C Language Course -- 追寻梦想＞编程乐园

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Lesson 4 operators of the language

I have mentioned that 'C' is a small language with most of the heavy work being done by explicit calls to library functions. There is however a rich mix of intrinsic operators which allow you to perform bit level operations, use pointers, and perform immediate operations on varables. In other words, most of a machine's instruction set is able to be used in the object program. At the time when 'C' was designed and first written these were unique for a high level language.

Lets start with a discussion about precedence.

This really means that the compiler puts invisable parentheses into your expression. Casting your mind back to Arithmetic in the primary school I expect you remember the nmemonic "My Dear Aunt Sally". The 'C' language does as well! So the following expression is correct

        15 + 4 * 11 = 59

The compiler has rendered the expression as:

        15 + ( 4 * 11 ) = 59

Now the 'C' language has a much larger collection of operators than just Multiply Divide Add Subtract, in fact much too big to try to remember the precedence of all of them. So my recomendation is to ALWAYS put in the parentheses, except for simple arithmetic. However, for the sake of completeness as much as anything else, here is the list.

First up come what are called the primary-expression operators:

                ()    Function.
                []    Array.
                .     struct member ( variable ).
                ->    struct member ( pointer ).

The unary operators:

                *     Indirection via a Pointer.
                &     Address of Variable.
                -     Arithmetic Negative.
                !     Logical Negation or Not.
                ~     Bit-wise One's Complement.
                ++    Increment.
                --    Decrement.
                sizeof  Which is self explanitary.

Now the binary operators:

Arithmetic Operators.

                *     Multiply.                                       My
                /     Divide.                                         Dear
                %     Modulo, or Remainder of Integer Division.
                +     Addition.                                       Aunt
                -     Subtraction.                                    Sally

The Shifting Operators.

                >>    Bit-wise Shift to the Right.
                <<    Bit-wise Shift to the Left.

Logical Relation Operators.

                <     Less Than.
                >     Greater Than.
                <=    Less Than or Equal.
                >=    Greater Than or Equal.
                ==    Equal.
                !=    Not Equal.

Bit-wise Boolean Operators.

                &     Bit-wise And.
                ^     Bit-wise Exclusive-or.
                |     Bit-wise Or.

The Logical Operators.

                &&    Logical And.
                ||    Logical Or.

The Assignment Operators. ( They all have the same priority. )

                =     The normal assignment operator.

The Self-referencing Assignment Operators.

Some explanation is in order here. The machine instructions in your computer include a suit of what are called "immediate operand" instructions. These instructions have one of the operands in a register and the other is either part of the instruction word itself ( if it is numerically small enough to fit ) or is the next word in the address space "immediately" after the instruction code word. 'C' makes efficient use of this machine feature by providing the above set of operations each of which translates directly to its corresponding machine instruction. When the variable in question is a 'register' one, or the optimiser is in use, the compiler output is just the one "immediate" machine instruction. Efficiency Personified!!!

These two lines will make things clearer.

        a = 8;
        a += 2;     /* The result is 10 */

The exclusive-or operation is very useful you can toggle any combination of bits in the variable using it.

        a = 7;
        a ^= 2;    /* Now a is 5 */
        a ^= 2;    /*  and back to 7. */

Naturally, you can use the other operations in exactly the same way, I'd like to suggest that you make a utterly simplistic little program and have a look at the assembler code output of the compiler. Don't be afraid of the assembler codes - they don't bite - and you will see what I was on about in the paragraph above.

Historical Note and a couple of Cautions.

In the Oldend Days when 'C' was first written all the self-referencing operations had the equals symbol and the operand around the other way. Until quite recently ( unix system V release 3.0 ) the 'C' compiler had a compatability mode and could cope with the old style syntax.

A sample or test program is probably in order here.

