Getting Segmentation Fault

Question

I saw many questions about getting segmentation fault in C program here in SO, and I thought it would be great to have a reference to those here, a question

Answer 1

All your examples are causing undefined behaviour, which might lead to a crash (or it might not appear to do any harm at all).

1. You're not allowed to change a string literal. (see e.g. here)

2. You forgot to allocate storage for the terminating nul byte, do malloc(strlen(str) + 1);

3. You're calling free() on a pointer you did not obtain from malloc (or similar functions). As you make the str pointer point to a string literal, you've lost the pointer to the memory allocated with malloc and leak memory here too.

4. You're calling free() twice on the same pointer, which is undefined behavior.

5. %s in the printf format string tells printf that the argument is a string (a char * pointing to a sequence of nul terminated characters) You're passing it a char, not a string. If you want to print the suffix of the string use printf("%s", &str[19]);

6. You're passing in an invalid pointer to free(), you already free'd s, you can't dereference it later when you do s->str. Reverse the order of deallocation: free(s->str); free(s);