Passing a string literal as a function parameter defined as a pointer

Question

I am reading the chapter on arrays and pointers in Kernighan and Richie's The C Programming Language.

They give the example:

/* strlen:  return length of string s */
int strlen(char *s)
{
    int n;

    for (n = 0; *s != '\0'; s++)
        n++;
    return n;
}

And then say:

“Since s is a pointer, incrementing it is perfectly legal; s++ has no effect on the character string in the function that called strlen, but merely increments strlen’s private copy of the pointer. That means that calls like

strlen("hello, world");  /* string constant */
strlen(array);           /* char array[100]; */
strlen(ptr);             /* char *ptr; */

all work.”

I feel like I understand all of this except the first call example: Why, or how, is the string literal "hello, world" treated as a char *s? How is this a pointer? Does the function assign this string literal as the value of its local variable *s and then use s as the array name/pointer?

Answer 1

To understand how a string like "Hello World" is converted to a pointer, it is important to understand that, the string is actually hexadecimal data starting at an address and moving along till it finds a NULL

So that means, every string constant such as "Hello World" is stored in the memory somewhere

Possibility would be:

0x10203040 : 0x48 [H]
0x10203041 : 0x65 [e]
0x10203042 : 0x6C [l]
0x10203043 : 0x6C [l]
0x10203044 : 0x6F [o]
0x10203045 : 0x20 [' ']
0x10203046 : 0x57 [W]
0x10203047 : 0x6F [o]
0x10203048 : 0x72 [r]
0x10203049 : 0x6C [l]
0x1020304A : 0x64 [d]
0x1020304B : 0x00 [\0]

So, when this function is called with the above values in the memory, [left side is address followed by ':' and the right side is ascii value of the character]

int strlen(const char *s)
{
    int n;

    for (n = 0; *s != ′\0′; s++)
        n++;
    return n;
}

strlen("Hello World");

at that time, what gets passed to strlen is the value 0x10203040 which is the address of the first element of the character array.

Notice, the address is passed by value.. hence, strlen has its own copy of the address of "Hello World". starting from n = 0, following uptil I find \0 in the memory, I increment n and also the address in s(which then gets incremented to 0x10203041) and so on, until it finds \0 at the address 0x1020304B and returns the string length.

Answer 2

"hello, world"

is an array of char (type is char[13]). The value of an array of char in an expression is a pointer to char. The pointer points to the first element of the array (i.e., the value of "hello, world" is &"hello, world"[0]).