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Oracle Solaris Studio 12.2: Debugging a Program With dbx
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Document Information


1.  Getting Started With dbx

2.  Starting dbx

3.  Customizing dbx

4.  Viewing and Navigating To Code

5.  Controlling Program Execution

6.  Setting Breakpoints and Traces

7.  Using the Call Stack

8.  Evaluating and Displaying Data

9.  Using Runtime Checking

10.  Fixing and Continuing

11.  Debugging Multithreaded Applications

Understanding Multithreaded Debugging

Thread Information

Viewing the Context of Another Thread

Viewing the Threads List

Resuming Execution

Understanding Thread Creation Activity

Understanding LWP Information

12.  Debugging Child Processes

13.  Debugging OpenMP Programs

14.  Working With Signals

15.  Debugging C++ With dbx

16.  Debugging Fortran Using dbx

17.  Debugging a Java Application With dbx

18.  Debugging at the Machine-Instruction Level

19.  Using dbx With the Korn Shell

20.  Debugging Shared Libraries

A.  Modifying a Program State

B.  Event Management

C.  Command Reference


Understanding Multithreaded Debugging

When it detects a multithreaded program, dbx tries to load, a special system library for thread debugging located in /usr/lib.

dbx is synchronous; when any thread or lightweight process (LWP) stops, all other threads and LWPs sympathetically stop. This behavior is sometimes referred to as the “stop the world” model.

Note - For information on multithreaded programming and LWPs, see the Solaris Multithreaded Programming Guide.

Thread Information

The following thread information is available in dbx:

(dbx) threads
    t@1 a l@1  ?()  running   in main()
    t@2      ?() asleep on 0xef751450  in_swtch()
    t@3 b l@2  ?()  running in sigwait()
    t@4     consumer()  asleep on 0x22bb0 in _lwp_sema_wait()
  *>t@5 b l@4 consumer()  breakpoint     in Queue_dequeue()
    t@6 b l@5 producer()     running       in _thread_start()

For native code, each line of information is composed of the following:

For Java code, each line of information is composed of the following:

Table 11-1 Thread and LWP States

Thread and LWP States
The thread has been explicitly suspended.
The thread is runnable and is waiting for an LWP as a computational resource.
When a detached thread exits (thr_exit)), it is in a zombie state until it has rejoined through the use of thr_join(). THR_DETACHED is a flag specified at thread creation time (thr_create()). A non-detached thread that exits is in a zombie state until it has been reaped.
asleep on syncobj
Thread is blocked on the given synchronization object. Depending on what level of support libthread and libthread_db provide, syncobj might be as simple as a hexadecimal address or something with more information content.
The thread is active on an LWP, but dbx cannot access the LWP.
dbx cannot determine the state.
A bound or active thread state has the state of the LWP associated with it.
LWP was running but was stopped in synchrony with some other LWP.
syscall num
LWP stopped on an entry into the given system call #.
syscall return num
LWP stopped on an exit from the given system call #.
job control
LWP stopped due to job control.
LWP suspended
LWP is blocked in the kernel.
single stepped
LWP has just completed a single step.
LWP has just hit a breakpoint.
fault num
LWP has incurred the given fault #.
signal name
LWP has incurred the given signal.
process sync
The process to which this LWP belongs has just started executing.
LWP death
LWP is in the process of exiting.

Viewing the Context of Another Thread

To switch the viewing context to another thread, use the thread command. The syntax is:

thread [-blocks] [-blockedby] [-info] [-hide] [-unhide] [-suspend] [-resume] thread_id

To display the current thread, type:


To switch to thread thread_id, type:

thread thread_id

For more information on the thread command, see thread Command.

Viewing the Threads List

To view the threads list, use the threads command. The syntax is:

threads [-all] [-mode [all|filter] [auto|manual]]

To print the list of all known threads, type:


To print threads normally not printed (zombies), type:

threads -all

For an explanation of the threads list, see Thread Information.

For more information on the threads command, see threads Command.

Resuming Execution

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 (see call Command).

Consider the following two scenarios when debugging a multithreaded application where many threads call the function lookup():

When calling strcmp(), dbx would resume all threads for the duration of the call, which 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: