How to load program reading stdin and taking parameters in gdb?

Question

I have a program that takes input from stdin and also takes some parameters from command line. It looks like this:

cat input.txt > myprogram -path "/home/user/work"

I try to debug the code with gdb inside emacs, by M-x gdb, I try to load the program with the command:

gdb cat input.txt > myprogram -path "/home/user/work"

However, gdb does not like it.

Question cribbed from here. Unfortunately I don't understand the solution and am not sure what to do beyond compiling with the -g option and running the command M-x gdb.

Solution 1

If you were doing it from a shell you'd do it like this:

% gdb myprogram
gdb> run params ... < input.txt

This seems to work within emacs too.

Solution 2

There are several ways to do it:

$ gdb myprogram
(gdb) r -path /home/user/work < input.txt

or

$ gdb myprogram
(gdb) set args -path /home/user/work < input.txt
(gdb) r

or

$ gdb -ex 'set args -path /home/user/work < input.txt' myprogram
(gdb) r

where the gdb run command (r) uses by default the arguments as set previously with set args.

Solution 3

For completeness' sake upon starting a debugging session there is also the --args option. ie)

gdb gdbarg1 gdbarg2 --args yourprog arg1 arg2 -x arg3

Solution 4

This is eleven years later, and this question has an answer already, but for someone just like me in the future, I just wanted to add some thing.

After you run gdb your_program, if you just type run < file_containing_input, the program will just run till the end, and you might not catch the problem, so before you do run < file_containing_input do a break. Something like this

$ gdb your_program
gdb> break main
gdb> run < file_containing_input

Solution 5

And if you do not need to debug from the very beginning you can also attach to an already running process by using:

$ gdb myprogram xxx

where xxx is the process id. Then you do not need to tell gdb the starting arguments.

View more solutions

Comments

[vinc456]

The redirection seems to work but I get some errors. Failed to read a valid object file image from memory. Program exited with code 042. Any ideas?

[Alnitak]

That's likely a general GDB error, and probably nothing to do with the fact you're running within emacs. Learn how to run GDB from a shell first (with a new question if necessary), and then worry about running it inside emacs.

[vinc456]

I figured it out. For some reason I typed void main(int argc, char *argv[]) instead of "int main..." and it slipped my eye. Anyways everything works fine now; thanks for your help!

[Deleted]

A belated thank you - the gdb manual is a pain in the butt to dredge through.

[Peter Ajtai]

How would you redirect input.txt as an input to yourprog upon starting a debugging session like this?

[Notinlist]

You missed to answer to the question title, at the part "reading stdin". I would make a good comment somewhere if it were shorter.

[cardiff space man]

When I try this with gdb in cygwin, it doesn't work. The "show args" command shows that I entered the args I wanted, but when I start the program with "r", the program waits until I type stuff instead of reading from the specified file.

[maxschlepzig]

@cardiffspaceman, well, I can't test it with Cygwin - perhaps their gdb version is somehow limited

[Ben Elgar]

@Peter: gdb --args yourprog.out input.txt

[Ruslan]

Why not simply gdb -ex 'r -path /home/user/work < input.txt' myprogram in the third variant?

[maxschlepzig]

@Ruslan, works as well - using 'set args ...' just gives you the chance to interactively define some break points etc. before running the program

[Ruslan]

True, but you can also set the breakpoint non-interactively, e.g. gdb -ex 'b main' -ex 'r -path /home/user/work < input.txt' myprogram.

[maxschlepzig]

@Ruslan, sure, nobody implied that you cannot do that. Sometimes you want to do it interactively. Sometimes you don't. I think the answer contains enough examples such that this is all pretty obvious.

[ixe013]

On Windows using msys64 I get < and input.txt as argv arguments to my program :( I'll keep digging around these answers with my gdb 8.2.1 : stackoverflow.com/questions/3544325/…

[Alnitak]

That only works if "yourprog" expects a file name to specify the input, not input redirection.