The internal mechanism of main function parameters, argc and argv

Occasionally interested in the parameters of the main function, write a program for verification.

Int main (int argc, char ** argv)

First, two points are explained:

First, the system will store the parameter list, that is, the command we input under shell, to a one-dimensional character array, and the user can be modified, there must be no constant area.

2. argv is actually a char * argv [], character pointer array. The number of elements is unclear. There are at least argc elements, and some others, such as HOSTNAME, SELINUX_ROLE_REQUESTED, TERM, SHELL, HISTSIZE, SSH_CLIENT, SELINUX_USE_CURRENT_RANGE, QTDIR, QTINC, SSH_TTY, SVN_EDITOR, etc,

 

Then, based on the space in the parameter list, divide the parameters, store the first address of each parameter to argv [I], and set * (argv [I]-1) to 0, use \ 0 instead of the original space to print the parameters.

The following shows my program. You can test it on your own.

/* Argcargv. c */

# Include <stdio. h>

# Include <string. h>

Int main (int argc, char ** argv)

{

Int I;

For (I = 0; I <argc/* 20, you can set a value greater than argc to see how many elements argv has */; I ++)

{

Printf ("% p, % s", argv [I], argv [I]);

Printf ("$ \ n ");

}

Printf ("\ n ");

Strcpy (argv [0], "meilidezhongguo, nihao! ");

For (I = 0; I <argc, I ++)

{

Printf ("% p, % s", argv [I], argv [I]);

Printf ("$ \ n ");

}

Printf ("\ n ");

}

Program running:./a. out abc 123 def 456 hello 789 world

My computer runs as follows:

0xbff64758,./a. out $

0xbff64760, abc $

0xbff64764, 123 $

0xbff64768, def $

0 x bff6476c, 456 $

0xbff64770, hello $

0xbff64776, 789 $

0xbff6477a, world $

(Nil), (null) $

.......

.......

.......

.......

 

0xbff64758, meilidezhongguo, nihao! $

0xbff64760, hongguo, nihao! $

0xbff64764, guo, nihao! $

0xbff64768, nihao! $

0xbff6476c, o! $

0xbff64770, hello $

0xbff64776, 789 $

0xbff6477a, world $

 

You can also use

Strncpy (argv [0], "meilidezhongguo, nihao! ", 28 );

Replace strcpy (argv [0], "meilidezhongguo, nihao! ");

Try,

We can also see that strncpy has copied 22 characters, strlen ("meilidezhongguo, nihao! "); Add \ 0 to the remaining six characters.

Understanding restrictions can only be added. Next time, you will have a new understanding and improvement!