The C Programming Language

这就是implicit function declaration的规定啊：

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.

这一章的收获很大be lumped with = put togethercryptic = secret, hidden meaninglexicographically a.字典序的cavalier a.随便的，傲慢的[note_1]

(*px)++;   // 表示px指向的数加1
*px++;     // 表示px的下一个地址(数组的下一个元素)的值

*t++ <==> *t, t++;[note_2]arithmetics for pointers:best use within one array, otherwise it will go wrong, badly.

type *p;
p+n;
/* n = 1 for char(8bt), 2 for int&short(16b), 3 for long&float(32b), 4 for double(64b) */
p++;    // no type restrictions
/* on PDP-11; other kind of arithmetic operations(comparison, etc.) are illegal */

[note_3]Multi-dimensional arrays

int day_tab[2][13];
<==>int day_tab[][13];
<==>int (*day_tab)[13];  // 数组指针(int [13]的指针)
<X=>int *day_tab[13];    // 这个表示指针数组(int* 的数组)

上面是非常重要的一个概念，说明[]的优先级高于*[note_4]5.9节的例子static var在function中permanent exist，无论调用多少次[note_5]5.11 command line args

main()
int argc;   //参数个数
char *argv[];  //字符串数组
{ ... }

(*++argv)[0]的解析：argv: pointer, 指向一个pointer数组，其中的每个元素(pointer)表示一个字符串++argv: pointer, 指向argv的下一个元素(pointer)*++argv: <==>*(++argv), 表示取出这个pointer的值(一个地址，代表一个字符串，仍然是一个pointer)(*++argv)[0]: 表示这个字符串中第1个元素(char)，也可以表示成*(*++argv)

A word of caution: newcomers to C occasionally write = when they mean ==. As we will see in Chapter 2, the result is usually a legal expression, so you will get no warning.

一个好的习惯是把常量放在左边，如

if ('\n' == c)

这样，当你把`==`误写为`=`的时候，编译器会报错。

We are now going to consider a family of related programs for processing character data. You will find that many programs are just expanded versions of the prototypes that we discuss here.

字符或者字符串操作算是使用C所能做到的最简单而有趣的事情。因为C当中的字符串并非immutable，所以给了我们可以操作字符串中单个字符的机会，这就可以做一些有趣的算法了。