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A collection of idioms in C.

As the title indicates this is a collection of C idioms, roughly categorized.

Command-Line Arguments

Check for first argument using strcmp.

if (*++argv && !strcmp(*argv, "-n")) {
        ++argv;
        // Do something,  usually set a flag.
}

Check for first argument pattern.

if (argc > 1 && argv[1][0] == '-' && argv[1][1] ==  'n') {        
}

Process optional arguments first

As seen in the original K&R, by convention optional arguments start with a minus sign.

while(--argc > 0 && (*++argv)[0] == '-') {
        // Process optional arguments
}

Processing arguments using while.

while(*argv) {
       // Do something
       if (*++argv)
        // Do something, considering there are more arguments to be processed.
}

Processing arguments using for.

for(int i = 1; i < argc; i++) {
        // Do something using argv[i]
}

Processing option arguments using getopt

Arguments are processed using while and switch. Most of the time argc and argv are the same arguments received by main. optarg is the variable set to to the value of the option argument. optind is the variable set to the index of next non-option argument.

while ((c = getopt (argc, argv, "f: :c")) != -1) { // Process option arguments
	  switch (c) {
	  case 't':
		  foo(optarg)  // Do something, optarg is the value of the option t.
		  break;
	  default:
	  }
}
argc -= optind;   // update   list  of   arguments  so   that  regular
		    // arguments (non-option) can be processed as usual.
argv += optind;

Control Flow

Nested ternary operator in order to chose an alternative.

In this idiom the consequent and alternative expressions of the ternary operator can be ternary expressions themselves. The conditions must be mutually exclusive and the final expression must be different for each case.

!x ? (!y ? upleft : (y == bottom ? lowleft : left)) :
    (x == last ? (!y ? upright : (y == bottom ? lowright : right)) :
                 (!y ? upper : (y == bottom ? lower : normal)));

goto expression as function termination.

A goto expression is used to execute code required in multiple parts of the function. This idiom presupposes multiple exit conditions and that the label should always be placed before the end of the function. Convenient for code reuse if preventing a function call is required for performance.

int some_function(int n) 
{
	  if ()
		  return 1;
	  if ()
		  goto message;
	  for() {
		  for () { // Nested for  to indicate the  usefulness of
			   // the  idiom  (a  break statement  won't  do
			   // here).
			  goto message;
		  }
	  }
	  // Handle common  exit condition. Ideally the  label should be
	  // the name of the operation.
	  :message
		   // print something
		   return -1;
}

Bit manipulation

Arithmetic

• left shift by n is equivalent to a division by 2^n.
• right shift by n is equivalent to a multiplication by 2^n.
• bitwise and by n is equivalent to a modulus operation by 2^n + 1, so the right operand must be odd.

// Multiplication

3 << 2; // multiply by 4.

3 << 4; // multiply by 16.

// Division

3 >> 2; // divide by 4.

3 >> 4; // divide by 16.

// Modulus - & 3 is equivalento to mod 3 + 1.

19 & 3; // equal to  19 % 4.

Check endianness

 unsigned int x;
 char *c;

x = 0x12345678; // set 4 byte integer
c = (char *)&x; // grab first byte

if (*c == 0x12) // check first byte
	  // big endian
	  // else little endian

// A more concise idiom
int n = 1;
if (*(char *)&n == 1)

Checking and Setting bits.

Check if some bit is set for a particular variable.

bool checked = ((some_var & (1 << 30)) != 0);

Use & to clamp a value to be of a particular maximum number of bits.

u64 some_var = (u64)(other_var & 0x00000003);

Build a bit pattern using the | operator.

uint32 color = ((uint8) (r * 255.0f)) << 16 |
	     ((uint8) (g* 255.0f)) << 8) |
	     ((uint8) (b * 255.0f) << 0));