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| How to get a human readable backgrace in Allegro 4 |
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larienna
Member #3,185
January 2003
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I ran into a buffer overflow bug this morning and it was hard to find why it crashed. The main problem is because we are in graphic mode, all the regular debugging tools does not work. strace: did not give much information that could be used. gdb: Freezes in graphic mode and never return to gdb, so I cannot display the backtrace or do any other commands. Valgrind: Ended up giving me a clue that I was writting outside a buffer. Allegro gave me a backtrace, but it looked like this: *** Error in `/mnt/data/git/glymmer/bin/glymmer': double free or corruption (!prev): 0x000056220431cea0 *** ======= Backtrace: ========= /lib/x86_64-linux-gnu/libc.so.6(+0x70bcb)[0x7f647571bbcb] /lib/x86_64-linux-gnu/libc.so.6(+0x76f96)[0x7f6475721f96] /lib/x86_64-linux-gnu/libc.so.6(+0x777de)[0x7f64757227de] /mnt/data/git/glymmer/bin/glymmer(+0x1bb3)[0x56220377ebb3] /mnt/data/git/glymmer/bin/glymmer(+0x10b8)[0x56220377e0b8] /mnt/data/git/glymmer/bin/glymmer(+0x11b0)[0x56220377e1b0] /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf1)[0x7f64756cb2b1] /mnt/data/git/glymmer/bin/glymmer(+0xeaa)[0x56220377deaa] It does not give me any filename, function name or lines of code. Not very useful. Debbuging symbols were activated. Else, I need to add tons of debugging message on the console and see where it fails. It's just long and annoying and it waste of time. Any suggestions? Enjoy! and have fun |
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Dizzy Egg
Member #10,824
March 2009
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Double free means you are trying to free a pointer to something that has already been free’d. So if you malloc something, then free it, then try and free it again. Set your pointers to null after freeing them, and then check pointers are not null before freeing them, and you should avoid double free errors.
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Chris Katko
Member #1,881
January 2002
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larienna said: gdb: Freezes in graphic mode and never return to gdb, so I cannot display the backtrace or do any other commands. That's not normal or right. You must be doing something pretty evil. I've never had it hang and I would immediately search for solutions to that. You can't solve problems if your tools don't work. When it does work, make sure you're looking at backtraces for the correct thread as it won't always start in the Allegro thread. Quote: *** Error in `/mnt/data/git/glymmer/bin/glymmer': double free or corruption (!prev): 0x000056220431cea0 *** Is "!prev" a function for a custom-made double linked list? Is this a C or C++ program? ala -----sig: |
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larienna
Member #3,185
January 2003
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I already found the bug, I was writting into a 2D dynamically allocated buffer, but I did not crop my values, so I was writting outside the limits of the buffer. I just clamped X and Y to the limits of the screen and it worked. It's just that I would have like to have a way to backtrace my program with meaningful information, but I can't. Only valgrind seems usable with graphic mode applications. Else I though that maybe if I run the program in windowed mode instead of fullscreen I could maybe have a debugger into another window. The idea is to avoid getting into a situation where you spend days trying to figure out a bug. So I was wondering if there was any tools that worked well with allegro? Enjoy! and have fun |
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Edgar Reynaldo
Major Reynaldo
May 2007
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Unless you are using two monitors and an extended desktop, you can't switch focus back to the gdb window am I correct? You have to use a window for debugging purposes, or an extended screen. My Website! | EAGLE GUI Library Demos | My Deviant Art Gallery | Spiraloid Preview | A4 FontMaker | Skyline! (Missile Defense) Eagle and Allegro 5 binaries | Older Allegro 4 and 5 binaries | Allegro 5 compile guide |
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larienna
Member #3,185
January 2003
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I made a test by using mode "GFX_AUTODETECT_WINDOWED" and run my program from gdb and it seems to work. I made a division by zero and could display a backtrace. So maybe I could have a kind of debug mode where I use the windowed mode instead to use GDB. Here is an example of backtrace results:
(gdb) bt
#0 0x0000555555555bc0 in gm_draw_dialog_stack (bmp=0x555555799af0,
engine=0x7fffffffde00) at /mnt/data/git/glymmer/src/lib/engine.c:112
#1 0x00005555555551b0 in main (argc=1, args=0x7fffffffdf58)
at /mnt/data/git/glymmer/src/demo/main.c:79
Enjoy! and have fun |
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