main function - 主函数
Every C program coded to run in a hosted execution environment contains the definition (not the prototype) of a function called main, which is the designated start of the program.
每个要在宿主环境运行的 C 编码程序都含有称作 main 的函数定义 (非原型)，它是程序的指定起始点。
int main (void) { body } (1)
int main (int argc, char *argv[]) { body } (2)
1. Parameters - 参数
argc - Non-negative value representing the number of arguments passed to the program from the environment in which the program is run.
代表程序所运行的环境传递给程序的参数数量。
argv - Pointer to the first element of an array of argc + 1 pointers, of which the last one is NULL and the previous ones, if any, point to strings that represent the arguments passed to the program from the host environment. If argv[0] is not a null pointer (or, equivalently, if argc > 0), it points to a string that represents the program name, which is empty if the program name is not available from the host environment.
指向 argc + 1 个指针的数组的首元素的指针。数组末元素为空指针，而若前面有元素，则它们指向表示从宿主环境传递给程序的参数的字符串。若 argv[0] 不是空指针 (或等价地 argc > 0)，则它指向表示程序名的字符串。若程序名从宿主环境不可用则该字符串为空。
The names argc and argv are arbitrary, as well as the representation of the types of the parameters: int main(int ac, char** av) is equally valid.
名称 argc and argv，以及这些参数的类型表示是任意的：int main(int ac, char** av) 同样合法。
A common implementation-defined form of main is int main(int argc, char *argv[], char *envp[]), where a third argument, of type char*[], points at an array of pointers to the host environment variables.
main 的常见实现定义形式是 int main(int argc, char *argv[], char *envp[])，其中第三参数类型为 char*[]，指向宿主环境变量的指针的数组。
2. Return value - 返回值
If the return statement is used, the return value is used as the argument to the implicit call to exit() (see below for details). The values zero and EXIT_SUCCESS indicate successful termination, the value EXIT_FAILURE indicates unsuccessful termination.
若使用返回语句，则返回值会用作隐式调用 exit() 的参数。值 zero and EXIT_SUCCESS 指示成功终止，值 EXIT_FAILURE 指示不成功终止。
4. Example
Demonstrates how to inform a program about where to find its input and where to write its results.
演示如何告知程序其寻找输入处及写结果处。
4.1 Example
//============================================================================
// Name : main
// Author : Yongqiang Cheng
// Version : Version 1.0.0
// Copyright : Copyright (c) 2019 Yongqiang Cheng
// Description : Hello World in C++, Ansi-style
//============================================================================
#include <stdio.h>
int main(int argc, char *argv[])
{
printf("argc = %d\n", argc);
for (int idx = 0; idx < argc; ++idx)
{
printf("argv[%d] --> %s\n", idx, argv[idx]);
}
printf("argv[argc] = %p\n", (void*)argv[argc]);
return 0;
}
argc = 1
argv[0] --> D:\visual_studio_workspace\yongqiang\Debug\yongqiang.exe
argv[argc] = 00000000
请按任意键继续. . .
4.2 Example
Invocation (调用): yongqiang.exe infile outfile
//============================================================================
// Name : main
// Author : Yongqiang Cheng
// Version : Version 1.0.0
// Copyright : Copyright (c) 2019 Yongqiang Cheng
// Description : Hello World in C++, Ansi-style
//============================================================================
#include <stdio.h>
int main(int argc, char *argv[])
{
printf("argc = %d\n", argc);
for (int idx = 0; idx < argc; ++idx)
{
printf("argv[%d] --> %s\n", idx, argv[idx]);
}
printf("argv[argc] = %p\n", (void*)argv[argc]);
return 0;
}
D:\visual_studio_workspace\yongqiang\Debug
Microsoft Windows [版本 10.0.17763.1098]
(c) 2018 Microsoft Corporation。保留所有权利。
C:\Users\cheng>d:
D:\>cd visual_studio_workspace
D:\visual_studio_workspace>cd yongqiang
D:\visual_studio_workspace\yongqiang>cd Debug
D:\visual_studio_workspace\yongqiang\Debug>dir
驱动器 D 中的卷是 DATA
卷的序列号是 90CB-45F2
D:\visual_studio_workspace\yongqiang\Debug 的目录
2020/03/21 21:10 <DIR> .
2020/03/21 21:10 <DIR> ..
2020/03/28 21:21 37,376 yongqiang.exe
2020/03/28 21:21 324,664 yongqiang.ilk
2020/03/28 21:21 651,264 yongqiang.pdb
3 个文件 1,013,304 字节
2 个目录 189,175,169,024 可用字节
D:\visual_studio_workspace\yongqiang\Debug>
D:\visual_studio_workspace\yongqiang\Debug>yongqiang.exe infile outfile
argc = 3
argv[0] --> yongqiang.exe
argv[1] --> infile
argv[2] --> outfile
argv[argc] = 00000000
D:\visual_studio_workspace\yongqiang\Debug>
4.3 Example
Invocation (调用): yongqiang.exe cheng yong qiang
//============================================================================
// Name : main
// Author : Yongqiang Cheng
// Version : Version 1.0.0
// Copyright : Copyright (c) 2019 Yongqiang Cheng
// Description : Hello World in C++, Ansi-style
//============================================================================
#include <stdio.h>
int main(int argc, char *argv[])
{
printf("argc = %d\n", argc);
for (int idx = 0; idx < argc; ++idx)
{
printf("argv[%d] --> %s\n", idx, argv[idx]);
}
printf("argv[argc] = %p\n", (void*)argv[argc]);
return 0;
}
D:\visual_studio_workspace\yongqiang\Debug
Microsoft Windows [版本 10.0.17763.1098]
(c) 2018 Microsoft Corporation。保留所有权利。
C:\Users\cheng>d:
D:\>cd visual_studio_workspace
D:\visual_studio_workspace>cd yongqiang
D:\visual_studio_workspace\yongqiang>dir
驱动器 D 中的卷是 DATA
卷的序列号是 90CB-45F2
D:\visual_studio_workspace\yongqiang 的目录
2020/03/28 23:37 <DIR> .
2020/03/28 23:37 <DIR> ..
2020/03/21 21:10 <DIR> Debug
2020/03/28 21:21 <DIR> yongqiang
2020/03/21 15:55 1,309 yongqiang.sln
2020/03/28 23:37 1,826,816 yongqiang.VC.db
2 个文件 1,828,125 字节
4 个目录 189,174,976,512 可用字节
D:\visual_studio_workspace\yongqiang>
D:\visual_studio_workspace\yongqiang>cd Debug
D:\visual_studio_workspace\yongqiang\Debug>dir
驱动器 D 中的卷是 DATA
卷的序列号是 90CB-45F2
D:\visual_studio_workspace\yongqiang\Debug 的目录
2020/03/21 21:10 <DIR> .
2020/03/21 21:10 <DIR> ..
2020/03/28 21:21 37,376 yongqiang.exe
2020/03/28 21:21 324,664 yongqiang.ilk
2020/03/28 21:21 651,264 yongqiang.pdb
3 个文件 1,013,304 字节
2 个目录 189,174,976,512 可用字节
D:\visual_studio_workspace\yongqiang\Debug>
D:\visual_studio_workspace\yongqiang\Debug>yongqiang.exe cheng yong qiang
argc = 4
argv[0] --> yongqiang.exe
argv[1] --> cheng
argv[2] --> yong
argv[3] --> qiang
argv[argc] = 00000000
D:\visual_studio_workspace\yongqiang\Debug>
References
https://en.cppreference.com/w/c/language/main_function
http://dbp-consulting.com/tutorials/debugging/linuxProgramStartup.html
