoints can also be set to occur only if a certain condition is met. By providing a command, the breakpoint will be taken only if the count is reached or the command returns zero. For example, a breakpoint that occurs only on certain I/O controls could be set in the driver's ioctl(9E) routine. This is the general syntax of conditional breakpoints:

    address,count:b command

In this example, address is the address at which to set the breakpoint. count is the number of times the breakpoint should be ignored (note that 0 means break only when the command returns zero). command is the kadb(1M) command to execute.

Here is an example of breaking only in the sdioctl() routine if the DKIOGVTOC (get volume table of contents) I/O control occurs.

kadb[0]: sdioctl+4,0:b <i1-0x40B
kadb[0]: $b
breakpoints
count   bkpt            type      len   command
0       sdioctl+4       :b instr  4     <i1-0x40B
kadb[0]: :c     

Adding four to sdioctl skips to the second instruction in the routine, bypassing the save instruction that establishes the stack. The `<i1' refers to the first input register, which is the second parameter to the routine (the cmd argument of ioctl(9E)). The count of zero is impossible to reach, so it stops only when the command returns zero, which is when `i1 - 0x40B' is true. This means i1 contains 0x40B (the value of the ioctl(9E) command, determined by examining the ioctl definition).

To force the breakpoint to be reached, the prtvtoc(1M) command, which is known to issue the following I/O control, is used:

# prtvtoc /dev/rdsk/c0t0d0s0
breakpoint at:
sdioctl+4:      mov     %i5, %l0
kadb[0]: $c
sdioctl(800000,40b,ffbefb54,100005,704a3ce8,4026bc7c) + 4
ioctl(3,40b,70ca27b8,40b,ffbefb54,0) + 1e0