C语言参考手册：Ritchie与PDP-11实现详解

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"C Reference Manual" 是由 Dennis M. Ritchie 编写的，他是 Bell Telephone Laboratories 的一位著名计算机科学家。这本手册专注于 C 语言，这是一种基于早期的 B 语言发展起来的计算机编程语言。C 与 B 的主要区别在于引入了类型的概念，并为此设计了额外的语法和语义规则。与仅能生成解释代码的 B 不同，C 是一个真正的编译器，能够将源代码转换为机器码。 C 语言在 UNIX 时间共享系统中占据了核心地位，包括操作系统本身。UNIX 时间共享系统广泛使用 C 进行软件开发，而手册特别关注的是 PDP-11 实现的 C 语言版本。尽管文档主要针对 PDP-11 平台，但作者偶尔也会提供关于依赖于特定实现特性的一些提示。 "UNIX Programmer's Manual" 是另一份重要的参考资料，它详细介绍了在 UNIX 系统下可用给 C 程序的库函数以及编译程序的方法。此外，Lesk 和 Barres 的 "The GCOS CLibrary" 也涵盖了该系统下的类似功能，特别是那些涉及 I/O 操作的函数。这些库函数极大地扩展了 C 程序的功能，使得开发者能够更高效地进行输入输出操作和系统交互。 C Reference Manual 提供了一个深入理解 C 语言基础和其在 UNIX 环境中的应用的宝贵资源，对于学习 C 编程、编写高效的 UNIX 程序以及利用其丰富的库功能至关重要。通过阅读和理解这份手册，程序员能够掌握如何充分利用 C 语言的特性来构建高性能的软件，并熟悉当时流行的开发环境和工具。

C Reference Manual - 6

7.2.4 ! expression

The result of the logical negation operator ! is 1 if the value of the expression is 0, 0 if the value of the expres-

sion is non-zero. The type of the result is int. This operator is applicable only to ints or chars.

7.2.5

expression

The

operator yields the one’s complement of its operand. The type of the expression must be int or char, and

the result is int.

7.2.6 ++ lvalue-expression

The object referred to by the lvalue expression is incremented. The value is the new value of the lvalue expres-

sion and the type is the type of the lvalue. If the expression is int or char, it is incremented by 1; if it is a pointer

to an object, it is incremented by the length of the object. ++ is applicable only to these types. (Not, for example, to

float or double.)

7.2.7 −− lvalue-expression

The object referred to by the lvalue expression is decremented analogously to the ++ operator.

7.2.8 lvalue-expression ++

The result is the value of the object referred to by the lvalue expression. After the result is noted, the object re-

ferred to by the lvalue is incremented in the same manner as for the prefix ++ operator: by 1 for an int or char, by

the length of the pointed-to object for a pointer. The type of the result is the same as the type of the lvalue-

expression.

7.2.9 lvalue-expression −−

The result of the expression is the value of the object referred to by the the lvalue expression. After the result is

noted, the object referred to by the lvalue expression is decremented in a way analogous to the postfix ++ operator.

7.2.10 sizeof expression

The sizeof operator yields the size, in bytes, of its operand. When applied to an array, the result is the total

number of bytes in the array. The size is determined from the declarations of the objects in the expression. This ex-

pression is semantically an integer constant and may be used anywhere a constant is required. Its major use is in

communication with routines like storage allocators and I/O systems.

7.3 Multiplicative operators

The multiplicative operators

, /, and % group left-to-right.

7.3.1 expression

expression

The binary

operator indicates multiplication. If both operands are int or char, the result is int; if one is

int or char and one float or double, the former is converted to double, and the result is double; if both

are float or double, the result is double. No other combinations are allowed.

7.3.2 expression / expression

The binary / operator indicates division. The same type considerations as for multiplication apply.

7.3.3 expression % expression

The binary % operator yields the remainder from the division of the first expression by the second. Both operands

must be int or char, and the result is int. In the current implementation, the remainder has the same sign as the

dividend.

7.4 Additive operators

The additive operators + and − group left-to-right.

C Reference Manual - 7

7.4.1 expression + expression

The result is the sum of the expressions. If both operands are int or char, the result is int. If both are float

or double, the result is double. If one is char or int and one is float or double, the former is converted to

double and the result is double. If an int or char is added to a pointer, the former is converted by multiplying

it by the length of the object to which the pointer points and the result is a pointer of the same type as the original

pointer. Thus if P is a pointer to an object, the expression ‘‘P+1’’ is a pointer to another object of the same type as

the first and immediately following it in storage.

No other type combinations are allowed.

7.4.2 expression − expression

The result is the difference of the operands. If both operands are int, char, float,ordouble, the same type

considerations as for + apply. If an int or char is subtracted from a pointer, the former is converted in the same

way as explained under + above.

If two pointers to objects of the same type are subtracted, the result is converted (by division by the length of the

object) to an int representing the number of objects separating the pointed-to objects. This conversion will in gen-

eral give unexpected results unless the pointers point to objects in the same array, since pointers, even to objects of

the same type, do not necessarily differ by a multiple of the object-length.

7.5 Shift operators

The shift operators << and >> group left-to-right.

7.5.1 expression << expression

7.5.2 expression >> expression

Both operands must be int or char, and the result is int. The second operand should be non-negative. The

value of ‘‘E1<<E2’’ is E1 (interpreted as a bit pattern 16 bits long) left-shifted E2 bits; vacated bits are 0-filled. The

value of ‘‘E1>>E2’’ is E1 (interpreted as a two’s complement, 16-bit quantity) arithmetically right-shifted E2 bit po-

sitions. Vacated bits are filled by a copy of the sign bit of E1. [Note: the use of arithmetic rather than logical shift

does not survive transportation between machines.]

7.6 Relational operators

The relational operators group left-to-right, but this fact is not very useful; ‘‘a<b<c’’ does not mean what it seems

to.

7.6.1 expression < expression

7.6.2 expression > expression

7.6.3 expression <= expression

7.6.4 expression >= expression

The operators < (less than), > (greater than), <= (less than or equal to) and >= (greater than or equal to) all yield 0

if the specified relation is false and 1 if it is true. Operand conversion is exactly the same as for the + operator ex-

cept that pointers of any kind may be compared; the result in this case depends on the relative locations in storage of

the pointed-to objects. It does not seem to be very meaningful to compare pointers with integers other than 0.

7.7 Equality operators

7.7.1 expression == expression

7.7.2 expression != expression

The == (equal to) and the != (not equal to) operators are exactly analogous to the relational operators except for

their lower precedence. (Thus ‘‘a<b == c<d’’ is 1 whenever a<b and c<d have the same truth-value).

7.8 expression & expression

The & operator groups left-to-right. Both operands must be int or char; the result is an int which is the bit-

wise logical and function of the operands.

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