GDB Scripting : A short article for a internal magazine
I wrote a small article for a internal magazine and few of my friends wanted me to post it to this blog.
This is for people who are new to GDB and still exploring itz features. So if you’ve used GDB for more than few weeks please ignore and skip🙂
GDB – Scripting
A good majority of novice programmers tend to use printf functions to trace function calls and to printout the debug data. This forces you to change the code and compile again and again. To eliminate these superfluous tasks from your day-to-day work, use GDB. The GDB has facilities for scripting and helps in saving plenty of your time.
Tracing Function Calls
If you want to know whether a function is called or not, create a break point and write a simple script.
#Set the breakpoint
(gdb) b mapi_sync
Breakpoint 1 at 0x7fffd75f36e2: file camel-mapi-folder.c, line 741
#Tell GDB what to do when the breakpoint is reached
Type commands for when breakpoint 1 is hit, one per line.
End with a line saying just “end”.
continue – Come out of break and continue
end – terminate command list
Run the program now. The GDB prints the function name when the breakpoint is hit and automatically continues running the program.
Breakpoint 1, mapi_sync (folder=0xc9c1a0, expunge=0, ex=0xf3a0c0) at camel-mapi-folder.c:741
741 CamelMapiStore *mapi_store = CAMEL_MAPI_STORE (folder->parent_store);
Using the GDB Scripts for Analyzing the Data
Suppose that you have a singly-linked list that has strings in it. At some point, you might want to know the contents of the list. To do this, use the GDB scripting instead of adding the debug statements in your code.
#Example for gslist traversal.
set $list = ($arg0)
while ((GSList *)$list->next != 0)
p (char *)(GSList *)$list->data
set $list = (GSList *)$list->next
p_gslist_str <list>: Dumps the strings in a GSList
Add the above snippet into a file and load it into the GDB as follows:
(gdb) source /home/jjohnny/scripts/gdb/gslist.gdb
Now, anywhere you want to take a look in the GSList, simply break and
(gdb) p_gslist_str server_uid_list
$17 = 0x7fffd81101b0 “7666BC1E000000015870BD1E00000001”
$18 = 0x7fffd810e330 “7666BC1E000000015970BD1E00000001”
$19 = 0x7fffd810cbe0 “7666BC1E000000015C70BD1E00000001”
Simple scripts thus can save you a lot of time from adding or removing the debugging statements from your code. Now go ahead and create a suite of scripts to aid the library you are writing.
More cool developer tricks later. Have fun !
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Thanks to Radhika for editing the article.