The C programming language (2nd) -- Notes -- 1.10

1.10 External variables and scopes        

        main Variables in functions （ Such as line,longest） yes main Private function private Or local local Variable . They are declared on main Inside , So no other function can directly access them . The same is true of function variables declared by other functions . example ,getline The variable declared by the function i And copy Functional i irrelevant . Each local variable in a function only when the function is called There is , When the function exits after execution disappear . This is also the reason why other languages often call such variables automatic variables . Later “ Automatic variable ”(automatic henceforth) representative “ local variable ”.（ Chapter four will discuss stastic Storage class , This kind of local variable keeps its value unchanged between multiple function calls .）

        Because automatic variables are only used in function calls 、 During execution , therefore , Between calls to a function , Automatic variables do not retain the assignment from the previous call , And it should be explicit every time you enter the function 【adv. Clearly , In detail ; Directly , Frankly ; Blatantly , Not implicitly 】 For the assignment . If the automatic variable is not assigned , Then the invalid value is stored in it .

        Except for automatic variables , You can also define variables outside all functions , namely , Variable names can be used in all functions 【 Come on 】 Access this type of variable （ This mechanism is the same as Fortran in COMMON or Pascal Variables declared by the outermost block of a language ）. Because external variables can be accessed globally , therefore , Functions can exchange data through external variables , Instead of using a parameter table . also , External variables exist during program execution , Instead of generating... When a function is called , Disappear when the function finishes executing . Even after the function assigned to the external variable returns , These variables will remain unchanged .

        External variables must be defined outside of all functions , And it can only be defined once , After definition, the compiler will allocate storage units for it . In each function that needs to access external variables , The corresponding external variable must be declared , In this case, the type . When declaring, you can use extern Statement explicitly declares , It can also be implicitly declared by context . To discuss external variables in more detail , Rewrite the above program for printing the longest text line , hold line、longest And max Declared as an external variable . This requires modifying the calls of these three functions 、 Declaration and function body .

int getline(void);
void copy(void)

/*print longest input line;  Specialized version.*/
main()
{
    int len;
    extern int max;
    extern char longest[];

    max = 0;
    while ((len = getline()) > 0)
        if (len > max) {max = len ;
        copy ();
        }
    if (max > 0)    /* There was a line.*/
        printf("%s", longest );
    return 0;
}

/*   getline: Specialized version*/
int getline(void)
{
    int c, i;
    extern char line[];

    for (i = 0; i < MAXLINE - 1
        && (c = getchar)) != EOF && c != '\n'; ++i)
            line [i] = c;
    if (c == '\n'){
        line[i] = c;
        ++i;
    }
    line [i] ='\0';
    return i;
}

/* copy  Specialized version */
void copy (void)
{
    int i;
    extern char line[], longest[];

    i = 0;
    while ((longest [i] = line[i]) != '\0')
        ++i;
}

  The first few lines of this example define main、 getline And copy Several external variables used by the function , Declare the type of each external variable , In this way, the compiler will allocate storage units for them . From the perspective of grammar , The definitions of external and local variables are the same , But because they are outside the functions , So these variables are external variables . Function before using external variables , You must know the name of the external variable . One way to achieve this is to use extern Declaration of type . This type of declaration , In addition to adding a keyword in front extern Outside , Other aspects are the same as the declaration of ordinary variables .

        In some cases, it can be omitted extern Statement . In the source file , If the definition of an external variable appears before the function that uses it , Then there is no need to use extern Statement . therefore ,main、getline And copy Some of them extern Statements are superfluous . In common practice , The definitions of all external variables are placed at the beginning of the source file , So you can omit extern Statement .

        If the program is contained in multiple source files , And a variable in file1 The document defines 、 stay file2 and file3 Use... In the document , So in the file file2 and file3 You need to use extern Declaration to establish the relationship between the variable and its definition . Usually, variables and functions extern The declaration is in a separate file （ It is customarily called header file ）, And use... At the beginning of each source file #include Statement to include the header file to be used . Suffix named .h The Convention is the extension of the header file name . For example, functions in the standard library are similar to <stdio.h> Declared in the header file of . For more details, see 4 Chapter , The first 7 Chapter 、 appendix B We will discuss function libraries .

        In the above special edition , because getline And copy Functions take no arguments , So logically , At the beginning of the source file , Their prototype should be getline() And copy(). But in order to match the old version C Language program compatibility ,ANSI C The language treats empty parameter tables as old versions C Language declaration , And do not check the parameter table any more . stay ANSI C in , If you want to declare an empty parameter table , Keywords must be used void Make an explicit statement . See Chapter 4 .

        Be careful , In this section, definitions are used cautiously when discussing external variables （define） And statement （ declaration） These two words . “ Definition ” Means to create variables or allocate storage units , and “ Statement ” It means describing the nature of the variable , But no storage units are allocated .

        Shun Ti , Now more and more people use everything they use as external variables , Because it seems that this can simplify the communication of data —— The parameter table is shorter , And you can always access these variables when you need them . however , Even when external variables are not used , They also exist . Over reliance on external variables can lead to certain risks , Because it will make the data relationship in the program ambiguous —— The value of an external variable may be accidentally or inadvertently modified , And the modification of the program becomes very difficult . Previously written   “ Print the longest line of text ” Of the program 2 The first version is not as good as the 1 A good version , There are two reasons , One is the use of external variables ; On the other hand , The first 2 The functions in the three versions write the names of the variables they manipulate directly into the functions , Thus, these two useful functions lose their generality .

        up to now , Have been to C The traditional core of the language is introduced . With the help of these few language elements , It has been able to write useful programs on a considerable scale . Readers are advised to spend some time writing programs as exercises . The following exercises are more complex than the program written earlier in this chapter .

        practice 1-20        Programming detab, Replace the tabs in the input with the appropriate number of spaces , Fill the space to the next tab stop . Suppose the position of the tab stop bit is fixed , Like every other n Column will have a tab stop .  n Should it be a variable or a symbolic constant ？

        practice 1-21         Programming entab, Replace the space string with the minimum number of tabs and spaces . But keep the space between words the same . Suppose the position of tab stop and practice 1-20 Of detab The procedure is the same . When a tab or a space can be used to reach the next tab , Which replacement character is better ？

        practice 1-22         Write a program , Put the longer input line “ fold ” In two or more shorter lines , The position of the break line is on the input line n After the last non space before the column . To ensure that the program can intelligently handle the situation when the input line is very long and there is no space or tab before the specified column .

        practice 1-23         Write a delete C All comment statements in language programs . To handle quoted strings and character constants correctly . stay C In language , Comments cannot be nested .

        practice 1-24         Write a program , lookup C Basic grammatical errors in language programs , Such as parentheses 、 square brackets 、 Curly braces do not deserve equivalence . Handle quotation marks correctly （ Include single and double quotes ）、 Escape character sequences and comments .（ If the reader wants to write a completely general program of this program , It will be more difficult .）