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I am a new C programmer and the pointers + arrays are messing me over a lot. I don't get any errors and the code runs fine...until somewhere in the middle the code starts acting up because some element in an array is set to something other than I wanted it to be. I wanted to know if there is a program that visualizes the memory of the code after it has finished running?

E.g after I run:

#include <stdio.h> int main(){ int array[2] = {0,1}; array[1] = 4; printf("%d\n",array[1]); }

It will show a block of memory where array has two elements with 0 and 4 .

Right now to avoid running into a problem where the array contains elements from previous action I clear the memory of that array by doing:

memset(tokenized,0,MAX_CHARS);

It seems to work, but I don't know if its actually doing what I think it's doing at the back end.

Edit: I am using Valgrind now and I just want to know, how do I know what line the error is referring to? For example I got this:

==24394== Source and destination overlap in strncpy(0x7ff000006, 0x7ff000006, 6) ==24394== at 0x4C2C236: strncpy (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so) ==24394== by 0x400D8A: tokenize_quotes (in /home/zee/Documents/seng265/repos/assignment2/a.out) ==24394== by 0x40184E: main (in /home/zee/Documents/seng265/repos/assignment2/a.out) ==24394== ==24394== Conditional jump or move depends on uninitialised value(s) ==24394== at 0x4C2C007: strcpy (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so) ==24394== by 0x400E06: tokenize_quotes (in /home/zee/Documents/seng265/repos/assignment2/a.out) ==24394== by 0x40184E: main (in /home/zee/Documents/seng265/repos/assignment2/a.out) ==24394==

But I don't know what line the error is on? I know its something about strcpy Thanks!

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    You can easily verify it by using a debugger, and check some random entries in tokenized before and after the call to memset. Commented Jun 18, 2012 at 10:35
  • I always run valgrind like this: valgrind -v --tool=memcheck --leak-check=full --show-reachable=yes <path_to_your_executable>. It always show line number, so it should help (I just don't remember which command switches that on so I copied it from one of my scripts). Commented Jun 18, 2012 at 11:53
  • Compile your code with -g to instruct the compiler to also produce debugging information. Tools like Valgrind and gdb can then use this information to map virtual addresses to lines in the source code. Commented Jun 18, 2012 at 12:08

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Use Valgrind:

Valgrind is a GPL'd system for debugging and profiling Linux programs. With Valgrind's tool suite you can automatically detect many memory management and threading bugs, avoiding hours of frustrating bug-hunting, making your programs more stable. You can also perform detailed profiling to help speed up your programs.​

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valgrind is nice tool try using that.

avinash@ubuntu:~$ valgrind ./test ==2559== Memcheck, a memory error detector ==2559== Copyright (C) 2002-2011, and GNU GPL'd, by Julian Seward et al. ==2559== Using Valgrind-3.7.0 and LibVEX; rerun with -h for copyright info ==2559== Command: ./test ==2559== 4 ==2559== ==2559== HEAP SUMMARY: ==2559== in use at exit: 0 bytes in 0 blocks ==2559== total heap usage: 0 allocs, 0 frees, 0 bytes allocated ==2559== ==2559== All heap blocks were freed -- no leaks are possible ==2559== ==2559== For counts of detected and suppressed errors, rerun with: -v ==2559== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0) avinash@ubuntu:~$
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I don't think Valgrind is the tool you are looking for: Valgrind is really really really good for checking uninitialised memory and invalid memory accesses (etc), but in this case, I don't Valgrind is going to answer the question.

If you want to check that memory is zero (i.e. you've reset it properly), you can use

char test = 0; unsigned i; for (i = 0; i < MAX_CHARS; i++) { test |= tokenized[i]; } printf("Memory %s zero\n", test == 0 ? "is" : "isn't");

(After the loop, test will be 0 only if every single byte in tokenized is zero.)

You won't want this permanently in your code though: you can trust memset does what you want if you pass it the right parameters.

The error you are getting from Valgrind is a different problem: you happen to be calling strncpy with two pointers that point to the same data, e.g.

 strncpy(ptr, ptr, 6);

and that data is uninitialised. As in, it is essentially random: your program hasn't written anything to that block of memory yet.

(Also, if you compile with -g (debugging symbols) Valgrind will give you more informative output, including line numbers.)

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