Berkeley CSUA MOTD:Entry 41104
Berkeley CSUA MOTD
 
WIKI | FAQ | Tech FAQ
http://csua.com/feed/
2025/05/24 [General] UID:1000 Activity:popular
5/24    

2005/12/21-23 [Computer/SW/Compilers, Computer/SW/Languages/C_Cplusplus] UID:41104 Activity:nil
12/21   I have a complex chuck of C code, and somewhere it in, I'm freeing
        a bad pointer.  However, this doesn't happen if I run it under
        gdb.  Does anyone know how I can figure out where it's crashing?
        \_ Valgrind is usually pretty good at finding this stuff.  It's slow,
           though.  If the program runs under FreeBSD, you can try running it
           with MALLOC_OPTIONS=AJX.
        \_ Is it running in gdb that causes the crash or is it running
           a debug build that causes the crash?  Debug builds null all
           memory at allocation, I believe there is a way you can tell
           gcc to set all the memory to some other value than null instead.
           If all memory is nulled then freeing that null value will do
           nothing, but in the non debug mode you will free a random int
           and boom, your code will crash.
           \_ No, running in gdb DOESN'T crash.  Running it normally does.
              \_ It's obviously freeing an uninitialized pointer.  In a
                 a debug build all allocated memory is nulled so you are
                 going to free null, which is valid and doesn't cause a
                 crash.  There is a way to make debug builds (or maybe gdb)
                 not null out memory.  It has been a while since I debugged
                 c, but I know for a fact that is your issue.
              \_ try attaching to the pid after it starts?
        \_ gdb the core dump
           \_ I don't seem to have a core dump.  It just seg faults. (on
              linux)  Is there a way to force a core dump when it frees a
              bad pointer?  (Sometimes it gives an error: free(): invalid
              pointer 0x9091420!)
              \_ Don't segfaults dump core?  Do you have ulimit set to 0?
                 \_ ulimit in unlimited.  Apprently they don't always
                    dump.
                    \_ Is it setuid?  -tom
                       \_ no.
        \_ Above suggestion, and this sort of failure-to-reproduce could mean
           the bug is in something time-dependant, either using a clock, net IO
           or bad synchronization between threads.  Heisenbugs suck.
           \_ I think it's because I have lots of if(x) {free(x);} stuff.
              I could see this happening if I didn't initialize all my
              pointers to NULL.  (I understand gdb helpfully
              automatically initilizes pointers to NULL for you.) However,
              I can't find any uninitialized variables.
              \_ if (x) { free(x); } is pointless if you're using the standard
                 C library.  free(NULL) is a no-op.  Also, if it's a Heisenbug,
                 check that you're not overflowing stack frames.  If you're
                 worried about uninitialized variables, make sure you compile
                 with all compiler warnings on.
                 \_ Good point.
        \_ How about doing some good old-fashioned trace logging to pinpoint
           the crash?  Unless trace logging prevents the crash from happening
           as well!
2025/05/24 [General] UID:1000 Activity:popular
5/24    

You may also be interested in these entries...
2014/1/14-2/5 [Computer/SW/Languages/C_Cplusplus] UID:54763 Activity:nil
1/14    Why is NULL defined to be "0" in C++ instead of "((void *) 0)" like in
        C?  I have some overloaded functtions where one takes an integer
        parameter and the other a pointer parameter.  When I call it with
        "NULL", the compiler matches it with the integer version instead of
        the pointer version which is a problem.  Other funny effect is that
        sizeof(NULL) is different from sizeof(myPtr).  Thanks.
	...
2011/3/7-4/20 [Computer/SW/Languages/C_Cplusplus] UID:54056 Activity:nil
3/7     I have a C question.  I have the following source code in two identical
        files t.c and t.cpp:
                #include <stdlib.h>
                int main(int argc, char *argv[]) {
                  const char * const * p1;
                  const char * * p2;
	...
