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4.9 Debugging Multiple Inferiors Connections and Programs

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). On some systems GDB may even let you debug several programs simultaneously on different remote systems. In the most general case, you can have multiple threads of execution in each of multiple processes, launched from multiple executables, running on different machines.

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. By default all inferiors are printed, but the argument id… – a space separated list of inferior numbers – can be used to limit the display to just the requested inferiors.

GDB displays for each inferior (in this order):

  1. the inferior number assigned by GDB
  2. the target system’s inferior identifier
  3. the target connection the inferior is bound to, including the unique connection number assigned by GDB, and the protocol used by the connection.
  4. the name of the executable the inferior is running.

An asterisk ‘*’ preceding the GDB inferior number indicates the current inferior.

For example,

(gdb) info inferiors
  Num  Description       Connection                      Executable
* 1    process 3401      1 (native)                      goodbye
  2    process 2307      2 (extended-remote host:10000)  hello

To get informations about the current inferior, use inferior:

inferior

Shows information about the current inferior.

For example,

(gdb) inferior
[Current inferior is 1 [process 3401] (helloworld)]

To find out what open target connections exist at any moment, use info connections:

info connections

Print a list of all open target connections currently being managed by GDB. By default all connections are printed, but the argument id… – a space separated list of connections numbers – can be used to limit the display to just the requested connections.

GDB displays for each connection (in this order):

  1. the connection number assigned by GDB.
  2. the protocol used by the connection.
  3. a textual description of the protocol used by the connection.

An asterisk ‘*’ preceding the connection number indicates the connection of the current inferior.

For example,

(gdb) info connections
  Num  What                        Description
* 1    extended-remote host:10000  Extended remote serial target in gdb-specific protocol
  2    native                      Native process
  3    core                        Local core dump file

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 ] [-no-connection ]

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.

By default, the new inferior begins connected to the same target connection as the current inferior. For example, if the current inferior was connected to gdbserver with target remote, then the new inferior will be connected to the same gdbserver instance. The ‘-no-connection’ option starts the new inferior with no connection yet. You can then for example use the target remote command to connect to some other gdbserver instance, use run to spawn a local program, etc.

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       Connection   Executable
* 1    process 29964     1 (native)   helloworld
(gdb) clone-inferior
Added inferior 2.
1 inferiors added.
(gdb) info inferiors
  Num  Description       Connection   Executable
* 1    process 29964     1 (native)   helloworld
  2    <null>            1 (native)   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.

Occasionally, 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):

  1. the program space number assigned by GDB
  2. the name of the executable loaded into the program space, with e.g., the 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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