The same lines in the array have the same memory address

Question

Why do the same lines in a char * array have the same address?

Is this because of compiler optimization?

Example:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define ARR_SIZE 7

int main(int argc, char** argv) {
  size_t i = 0, j = 0;

  char * myArr[ARR_SIZE] = {
    "This is the first string",
    "This is the second string",
    "This is Engie",
    "This is the third string",
    "This is Engie",
    "This is the fifth string",
    "This is Engie"

  };

  for (i = 0; i < ARR_SIZE; ++i){
    for (j = i + 1; j < ARR_SIZE; ++j){
      if (memcmp((myArr + i), (myArr + j), sizeof(char*)) == 0){
      fprintf(stdout, "%p, %p\n", *(myArr + i), *(myArr + j));
      fprintf(stdout, "found it start index: %lu, search index: %lu\n", i, j);
      }
    }
  }
  return 0;
}

Gdb:

(gdb) x/7w myArr
0x7fffffffdd10: U"\x4007a8"
0x7fffffffdd18: U"\x4007c1"
0x7fffffffdd20: U"\x4007db"
0x7fffffffdd28: U"\x4007e9"
0x7fffffffdd30: U"\x4007db"
0x7fffffffdd38: U"\x400802"
0x7fffffffdd40: U"\x4007db"


(gdb) x/7s *myArr
0x4007a8:   "This is the first string"
0x4007c1:   "This is the second string"
0x4007db:   "This is Engie"
0x4007e9:   "This is the third string"
0x400802:   "This is the fifth string"
0x40081b:   "%p, %p\n"
0x400823:   ""

Answer 1

It is called continuous merging. It is usually supported at higher optimization levels. The compiler simply takes all the unique constant values ​​and pushes them down. Good for memory usage and caching efficiency.

gcc owns -fmerge-constantsor uses -O and company

Other compilers may or may not do this. It is specific to the compiler.

, , ++ .

, :

• , ( undefined)
• ( undefined)

( ), , const.