2010/1/22-30 [Computer/HW/Laptop, Computer/SW/OS/OsX] UID:53655 Activity:high
1/22    looking to buy a new development laptop
        needs ssdrive, >6 hr possible batt life, and runs linux reasonably
        Anyone have a recommendation? Thx.
        \_ thinkpad t23 w ssdrive and battery inplace of drive bay
        \_ Ever wondered what RICHARD STALLMAN uses for a laptop?  Well,
           wonder no more!
	...
2009/8/31-9/9 [Computer/SW/Compilers] UID:53312 Activity:nil
8/31    I'm trying to learn ActionScript, like a step by step tutorial.
        The site at http://www.actionscript.org/resources/categories/Tutorials/Flash/Beginner
        isn't well organized. It doesn't explain how to get started with
        an editor, compiler, IDE. And should I even learn AS2 when you can
        learn AS3? Is Adobe Flash CS4 >>> CS3 or just CS4 > CS3?
	...
2009/2/28-3/11 [Computer/SW/Compilers] UID:52661 Activity:nil
2/28    I'm looking for a recommendation of a compiler/IDE to use to
        develop C/C++ code under Linux. In school, we used jove/gcc and
        I still use emacs/vi and gcc to this day. However, it is really
        lacking. Under Windows I tried Visual Studio and there were some
        really nice things about it, although it was so overwhelming that
        after 6 months of occasional use I still didn't really know what I
	...
2008/6/9-12 [Computer/SW/Languages/C_Cplusplus, Computer/SW/Security] UID:50194 Activity:nil
6/8     CSUA code guru please help. I need to see my random number
        generator with a good seed (I just need random 18 bit
        identifiers). The usual time(NULL) is OK, except my program
        might be invoked faster than once a second, and seeding using
        time() produced the same result. I tried clock() but it seems
        to return 0. My program needs to be run in Linux/DOS (Watcom
	...
2008/5/2-8 [Computer/SW/Compilers] UID:49874 Activity:low
5/2     How do I get the L1/L2 cache size and cache line size on my machine?
        Can I find this stuff out at compile time somehow?
        \_ You aren't planning on running your code on any other processors?
        \_ May I ask what it is you want to achieve ultimately? If you don't
           know your architecture and want to find out dynamically, there are
           tools that can peek/poke to give you definitive answers, plus you get
	...
2008/4/2-6 [Computer/SW/Languages/C_Cplusplus] UID:49645 Activity:moderate
4/2     Is there an interpreted version of C or C++ that can be used for
        educational purposes? It doesn't have to be full-featured or
        strictly adhere to the standards, but it's painful for students
        to change a variable in a for loop and then wait for a compile
        to see how it changes the result. Something really lightweight
        would encourage them to play around a lot more and learn more in
	...
2007/11/30-12/6 [Computer/SW/Compilers, Computer/HW/CPU] UID:48719 Activity:moderate
11/29   From the CSUA minutes:
        - Next Gen Console
        -- If we have $1800 in our accounts, should we buy a console:
           4 votes passes.
        -- Console voting: 2 votes each, neither passes
           * 360 = 600, more games
	...
2007/11/27-30 [Computer/SW/Languages/C_Cplusplus, Computer/SW/OS/Solaris] UID:48701 Activity:high
11/27   I'm using select to do a nonblocking check to see if a single socket
        has anything to read off it.  Problem is, I can have up to 12228
        file descriptors, and Linux fd_set only supports up to 4096.  Any idea
        what I can do about this?  (Or a better solution?) -jrleek
        \- 1. who are you
           2. i am busy this week and you didnt mention language
	...
2006/11/10-12 [Computer/SW/Compilers] UID:45316 Activity:nil
11/10   Is there anyway to get C/C++ compilers to automatically compile
        different code for different processors?  I'd like to be able to
        say something like:
          #if defined X86 ...
          #elif defined SPARC ...
