1. When passing a string as input, add const in general

For example:

int strlen(const char * str);

Here, the string str does not need to be modified inside this function, so it can be qualified with const.

This way, when we call strlen, we can write:

strlen("abc");

Without const, a warning will show up:

Note the difference:

char *strcpy(char *dest,const char *src);

The first parameter is for output, and the second is for input.

You can pass data directly as an input parameter, but an output parameter requires a pre-allocated buffer:

C
char dest[200];
strcpy(dest,"abc");

Only then will it work.

2. const means the object it qualifies is read-only

C
#include <stdio.h>

int main()
{
    const char *p = "abc";
    char str[]="def";
    p = str;
    printf("%s",p);
    return 0;
}

What is the result when you compile and run it?

The answer:

No errors; it prints def.

After being qualified with const, the memory that p points to is read-only, but the value of p itself can still change.