gdb to list process/thread information (state, command, stack..etc )

Do you know, how powerful are debuggers ? It has lots of capabilities to track debugged program.. Here in this blog, I will describe some of the commands which are very handy with “GDB”..

I was tracking down a running program in my setup with gdb ( gdb -p), it is nothing but “qemu-kvm”.. which is a “multithreaded application”..

I will show some example gdb commands below.

“info threads” will show you the threads of debugged program…

(gdb) info threads
 4 Thread 0x7f7a6ee2b700 (LWP 8120)  0x000000392a8d82a7 in ioctl () from /lib64/libc.so.6
 3 Thread 0x7f7a6e62a700 (LWP 8121)  0x000000392a8d82a7 in ioctl () from /lib64/libc.so.6
 2 Thread 0x7f7a67875700 (LWP 8127)  0x000000392ac0b82f in pthread_cond_timedwait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
 * 1 Thread 0x7f7a6f039980 (LWP 8119)  0x000000392a8d89a3 in select () from /lib64/libc.so.6

below is shortform of "thread apply all bt" command which will show you back trace of all the threads..

(gdb) thr a a bt

Thread 4 (Thread 0x7f7a6ee2b700 (LWP 8120)):
 #0  0x000000392a8d82a7 in ioctl () from /lib64/libc.so.6
 #1  0x0000000000429869 in kvm_run (env=0x1a50580) at /usr/src/debug/qemu-kvm-0.13.0/qemu-kvm.c:610
 #2  0x000000000042a669 in kvm_cpu_exec (env=) at /usr/src/debug/qemu-kvm-0.13.0/qemu-kvm.c:1238
 #3  0x000000000042a8a5 in kvm_main_loop_cpu (_env=0x1a50580) at /usr/src/debug/qemu-kvm-0.13.0/qemu-kvm.c:1495
 #4  ap_main_loop (_env=0x1a50580) at /usr/src/debug/qemu-kvm-0.13.0/qemu-kvm.c:1541
 #5  0x000000392ac07b41 in start_thread () from /lib64/libpthread.so.0
 #6  0x000000392a8df49d in clone () from /lib64/libc.so.6

Thread 3 (Thread 0x7f7a6e62a700 (LWP 8121)):
 #0  0x000000392a8d82a7 in ioctl () from /lib64/libc.so.6
 #1  0x0000000000429869 in kvm_run (env=0x1a6c080) at /usr/src/debug/qemu-kvm-0.13.0/qemu-kvm.c:610
 #2  0x000000000042a669 in kvm_cpu_exec (env=) at /usr/src/debug/qemu-kvm-0.13.0/qemu-kvm.c:1238
 #3  0x000000000042a8a5 in kvm_main_loop_cpu (_env=0x1a6c080) at /usr/src/debug/qemu-kvm-0.13.0/qemu-kvm.c:1495
 #4  ap_main_loop (_env=0x1a6c080) at /usr/src/debug/qemu-kvm-0.13.0/qemu-kvm.c:1541
 #5  0x000000392ac07b41 in start_thread () from /lib64/libpthread.so.0
 #6  0x000000392a8df49d in clone () from /lib64/libc.so.6
Thread 2 (Thread 0x7f7a67875700 (LWP 8127)):
 #0  0x000000392ac0b82f in pthread_cond_timedwait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
 #1  0x000000000043d4bb in cond_timedwait (unused=) at posix-aio-compat.c:103
 #2  aio_thread (unused=<value optimized out>) at posix-aio-compat.c:324
 #3  0x000000392ac07b41 in start_thread () from /lib64/libpthread.so.0
 #4  0x000000392a8df49d in clone () from /lib64/libc.so.6
Thread 1 (Thread 0x7f7a6f039980 (LWP 8119)):
 #0  0x000000392a8d89a3 in select () from /lib64/libc.so.6
 #1  0x00000000004196d4 in main_loop_wait (nonblocking=) at /usr/src/debug/qemu-kvm-0.13.0/vl.c:1291
 #2  0x000000000042b067 in kvm_main_loop () at /usr/src/debug/qemu-kvm-0.13.0/qemu-kvm.c:1710
 #3  0x000000000041a51e in main_loop (argc=, argv=, envp=)

"thread <thread number>" this will help you to switch between "threads"...

(gdb) thread 3

[Switching to thread 3 (Thread 0x7f7a6e62a700 (LWP 8121))]#0 0x000000392a8d82a7 in ioctl () from /lib64/libc.so.6

(gdb)

Suppose you want to show "stat" or any related information of a process, you can print it in "gdb" as shown below..

** 8127" is the process id.. Here "STATE" says "t" ..

because it is being traced by the debugger..

(gdb) info proc 8127 stat

process 8127

cmdline = '/usr/bin/qemu-kvm'

cwd = '/'

exe = '/usr/bin/qemu-kvm'

Process: 8127

Exec file: qemu-kvm

State: t

Parent process: 1

Process group: 8118

Session id: 8118

TTY: 0

TTY owner process group: -1

Flags: 0x84020c0

Minor faults (no memory page): 11392

Minor faults, children: 0

Major faults (memory page faults): 1

Major faults, children: 0

utime: 344

stime: 427

utime, children: 0

stime, children: 0

jiffies remaining in current time slice: 20

'nice' value: 0

jiffies until next timeout: 4

jiffies until next SIGALRM: 0

start time (jiffies since system boot): 1005403

Virtual memory size: 892162048

Resident set size: 10937

rlim: 18446744073709551615

Start of text: 0x400000

End of text: 0x659d4c

Start of stack: 0x7fff86b785b0

(gdb) info proc 1 stat

process 1

cmdline = '/sbin/init'

cwd = '/'

exe = '/sbin/upstart'

Process: 1

Exec file: init

State: S

Parent process: 0

Process group: 1

Session id: 1

TTY: 0

TTY owner process group: -1

Flags: 0x402100

Minor faults (no memory page): 2662

Minor faults, children: 4307085

Major faults (memory page faults): 11

Major faults, children: 808

utime: 6

stime: 116

utime, children: 210588

stime, children: 24321

jiffies remaining in current time slice: 20

'nice' value: 0

jiffies until next timeout: 1

jiffies until next SIGALRM: 0

start time (jiffies since system boot): 2

Virtual memory size: 19865600

Resident set size: 381

rlim: 18446744073709551615

Start of text: 0x400000

End of text: 0x421dc6

 Start of stack: 0x7fffa2558780

           I thought of printing information of "init" process :

(gdb) info proc 1 stat

process 1

cmdline = '/sbin/init'

cwd = '/'

exe = '/sbin/upstart'

Process: 1

Exec file: init

State: S

Parent process: 0

Process group: 1

Session id: 1

TTY: 0

TTY owner process group: -1

Flags: 0x402100

Minor faults (no memory page): 2662

Minor faults, children: 4307085

Major faults (memory page faults): 11

Major faults, children: 808

utime: 6

stime: 116

utime, children: 210588

stime, children: 24321

jiffies remaining in current time slice: 20

'nice' value: 0

jiffies until next timeout: 1

jiffies until next SIGALRM: 0

start time (jiffies since system boot): 2

Virtual memory size: 19865600

Resident set size: 381

rlim: 18446744073709551615

Start of text: 0x400000

End of text: 0x421dc6

 Start of stack: 0x7fffa2558780

Thats it guys.. I hope this helps you when playing with debuggers.!!! Njoy!!!

[Note: modified..]

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