          #else ...
	...
2013/4/9-5/18 [Computer/SW/Languages/C_Cplusplus, Computer/SW/Apps, Computer/SW/Languages/Perl] UID:54650 Activity:nil
4/04    Is there a good way to diff 2 files that consist of columns of
        floating point numbers, such that it only tells me if there's a
        difference if the numbers on a given line differ by at least a given
        ratio?  Say, 1%?
        \_ Use Excel.
           1. Open foo.txt in Excel.  It should convert all numbers to cells in
	...
2013/4/29-5/18 [Computer/SW/Languages/C_Cplusplus, Computer/SW/Compilers] UID:54665 Activity:nil
4/29    Why were C and Java designed to require "break;" statements for a
        "case" section to terminate rather than falling-through to the next
        section?  99% of the time poeple want a "case" section to terminate.
        In fact some compilers issue warning if there is no "break;" statement
        in a "case" section.  Why not just design the languages to have
        termination as the default behavior, and provide a "fallthru;"
	...
2012/7/19-11/7 [Computer/SW/Languages/C_Cplusplus] UID:54439 Activity:nil
7/19    In C or C++, how do I write the code of a function with variable
        number of parameters in order to pass the variable parameters to
        another function that also has variable number of parameters?  Thanks.
        \_ The usual way (works on gcc 3.0+, Visual Studio 2005+):
               #define foo(fmt, ...) printf(fmt, ##__VA_ARGS__)
           The cool new way (works on gcc 4.3+):
	...
2011/2/5-19 [Computer/SW/Languages/C_Cplusplus] UID:54027 Activity:nil
2/4     random C programming/linker fu question.  If I have
        int main() { printf("%s is at this adddr %p\n", "strlen", strlen); }
        and soda's /proc/sys/kernel/randomize_va_space is 2 (eg; on)
        why is strlen (or any other libc fn) at the same address every time?
        \_ I don't pretend to actually know the right answer to this, but
           could it have something to do with shared libraries?
	...
2010/2/12-3/9 [Computer/SW/Languages/C_Cplusplus] UID:53708 Activity:nil
2/12    I need a way to make a really big C++ executable (~200MBs) that does
        nothing.  No static initialization either.  Any ideas?
        \_ static link in lots of libraries?
        \_ #define a   i=0; i=0; i=0; i=0; i=0; i=0; i=0; i=0; i=0; i=0;
           #define b   a a a a a a a a a a
           #define c   b b b b b b b b b b
	...
2009/9/28-10/8 [Computer/SW/Languages/C_Cplusplus] UID:53409 Activity:nil
9/28    http://www.tiobe.com/index.php/content/paperinfo/tpci/index.html
        Java is #1!!! Followed by C, PHP, C++, Visual Basic, Perl,
        C#, Python, Javascript, then finally Ruby. The good news is
        Pascal is going waaaay back up!
        \_ C is still more popular than C++?  I feel much better about myself
           now.
	...
2009/8/7-14 [Computer/SW/Languages/C_Cplusplus, Computer/SW/Languages/Java] UID:53252 Activity:high
8/6     In C one can do "typedef int my_index_t;".  What's the equivalent in
        C#?  Thanks.
        \_ C#? Are you serious? Is this what the class of 2009 learn?
           \_ No.  I have to learn .NET code at work.  I am Class of '93.
           \_ python is what 2009 learns, see the motd thread about recent
              cal courses and languages
	...
2009/7/21-24 [Computer/SW/Languages/Java] UID:53168 Activity:moderate
7/20    For those who care btw, it looks like eclipse is now A Standard Tool
        at UCB ugrad cs, probably replaced emacs.  Furthermore, people get
        angry at seeing Makefiles, (since eclispe takes care of that).  I
        guess it's just a sign of the times.
        \_ The more people at my work use eclipse the less the code is
           managable in emacs.  I'm not sure which application's fault
	...