C语言程序在内存中布局图

High Address +-------------------------+
   ↑         | Command-line arguments  |
   |         | & environment variables |
   |         +-------------------------+
   |         |        Stack area       |
   |         |            |            |
   |         |            |            |
   |         |            ↓            |
   |         +-------------------------+
   |         |            ↑            |
   |         |            |            |
   |         |            |            |
   |         |        Heap area        |
   |         +-------------------------+
   |         |   uninitialized data    | // BSS (Block Started by Symbol) Segment
   |         +-------------------------+
   |         |    initialized data     | // Data Segment
   |         +-------------------------+
   |         |    code/text segment    |
Low Address  +-------------------------+

main函数栈帧图

        +---------------------+
        |   Null-terminated   |
        |   env var strings   |<------+
        +---------------------+       |
        |   Null-terminated   |       |
        | cmdline arg strings |<--+   |
        +---------------------+   |   |
        |        unused       |   |   |
        +---------------------+   |   |
        |   envp[n] == NULL   |   |   |
        +---------------------+   |   |
        |      envp[n-1]      |   |   |
        +---------------------+   |   |
        |         ...         |   |   |
        +---------------------+   |   |
+------>|       envp[0]       |-------+
|       +---------------------+   |
|       |  argv[argc] == NULL |   |
|       +---------------------+   |
|       |    argv[argc-1]     |   |
|       +---------------------+   |
|       |         ...         |   |
|       +---------------------+   |
|   +-->|       argv[0]       |---+
|   |   +---------------------+
|   |   |     Linker vars     |
|   |   +---------------------+
+-------|         envp        |
    |   +---------------------+
    +---|         argv        |
        +---------------------+
        |         argc        |
        +---------------------+
        |   Stack frame for   |
        |    main function    |
        +---------------------+

输出所有命令行参数：

#include <stdio.h>

int
main(int argc, char *argv[])
{
    int     i;

    /* echo all command-line args */
    // for (i = 0; argv[i] != NULL; i++) {
    for (i = 0; i < argc; i++) {
        printf("argv[%d]: %s\n", i, argv[i]);
    }

    /* `exit(0);` is the same as `return(0);` from the `main` function. */
    return 0;
}

APUE 摘录：Returning an integer value from the main function is equivalent to calling exit with the same value. Thus exit(0); is the same as return(0); from the main function.

APUE 摘录：We are guaranteed by both ISO C and POSIX.1 that argv[argc] is a null pointer. This lets us alternatively code the argument-processing loop as for (i = 0; argv[i] != NULL; i++).