/* ----------------------------------------- */

#include 

char *mes[] =
{
        "Your compiler",
        " understands",
        " does not understand",
        " the old-fashioned self-referencing style."
        };

main()
{
        int a;

        a = 5;
        a=-2;
        printf ( "%s %s %s\n", mes [ 0 ], mes [ ( a == -2 ) ? 2 : 1 ], mes [ 3
] );
        }

/* ----------------------------------------- */

The 'C' compiler issued with unix System V release 3.2 seems to have ( thankfully ) dropped the compatability mode. However a collegue, who was using an old compiler, and I spent hours trying to find this strange bug! The cure for the problem is either to put spaces on either side of the '=' sign or to bracket the unary minus to the operand.

        a=(-2);
        a = -2;

Either is acceptable, and might save you a lot of spleen if sombody tries to install your work of art program on an ancient machine.

The other caution is the use of the shifting instructions with signed and unsigned integers.

If you shift a signed integer to the right when the sign bit is set then in all probability the sign will be extended. Once again a little demo program. Please cut it out of the news file with your editor and play with it.

/* ----------------------------------------- */

#ident "#(@) shifts.c - Signed / Unsigned integer shifting demo."
#include 

#define WORD_SIZE ( sizeof ( INTEGER int ) * 8 )
#define NIBBLE_SIZE 4
#define NIBBLES_IN_WORD (( WORD_SIZE ) / NIBBLE_SIZE )
#define SIGN_BIT ( 1 << ( WORD_SIZE - 1 ))

char *title[] =
{ "       Signed             Unsigned",
        "                 Signed                                 Unsigned"
        };

main ()
{
        INTEGER int a;
        unsigned INTEGER int b, mask;
        int ab, i, j, bit_counter, line_counter;

        a = b = SIGN_BIT;
        printf ( "%s\n\n", title [ ( WORD_SIZE == 16 ) ? 0 : 1 ] );

        for ( line_counter = 0; line_counter < WORD_SIZE; line_counter++ )
        {
                for ( ab = 0; ab < 2; ab++ )
                {
                        mask = SIGN_BIT;
                        for ( i = 0; i < NIBBLES_IN_WORD; i++ )
                        {
                                for ( j = 0; j < NIBBLE_SIZE; j++ )
                                {
                                        printf ( "%c", ((( ab ) ? b : a ) &
mask ) ? '1' : '0' );
                                        mask >>= 1;
                                        }
                                printf ( " " );
                                }
                        printf ( "%s", ( ab ) ? "\n" : " " );
                        if ( ab )
                        {
                          b >>= 1;
                                }
                        else
                        {
                                a >>= 1;
#if defined(FIX_COMPILER_BUG)
# if (INTEGER == long)
                                a |= SIGN_BIT;    /* This is a work-around for
the 3b2 compiler bug. */
# endif
#endif
                                }
                        }
                }
        }

/* ----------------------------------------- */

This little program might well produce some interesting surprises on your machine in the same way it did on mine. I have an AT&T 3b2/400 and use the K & R style compiler. Interestingly, the above program did what I expected it to do when the integers were short, the sign bit is extended, but when the integers are long the sign bit is NOT extended. In this case the different behaviour is caused by the compiler always issuing a Logical Shift instruction, when it should issue a Arithmetic Shift instruction for signed integers and a Logical Shift instructon for unsigned ones. In the case of the short int the varable is loaded from memory into the register with a sign extend load instruction, this makes the Logical Shift instruction right work correctly for short ints, but not for longs. I had to examine the assember codes output by the compiler in order to discover this.

Here are the compiler invocation lines.

cc -olong.shifts -DFIX_COMPILER_BUG -DINTEGER=long shifts.c

        and

cc -oshort.shifts -DINTEGER=short shifts.c

Experiment with the "-DFIX_COMPILER_BUG" and see what your compiler does.

I assert the right to be known as the author, and owner of the intellectual property rights of all the files in this material, except for the quoted examples which have their individual copyright notices. Permission is granted for onward copying, but not modification, of this course and its use for personal study only, provided all the copyright notices are left in the text and are printed in full on any subsequent paper reproduction.

 +----------------------------------------------------------------------+
  NAME   Christopher Sawtell                                           
  SMAIL  215 Ollivier's Road, Linwood, Christchurch, 8001. New Zealand.
  EMAIL  chris@gerty.equinox.gen.nz                                    
  PHONE  +64-3-389-3200   ( gmt +13 - your discretion is requested )   
 +----------------------------------------------------------------------+