gdb useful tidbits/bookmarks

Some of these are hard to google for and I’ve used more than once now, so to save time I’m bookmarking them here.

1. Setting watch points in gdb
2. Displaying the whole string in gdb
3. Setting gdb to break at a particular file/line
4. Setting the coredump limit of a running process to catch a core

None of this is rocket science, but still useful.

1) Watchpoints

Setting a watchpoint;

(gdb) watch mm_dictionary.mmdict_head->index 
Hardware watchpoint 3: mm_dictionary.mmdict_head->index
(gdb) cont
Continuing.
[Switching to Thread 0x7ffff565c700 (LWP 15147)]
Hardware watchpoint 3: mm_dictionary.mmdict_head->index

Old value = 20
New value = 0
reporter_thread () at x_reporter.c:148

watch -l didn’t seem to work on my gdb, so I had to set the watch on the global scope variable.

Listing and removing watchpoints

(gdb) info watchpoints
Num     Type           Disp Enb Address            What
2       hw watchpoint  keep y                      *current->index
(gdb) clear
No breakpoint at this line.
(gdb) clear 2
No breakpoint at 2.
(gdb) delete 2
(gdb) info watchpoints
No watchpoints

reference:

https://sourceware.org/gdb/onlinedocs/gdb/Delete-Breaks.html#Delete-Breaks

https://sourceware.org/gdb/onlinedocs/gdb/Set-Watchpoints.html#Set-Watchpoints

2) Viewing the whole string

(gdb) set print elements 0

3) Setting GDB to break at a file:line number

This is obvious, in hindsight;

(gdb) break x_symdict.c:149
Breakpoint 5 at 0x402038: file x_symdict.c, line 149.
(gdb) info breakpoints
Num     Type           Disp Enb Address            What
3       hw watchpoint  keep y                      mm_dictionary.mmdict_head->index
    breakpoint already hit 1 time
5       breakpoint     keep y   0x0000000000402038 in add_mmdict at x_symdict.c:149
(gdb) delete 5
(gdb) info breakpoints
Num     Type           Disp Enb Address            What
3       hw watchpoint  keep y                      mm_dictionary.mmdict_head->index
    breakpoint already hit 1 time

4) Setting a core limit on a running process dynamically

echo -n "Max core file size=unlimited:unlimited" >/proc/12345/limits 

 Thank you; http://blogs.kent.ac.uk/unseenit/2013/06/06/changing-ulimit-for-running-processes/

 You were hard to find, so I’m putting you here for safe keeping.