calloc(n, size) allocates n blocks of size bytes and initializes to 0. malloc(size) allocates size bytes without initialization. calloc() is typically slower due to initialization.

/* */ is used for multi-line comments in C. // is used for single-line comments and was introduced in C99.

Since both 5 and 2 are integers, integer division occurs (5/2 = 2). The result 2 is then converted to float as 2.000000.

In structures, each member has its own memory location. In unions, all members share the same memory location, so only one can hold a value at a time.

When static is used with a global variable, it restricts its scope to the file where it is declared. Without static, global variables are accessible across files using extern.

Array name decays to pointer. p[5] is equivalent to *(p+5) which is *(arr+5) or arr[5]. This demonstrates pointer arithmetic.

Declaration tells the compiler about a variable's name and type (e.g., 'extern int x;'). Definition actually allocates memory (e.g., 'int x = 5;'). A variable can be declared multiple times but defined only once.

++a increments a to 6 and returns 6. Then a++ returns 6 and increments a to 7. So b = 6 + 6 = 12.

An empty structure in C has a size of 1 byte. This is to ensure each structure instance has a unique address in memory.

strlen() counts characters excluding the null terminator. "Hello" has 5 characters, so strlen(s) returns 5.