Why string assignment require new memory location?

Question

Why string assignment require new memory location?

Debojit Acharjee 455

When any integer or a single character is assigned to a pointer, it uses the same memory location, but why a sting value is stored in different memory locations in every assignment?

IN the following program integer 30 and character 'c' takes the same memory location but why the string "C" and "D" takes different memory locations?

#include <stdio.h>

int main()

{

    char *c;

    *c = 30;
    printf("The memory location of 30 is: %x\n", c);

    *c = 'c';
    printf("The memory location of 'c' is: %x\n", c);

    c = "C";
    printf("The memory location of \"C\" is: %x\n", c);

    c = "D";
    printf("The memory location of \"D\" is: %x\n", c);

    return 0;
}

Output:

The memory location of 30 is: 265000

The memory location of 'c' is: 265000

The memory location of "C" is: 4050ab

The memory location of "D" is: 4050d3

1 answer

Your answer

Answer 1

didier bagarry 160

Until the line c = "C"; the pointer c has no given values, so lines before are undefined behaviors! You try to write value at the address given by c.

The address displayed before is the random value in c.

After, your code displays c value which is the address of the array "C" then displays the address of the array "D". The strings "C" and "D" are string literals their have each a fixed address displayed by your code and their address is obviously different.

A pointer is simply a variable which store an address, like an int can store a integral number. We can read or write where the c pointer points using a *. Nether try to do *c before having set c.

Debojit Acharjee 455 Reputation points

2023-06-04T13:37:18.6866667+00:00

But I didn't get the answer--Why only the strings stored in different locations?
didier bagarry 160 Reputation points

2023-06-04T16:20:27.8566667+00:00

All objects are stored somewhere and all objects have a specific address. For the 2 first, you decided to store somewhere, you use the *c = ... syntax so address displayed is c, for the 2 last you use the c = ... syntax so c has new value, the address of the object.
Try to revert the order (the string first), you will see an error, because 10 cannot be stored over a constant string.
Debojit Acharjee 455 Reputation points

2023-06-05T05:37:25.6066667+00:00

Why 10 are you referring to because in the last program I included 30. And for the last 2 i used c instead of *c because for a string pointer you can't assign to *c because as per the syntax it should be written as c = "C". Whether you use *c or c it's going to mean the same thing because c has been declared as character pointer. But my question is why different addresses were used only for the strings?
Minxin Yu 13,501 Reputation points Microsoft External Staff

2023-06-05T09:51:57.3333333+00:00
String literals are stored in the read-only memory segment. Each string has its own address. Print the char* a and it is the same as char* c

char* a = "C";
Debojit Acharjee 455 Reputation points

2023-06-05T11:42:54.8933333+00:00

There is nothing like real-only memory in RAM. Why other types are not stored in different locations?
Xiaopo Yang - MSFT 12,731 Reputation points Microsoft External Staff

2023-06-06T02:54:00.88+00:00

Apart from literal values like the string type you said, the constant global variables are also stored in data segment.

didier bagarry 160

@Debojit Acharjee, you say "Whether you use *c or c it's going to mean the same thing because c has been declared as character pointer".
No, *c and c do not do the same think. The first is an access the data POINTED by c(cannot modify c value), the second is an access to the POINTER c.

char*  pt;
int x = 'A';
pt = &x;     // now pt points to x
*pt = 'Z';   // change x
printf( "%p %c\n", pt, *pt ); // display address and value of x

*pt = 100;   // also change x, not pt
printf( "%p %c\n", pt, *pt ); // address is same, of course pt still contains address of pt

char msg[] = "ABC";
pt = msg;  // or pt = &msg[0], now pt points to first char of msg
*pt = 'X';  // modify 1st char of msg
printf( "%p %s\n", pt, pt ); // display address and msg (no * for 2nd param, because print needs the address to see access to each successives chars)

char* const_txt_ptr = "CDE";
pt = const_txt_ptr; // same as pt = "CDE"
// here *pt cannot be written ("CDE" is only readable)
printf( "%p %s\n", pt, pt ); // display address and value of const_txt_ptr (the same as "CDE")

A pointer is nothing magic. It's a variable, you can change its value modifying it. Its special capability is to access something pointed with the notation *pt or pt[0].

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Why string assignment require new memory location?

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