Coding FAQ - goofycoder/knowdb GitHub Wiki

Q: stack vs heap allocation

Stack

  • (+) faster (just move the stack pointer (esp))
    • Use memory pools, you can get comparable performance out of heap allocation
  • (-) stack's size is very limited
  • lifetime of function frame
  • usually stack-allocated memory is guaranteed in the cache (better cache locality)

Q: How do you compare structs for equality in C?

A: C provides no language facilities to do this - you have to do it yourself and compare each structure member by member.

Reasons that memcmp() would not work:

  • The padding in the struct could be different.

  • Consider a "BOOL" field. In terms of equality, any non-zero BOOL is equal to every non-zero BOOL value. So while 1 and 2 may both be TRUE and therefore equal, memcmp will fail.

Reference: http://stackoverflow.com/questions/141720/how-do-you-compare-structs-for-equality-in-c

Q: Is it legal to do this:

char *str = "hello world\n";
char *p = (char*)malloc(strlen(str)+1);
strcpy(p, str, strlen(str)+1);
p = p+5;        // advance the pointer
free(p);

A: Illegal. The output is likely to be:

*** glibc detected *** free(): invalid pointer: 0x0000000000XXXXXX ***
Abort

You are not allowed to free pointers not obtained from malloc()according to http://c-faq.com/malloc/crash.html