$ Linux Command

Processes

pstack

Print stack traces of a running process.

stacktraceprocessdebuggingthreads

Additional Notes

pstack attaches to a running process and prints a stack trace for each thread. It shows the function call chain at the point of inspection, including function names and addresses. This is useful for understanding what a process is doing at a specific moment, diagnosing hangs, deadlocks, or unexpected behavior.

pstack works by using ptrace to attach to the target process, reading its stack frames, and symbolizing the addresses using the process's symbol table. On modern systems, pstack is often a shell script that invokes gdb with a batch command to dump thread backtraces.

Syntax

pstack [options] [pid...]

Parameters

  • pid: Process ID(s) to examine.

Common Options

  • -p pid: Specify the PID (alternative syntax).
  • -w: Wait if the process is stopped.
  • -D: Show debug information.
  • -M: Enable multi-process mode.
  • --help: Show help and exit.
  • --version: Show version information.

Examples

pstack 1234

Print the stack trace of PID 1234.

pstack $(pidof mysqld)

Stack trace of all mysqld processes.

pstack -p 5678

Stack trace using the -p syntax.

for pid in $(pidof java); do pstack $pid >> java_stacks.log; done

Collect stack traces from all Java processes over time for analysis.

Practical Notes

  • pstack is part of the gdb or pstack package. Install with sudo apt install gdb or sudo apt install pstack.
  • The target process must have read permissions. pstack typically needs the same user or root.
  • Stacks from optimized code may show incomplete or incorrect backtraces due to inlining and frame pointer omission.
  • For Java processes, use jstack instead, which understands JVM internals and produces Java-level stack traces.
  • Continuous sampling with pstack can help identify where a process is spending time. Collect multiple samples over a period.
  • For kernel-level stack traces of threads, use /proc/PID/stack or cat /proc/PID/wchan.
  • The gdb-based pstack script is more portable than compiled alternatives, but it is slower because it starts gdb for each invocation.