| Debugging with GDB: Inferiors and Programs |
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GDB lets you run and debug multiple programs in a single session. In addition, GDB on some systems may let you run several programs simultaneously (otherwise you have to exit from one before starting another). In the most general case, you can have multiple threads of execution in each of multiple processes, launched from multiple executables.
GDB represents the state of each program execution with an object called an inferior. An inferior typically corresponds to a process, but is more general and applies also to targets that do not have processes. Inferiors may be created before a process runs, and may be retained after a process exits. Inferiors have unique identifiers that are different from process ids. Usually each inferior will also have its own distinct address space, although some embedded targets may have several inferiors running in different parts of a single address space. Each inferior may in turn have multiple threads running in it.
To find out what inferiors exist at any moment, use info inferiors
:
info inferiors
Print a list of all inferiors currently being managed by GDB.
GDB displays for each inferior (in this order):
An asterisk ‘*’ preceding the GDB inferior number indicates the current inferior.
For example,
(gdb) info inferiors Num Description Executable 2 process 2307 hello * 1 process 3401 goodbye
To switch focus between inferiors, use the inferior command:
inferior infno
Make inferior number infno the current inferior. The argument infno is the inferior number assigned by GDB, as shown in the first field of the ‘info inferiors’ display.
The debugger convenience variable ‘$_inferior’ contains the number of the current inferior. You may find this useful in writing breakpoint conditional expressions, command scripts, and so forth. See Convenience Variables, for general information on convenience variables.
You can get multiple executables into a debugging session via the
add-inferior and
clone-inferior
commands. On some
systems GDB can add inferiors to the debug session
automatically by following calls to fork and exec. To
remove inferiors from the debugging session use the
remove-inferiors
command.
add-inferior [ -copies n ] [ -exec executable ]
Adds n inferiors to be run using executable as the
executable; n defaults to 1. If no executable is specified,
the inferiors begins empty, with no program. You can still assign or
change the program assigned to the inferior at any time by using the
file command with the executable name as its argument.
clone-inferior [ -copies n ] [ infno ]
Adds n inferiors ready to execute the same program as inferior infno; n defaults to 1, and infno defaults to the number of the current inferior. This is a convenient command when you want to run another instance of the inferior you are debugging.
(gdb) info inferiors Num Description Executable * 1 process 29964 helloworld (gdb) clone-inferior Added inferior 2. 1 inferiors added. (gdb) info inferiors Num Description Executable 2 <null> helloworld * 1 process 29964 helloworld
You can now simply switch focus to inferior 2 and run it.
remove-inferiors infno…
Removes the inferior or inferiors infno…. It is not
possible to remove an inferior that is running with this command. For
those, use the kill or detach command first.
To quit debugging one of the running inferiors that is not the current
inferior, you can either detach from it by using the detach inferior
command (allowing it to run independently), or kill it
using the kill inferiors
command:
detach inferior infno…
Detach from the inferior or inferiors identified by GDB
inferior number(s) infno…. Note that the inferior’s entry
still stays on the list of inferiors shown by info inferiors,
but its Description will show ‘<null>’.
kill inferiors infno…
Kill the inferior or inferiors identified by GDB inferior
number(s) infno…. Note that the inferior’s entry still
stays on the list of inferiors shown by info inferiors, but its
Description will show ‘<null>’.
After the successful completion of a command such as detach,
detach inferiors, kill or kill inferiors, or after
a normal process exit, the inferior is still valid and listed with
info inferiors, ready to be restarted.
To be notified when inferiors are started or exit under GDB’s
control use set print inferior-events
:
set print inferior-events
set print inferior-events on
set print inferior-events off
The set print inferior-events command allows you to enable or
disable printing of messages when GDB notices that new
inferiors have started or that inferiors have exited or have been
detached. By default, these messages will not be printed.
show print inferior-events
Show whether messages will be printed when GDB detects that inferiors have started, exited or have been detached.
Many commands will work the same with multiple programs as with a
single program: e.g., print myglobal will simply display the
value of myglobal in the current inferior.
Occasionaly, when debugging GDB itself, it may be useful to
get more info about the relationship of inferiors, programs, address
spaces in a debug session. You can do that with the maint info program-spaces
command.
maint info program-spaces
Print a list of all program spaces currently being managed by GDB.
GDB displays for each program space (in this order):
file command.
An asterisk ‘*’ preceding the GDB program space number indicates the current program space.
In addition, below each program space line, GDB prints extra information that isn’t suitable to display in tabular form. For example, the list of inferiors bound to the program space.
(gdb) maint info program-spaces
Id Executable
* 1 hello
2 goodbye
Bound inferiors: ID 1 (process 21561)
Here we can see that no inferior is running the program hello,
while process 21561 is running the program goodbye. On
some targets, it is possible that multiple inferiors are bound to the
same program space. The most common example is that of debugging both
the parent and child processes of a vfork call. For example,
(gdb) maint info program-spaces
Id Executable
* 1 vfork-test
Bound inferiors: ID 2 (process 18050), ID 1 (process 18045)
Here, both inferior 2 and inferior 1 are running in the same program
space as a result of inferior 1 having executed a vfork call.
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