Use the cont command to resume program execution. Currently, threads use synchronous breakpoints, so all threads resume execution. However, you can resume a single thread using the call command with the –resumeone option.
Consider the following two scenarios when debugging a multithreaded application where many threads call the function lookup():
You set a conditional breakpoint:
stop in lookup -if strcmp(name, "troublesome") == 0
When t@1 stops at the call to lookup(), dbx attempts to evaluate the condition and calls strcmp().
You set a breakpoint:
stop in lookup
When t@1 stops at the call to lookup(), you issue the command:
call strcmp(name, "troublesome")
When calling strcmp(), dbx would resume all threads for the duration of the call, which is similar to what dbx does when you are single-stepping with the next command. It does so because resuming only t@1 has the potential to cause a deadlock if strcmp() tries to grab a lock that is owned by another thread.
A drawback to resuming all threads in this case is that dbx cannot handle another thread, such as t@2, hitting the breakpoint at lookup() whilestrcmp() is being called. It emits a warning like one of the following:
event infinite loop causes missed events in following handlers:
Event reentrancy first event BPT(VID 6, TID 6, PC echo+0x8) second event BPT(VID 10, TID 10, PC echo+0x8) the following handlers will miss events:
In such cases, if you can ascertain that the function called in the conditional expression will not grab a mutex, you can use the -resumeone event modifier to force dbx to resume only t@1:
stop in lookup -resumeone -if strcmp(name, "troublesome") == 0
Only the thread that hit the breakpoint in lookup() would be resumed in order to evaluate strcmp().
This approach does not help in cases such as the following examples:
If the second breakpoint on lookup() happens in the same thread because the conditional recursively calls lookup()
If the thread on which the conditional runs yields, sleeps, or in some manner relinquishes control to another thread