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14.13 Debugging multiple processes ( programs with fork and vfork
calls)
14.13.1 Ask mode for set follow-fork-mode
The ask command prompts the user to select between parent and child as the debugger
response to a program call of fork/vfork. Based on the user selection, the parent or
the child process is debugged.
For example,
(gdb) set follow-fork-mode ask (gdb) show follow-fork-mode
The debugger response to a program call to fork or vfork is ask.
(gdb) run Starting program: sample [New process 4941] Select follow-fork-
mode: [0] parent [1] child
14.13.2 Serial mode for set follow-fork-mode
The option serial, for the follow-fork-mode command, enables debugging of a
parent and child process within a debugger session. During a debug session, if the
parent process forks a child, the debugger starts debugging the child process. When
the child process exits, the debugger switches back to the parent process. The
follow-fork-mode will work only if there is a wait() call by the parent process. This
feature is enabled by setting the follow-fork-mode flag to serial, as specified in the
following example:
(gdb) set follow-fork-mode serial
The follow-fork-mode is not supported under following conditions:
• MxN threaded programs
• Parent process is 32-bit and child process is 64-bit and vice versa. For the follow-
fork-mode to work both parent and child process must be of the same mode.
14.13.3 Support for showing unwind info
The maint info unwind command prints the unwind information for the regions
unwinded at the given address expression. Usage:
maint info unwind exp
where, exp is an address expression.
For example,
(gdb) maint info unwind $pc
modsched:
0x4000930 .. 0x4000a20, end_prologue@0x4000970
Info block version:0x0, flags:0x0, length:4 * 4 == 16
0x40172b20: (0c) R1prologue rlen=12
0x40172b21: (e8) P7preds_when t=11
0x40172b23: (b1) P3preds_gr gr=41
0x40172b25: (ea) P7lc_when t=7
14.13 Debugging multiple processes ( programs with fork and vfork calls) 195