C 語言中 int main() 和 void main() 有何區別？

第一次接觸 C 語言，嚴格的說，是第一次接觸計算機程序設計語言。第一堂課就是這個 Hello,world! 程序，第二節課是 int, void 等等了，我自己就去敲代碼了，用了 void main()，我是第一個寫出程序運行正確並上交的(班級領頭羊已經不管這種小兒科了，自己學到後面了)，很驕傲，交上去馬上得到，老師一頓訓，非要說我寫 void main 是錯的，要我改成 int main 加上 return 0; 雖然現在已經習慣使用 int main，但是，一直不知道區別。誰來解釋解釋區別？

搬運一下自己博客的文章： main原型考證及程序終止行為

在C和C++中流傳著很多版本的main函數聲明，不同的書里也有不同的寫法。今天我從幾種標準(C89/99/11以及C++98/03/11/14)的角度來尋找一下什麼是「標準行為」以及在主函數中return後發生了什麼。

比較常見的是下面幾種：

void main()
main()
int main()
int main(void)
int main(int argc,char *argv[])


void main()

首先，從標準角度(所有版本)來說，void main()肯定是錯的，沒有任何標準(C89/99/11以及C++98/03/11/14)中允許過這種寫法。

但是我在APUE里看到了一種把主函數寫為void main()的原因，不知道是不是有人從這個角度說的然後就以訛傳訛了。

The problem is that these compilers don』t know that an exit from main is the same as a return. One way around these warnings, which become annoying after a while, is to use return instead of exit from main. But doing this prevents us from using the UNIX System』s grep utility to locate all calls to exit from a program. Another solution is to declare main as returning void, instead of int, and continue calling exit. This gets rid of the compiler warning but doesn』t look right (especially in a programming text), and can generate other compiler warnings, since the return type of main is supposed to be a signed integer.

評論區 @James Swineson 提到了另一種可能：

void main 的寫法可能是從嵌入式來的……沒有操作系統，入口點是硬體實現，返回任何東西都沒意義 。

main()

在KR C與C89里，函數沒有顯式聲明返回類型，則默認是int:

C89對函數定義的語法(Syntax)描述如下(注意declaration-specifiers的opt下標符號)：

$$declaration specifiers{opt}hspace{2mm}declaratorhspace{2mm}declaration list{opt}hspace{2mm}compound statement$$

C89中declaration-specifiers在Syntax上為：

• storage-class-specifier
• type-specifier
• type-qualifier

表明在C89中函數的return type可以省略。

KR C里的描述如下：

Various parts may be absent; a minimal function is

dummy() {}


which does nothing and returns nothing. A do-nothing function like this is sometimes useful as a place holder during program development. If the return type is omitted, int is assumed.

所以說：

func(){}
// 等價於
int func(){}


但是這種方式在C99之後就被廢除掉了(注意declaration-specifiers沒有opt下標了)：

[Math Processing Error]declaration.specifiersdeclaratordeclaration.listoptcompound.statement

綜上，在C89中，函數的返回類型可以省略，但默認為int，即

主函數聲明main()隱式是int main()。

int main()

int main()和int main(void)在C語言中是有區別的：

int main()
// 不等價於
int main(void)


在C語言中參數列表為空(即不提供參數列表也不為void)，表示不提供參數數量和參數類型信息：

int func(){
print("func()
");
return 0;
}
int main(void){
func(1,2,3,4);// call func();
}


The empty list in a function declarator that is not part of a definition of that function specifies that no information about the number or types of theparameters is supplied.

C99/11 Standard

在C99/11標準中，明確定義了對於標準的main函數的兩個原型：

The function called at program startup is named main. The implementation declares no prototype for this function. It shall be defined with a return type of int and with no parameters:

int main(void) { /* ... */ }


or with two parameters (referred to here as argc and argv, though any names may be used, as they are local to the function in which they are declared):

int main(int argc, char *argv[]) { /* ... */ }


or equivalent;or in some other implementation-defined manner.
If they are declared, the parameters to the main function shall obey the following constraints:

• The value of argc shall be nonnegative.
• argv[argc] shall be a null pointer.
• If the value of argc is greater than zero, the array members argv[0] through argv[argc-1] inclusive shall contain pointers to strings, which are given implementation-defined values by the host environment prior to program startup. The intent is to supply to the program information determined prior to program startup from elsewhere in the hosted environment. If the host environment is not capable of supplying strings with letters in both uppercase and lowercase, the implementation shall ensure that the strings are received in lowercase.
• If the value of argc is greater than zero, the string pointed to by argv[0]
  represents the program name; argv[0][0] shall be the null character if the program name is not available from the host environment. If the value of argc is greater than one, the strings pointed to by argv[1] through argv[argc-1]
  represent the program parameters.
• The parameters argc and argv and the strings pointed to by the argv array shall be modifiable by the program, and retain their last-stored values between program startup and program termination.

C++ Standard

由於C和C++中對於函數參數列表的規則並不一致(C++中參數列表為空代表不接收任何參數)。所以C++中main的原型和ISO C也並不太一樣：

• a function of () returning int and
• a function of (int, pointer to pointer to char) returning int

main return value

main必須要有返回值的原因是：在C和C++中使用return-statement都是將return的值作為參數來調用exit/std::exit來終止程序。

If status is zero or EXIT_SUCCESS, an implementation-defined form of the status successful termination is returned.

ISO C99/11:
If the return type of the main function is a type compatible with int, a return from the initial call to the main function is equivalent to calling the exit function with the value returned by the main function as its argument;reaching the } that terminates the main function returns a value of 0. If the return type is not compatible with int, the termination status returned to the host environment is unspecified.

ISO C++11/14:
A return statement in main has the e?ect of leaving the main function (destroying any objects with automatic storage duration) and calling std::exit with the return value as the argument. If control reaches the end of main without encountering a return statement, the e?ect is that of executing

return 0;


exit

#include &

The exit function causes normal program termination to occur. If more than one call to the exit function is executed by a program, the behavior is undefined.