Custom/local header files are included using double quotes: #include "filename". Standard library headers use angle brackets: #include <filename>. The compiler searches for quoted files in the current directory first.

Variable names in C cannot start with a digit. They must begin with a letter (a-z, A-Z) or underscore (_). '123myVar' is invalid.

Operator precedence: multiplication (*) is performed before addition (+). So: 3 * 2 = 6, then 5 + 6 = 11.

The asterisk (*) symbol is used to declare pointer variables. For example: int *ptr; declares a pointer to an integer.

The correct syntax is 'int *p;' where int is the data type, * indicates pointer, and p is the pointer variable name.

getchar() reads a single character from standard input (stdin). While fgetc() can also read a character, getchar() is the standard dedicated function for this purpose.

In C, array indices are 0-based. arr[1][2] refers to the element in the second row (index 1) and third column (index 2) of the 2D array.

The += operator adds 5 to a (a = a + 5 = 10 + 5 = 15), and printf prints the updated value 15.

The strlen() function returns the number of characters in a string, excluding the null terminator ('\0').