If a name that has not been previously declared occurs in an expression and is followed by a left parenthesis, it is declared by context to be a function name, the function is assumed to return an int, and nothing is assumed about its arguments. Furthermore, if a function declaration does not include arguments, as in
double atof();
that too is taken to mean that nothing is to be assumed about the arguments of atof; all parameter checking is turned off. This special meaning of the empty argument list is intended to permit older C programs to compile with new compilers. But it’s a bad idea to use it with new programs. If the function takes arguments, declare them; if it takes no arguments, use void. (查看原文)
BCPL and B are "typeless" languages. By contrast, C provides a variety of data types. The fundamental types are characters, and integers and floating-point numbers of several sizes. In addition, there is a hierarchy of derived data types created with pointers, arrays, structures, and unions. Expressions are formed from operators and operands; any expression, including an assignment or a function call, can be a statement. Pointers provide for machine-independent address arithmetic. (查看原文)
Declaring the argument x to be unsigned ensures that when it is right-shifted, vacated bits will be filled with zeros, not sign bits, regardless of the machine the program run on. (查看原文)
char *fgets(char *line, int maxline, FILE *fp)
fgets reads the next input line (including the newline) from file fp into the character array
line; at most maxline-1 characters will be read. The resulting line is terminated with '\0'.
Normally fgets returns line; on end of file or error it returns NULL. (查看原文)
void *realloc(void *p, size_t size)
realloc changes the size of the object pointed to by p to size. The contents will be
unchanged up to the minimum of the old and new sizes. If the new size is larger, the
new space is uninitialized. realloc returns a pointer to the new space, or NULL if the
request cannot be satisfied, in which case *p is unchanged. (查看原文)
char *strtok(s,ct) strtok searches s for tokens delimited by characters from ct; see
below.
A sequence of calls of strtok(s,ct) splits s into tokens, each delimited by a character from
ct. The first call in a sequence has a non-NULL s, it finds the first token in s consisting of
characters not in ct; it terminates that by overwriting the next character of s with '\0' and
returns a pointer to the token. Each subsequent call, indicated by a NULL value of s, returns the
next such token, searching from just past the end of the previous one. strtok returns NULL
when no further token is found. The string ct may be different on each call. (查看原文)
The valid pointer operation are assignment of pointers of the same type,adding or subtracting a pointer and an integer,subtracting or comparing two pointers to members of the same array,and assigning or comparing to zero.All other pointer arithmetic is illegal.It is not legal to add two pointers,or to multiply or divide or shift or mask them,or to add float or double to them,or even,except for void *,to assign a pointer of one type to pointer of another type without a cast. (查看原文)
A character written between single quotes represents an integer value equal to the numerical value of the character in the machine's character set. (查看原文)
The fundamental types are charaters, and integers and floatingpoint number of serveral sizes.
The language does not define any storage allocation facility other than static definiction and the stack discipline provided by the local variables of functions, there is no heap or grabage collection.
Similarly, C offers only straightfowward, single-thread control flow: tests, loops, grouping and subprograms, but not multiprogramming, parallel operations, synchronization, or coroutines.
.. keeping the lalnguage down to the modest size has real benefits. (查看原文)
The only way to learn a new programming language is by writing programs in it. The first program to write is the same for all languages.
C functions are like the subroutines and functions of Fortan or the procedures and functions of Pascal.
... "main" is special -- your program begins executing at the beginning of main.
Notice that \n represents only a single character. An escape sequence like \n provides a general and extensible mechanism for representing hard-to-type or invisible characters.
... a comment, which in this case explains briefly what that program does.
Comments may appear anywhere a blank or tab or newline can.
In C, all variables must be delcared before they are used...
The range of both int and float depends on the machine you are using.
The sizes of these objects ... (查看原文)
Chapter 7 describes the standard library, which provides a common interface to the operating system. This library is defined by the ANSI standard. (查看原文)
Although variables of enum types may be declared, compilers need not check that what you store in such a variable is a valid value for the enumeration. Nevertheless, enumeration variables offer the chance of checking and so are often better than #define s. (查看原文)
作者: Brian W. Kernighan, Dennis M. Ritchie 副标题: Second Edition isbn: 0131103628 书名: The C Programming Language 页数: 274 定价: USD 67.00 出版社: Prentice Hall 出版年: 1988-4-1 装帧: Paperback 又名: C Programming Language, K&R
