using the "return" command in a gdb script

Question

I'm trying to automate a test where I make a function return an error. But the gdb "return (int) 22" command does not seem to be executed, and the program exits with a success code. Is there something special that has to be done to make the "return" command work in a gdb script? Here is the gdb script I'm using:

set width 0
set height 0
set confirm off
set pagination off
set verbose off
set breakpoint pending on

set args the program args

break file.c:function_in_which_to_break
commands 1
    return 22
    continue
end

run the program args
quit

I would run the program with the following gdb command line:

gdb --batch --command=rc my_program

The desired behaviour is that "function_in_which_to_break" returns 22. The rest of the code will keep passing that up the call stack until the program exits and the exit code of the program should be 22.

The actual behaviour is that the exit code of the program is success.

When I run the program under gdb in a terminal (with command

gdb --args my_program the program args

), break at file.c:function_in_which_to_break by typing in "break file.c:function_in_which_to_break" and run, the program does break there. Then when I type in "return 22" and "continue", the program behaves as I expect, the function returns 22, the program returns 22 and the failure that I expect is shown.

UPDATE: When I said the program returns "success", I meant gdb reported that the child returned success. And when I said the program returns 22, I mean gdb reports that the child returns 22 (actually it says "exited with code 026"). gdb itself returns success in both cases.

UPDATE 2: I found some errors in the automation around the gdb invocation - looking for the "exited with code 026" in a file with misspelled name, calling a misspelled gdb-rc script, stupid things like that. Once those were fixed, the gdb script file with the return statement among the commands seems to work. So as soon as someone writes up an answer like "return commands should work the same as all the other commands" I will accept it. @ks1322

Community · Accepted Answer · 2020-06-20 09:12:55Z

1

The actual behaviour is that the exit code of the program is success.

According to documentation, you should also run gdb with -return-child-result option to get return code of the process being debugged:

-return-child-result

    The return code from GDB will be the return code from the child process (the process being debugged), with the following exceptions:

        GDB exits abnormally. E.g., due to an incorrect argument or an internal error. In this case the exit code is the same as it would have been without ‘-return-child-result’.
        The user quits with an explicit value. E.g., ‘quit 1’.
        The child process never runs, or is not allowed to terminate, in which case the exit code will be -1. 

    This option is useful in conjunction with ‘-batch’ or ‘-batch-silent’, when GDB is being used as a remote program loader or simulator interface.

UPDATE:
It appears that the real reason of success exit code of the program were some bugs in parsing gdb output. Anyway using -return-child-result and analyzing gdb exit code instead, is less error prone than parsing text output, I guess.

Is there something special that has to be done to make the "return" command work in a gdb script?

It appears that no, but there are exceptions for commands that resume execution, see How to print Entering and Leaving for a function in gdb command? for details.

edited Jun 20, 2020 at 9:12

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answered Sep 6, 2018 at 14:30

ks1322

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5 Comments

Brenda J. Butler Over a year ago

Thanks @ks1322, I updated the question. Although this answer does not answer my question it will be useful in the automation task I have ... If I understand correctly I will be able to make gdb return with the return code of the inferior.

Brenda J. Butler Over a year ago

To be clear: currently my test automation looks for the string 'exited with code 026' in the stdout of the gdb process, to determine if the desired outcome happened. If the child returns success, that string does not appear in the output and the test fails -- as I want, since I am testing behaviour under problematic conditions.

ks1322 Over a year ago

Try to run without -batch: it suppresses some of gdb output. Also what is the exit code of gdb when run with -return-child-result? Is it what you expect?

Brenda J. Butler Over a year ago

The exit code of gdb when run with -return-child-result is indeed the return code of the child being debugged. I saw it return 22 when the child returned 026. Running without --batch gave pretty much the same results. - In other news, I now have some tests working with the "return 22" in the script. There were various errors in the automation for the various attempts which have been corrected. If you were to write in an answer "the return function should work as well as any other gdb command in a script, look for your errors elsewhere" then I would accept it.

phil294 Over a year ago

It is really frustrating to open up man gdb and see no information about this option anywhere. Thank you

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