Johannes Buchner

So your C code has invalid allocations, double frees, pointer arithmetic errors and generally crashes. That’s ok.

First of all, check the return value of every malloc/calloc/realloc.

#include <assert.h>
void myfunction(int n) {
int * arr = (int*) malloc(n, sizeof(int));
/* assert(arr != NULL); -- for development */
if(arr == NULL) {
perror("memory could not be allocated");
// error handling
}
...

Your code will crash sooner or later, but if you handle it now, you will know why.

Valgrind:

You will not be able to get everything right, except with excessive effort. Try valgrind, it prints out what memory you illegally accessed, left unfreed (leaks), etc.. Best of all: It tells you where exactly in your code.

Example output.

==19182== Invalid write of size 4
==19182==    at 0x804838F: f (example.c:6)
==19182==    by 0x80483AB: main (example.c:11)
==19182==  Address 0x1BA45050 is 0 bytes after a block of size 40 alloc'd
==19182==    at 0x1B8FF5CD: malloc (vg_replace_malloc.c:130)
==19182==    by 0x8048385: f (example.c:5)
==19182==    by 0x80483AB: main (example.c:11)

call it like this:

$ valgrind --leak-check=full ./myprog.exe

DUMA:

Beside having a silly acronym (Detect Unintended Memory Access), this library protects you from illegal memory access. The previous name is more accurate: “ElectricFence”. Usually the OS is very lenient about memory access (no checks make stuff fast), except when you run into other programs space. To make the usual output (“Segmentation fault.”) more colorful, and to terminate early (so that your debug output matches the access), run your (or any) program with

$ LD_PRELOAD=libduma.so.0.0.0 ./myprog.exe

The LD_PRELOAD is a bash feature (read about it elsewhere), it saves you from linking your program with the library.
Last word I wanted to say: With DUMA or valgrind, your program will run slower (all allocated memory is registered). Be a good programmer and make your programs pointer perfect (what’s up with the alliterations by the way), so that the OS doesn’t have to do it.