using STDOUT from within gdb

Question

I have a function that pretty prints a data structure, its function prototype is:

void print_mode(FILE *fp, Mode *mode);

the FILE* allows you to redirect the output to anywhere you want, e.g. stdout, stderr, a file etc. Mode is the data structure

I am trying to call this function from within gdb and want the output to be directed to the gdb console window, stdout?

I have tried:

(gdb) p print_mode(STDOUT,fragment_mode)
No symbol "STDOUT" in current context.
(gdb) p print_mode(stdout,fragment_mode)
$17 = void

neither of which work

any ideas how i can get the output of the function to display in the gdb console?

should add - I am using gdb within emacs 24.2.1 under linux

Try looking at info terminal, run > redirect-file and tty device-name. There will be device names that are the equivalent of stdin, stdout, ala /dev/tty — Brian Tiffin
– Brian Tiffin, Commented Dec 11, 2013 at 15:16
Try call print_mode(stdout,fragment_mode) instead of p print_mode(stdout,fragment_mode)? — anishsane
– anishsane, Commented Dec 11, 2013 at 15:16
@anishsane: In terms what the program itself prints out print and call shouldn't make any difference. — alk
– alk, Commented Dec 11, 2013 at 18:30
Let's check whether stdout has a sensible value. Does call fprintf(stdout, "test %d\n", 5) within gdb produce output? — Mark Plotnick
– Mark Plotnick, Commented Dec 11, 2013 at 18:44

alk · Accepted Answer · 2019-10-11 10:20:24Z

2

STDOUT seems to be macro, which is not know to GDB, as handled prior to compilation by the pre-preprocessor.

Using stdout should do the job.

However the function print_mode() simply does not seem to print out anything.

In terms what's being printed to the console by the program being debugged, GDB's commands printand call should not make a difference.

For details on this you might like to read here: https://sourceware.org/gdb/onlinedocs/gdb/Calling.html

An issue might be that stdout by default is line buffered, so output would not occur before detecting a linefeed and print_mode() perhaps does not send a linefeed (\n).

To test this just use stderr as output file, as the latter isn't buffered:

p print_mode(stderr, fragment_mode)

edited Oct 11, 2019 at 10:20

answered Dec 11, 2013 at 18:07

alk

71.2k10 gold badges112 silver badges274 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

bph · Accepted Answer · 2013-12-12 10:32:22Z

0

Oh dear - silly mistake. You're right, stdout does do the job.

I forgot that having upgraded from emacs 23 to 24, the way gdb works has changed in as much as it now opens a separate buffer *input/output of program-name* to which it redirects the output of the program being debugged. In the prior version of emacs it was all displayed in the same, single gdb buffer.

So my second attempt was actually working, I was just looking in the wrong place so didn't see the output

answered Dec 12, 2013 at 10:32

bph

11.4k20 gold badges77 silver badges140 bronze badges

Collectives™ on Stack Overflow

using STDOUT from within gdb

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related