Does indexing of the array of command line argument pointers start from 1 or from 0?

Question

Programming

To be precise, I'm concerned about the line mem_size = atoi(argv[1]);

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

int main(int argc, char *argv[])
{
  char *char_ptr;  // Указатель на символьный тип
  int *int_ptr;    // Указатель на целый тип
  int mem_size;

  if (argc < 2)		// Если нет аргументов командной строки,
    mem_size = 50;  // исползуется 50 - значение по умолчанию
  else
    mem_size = atoi(argv[0]);

  printf("\t[+] allocating %d bytes of memory on the heap for char_ptr\n", mem_size);
  
  char_ptr = (char *) malloc(mem_size);  // Выделение памяти в куче

  if(char_ptr == NULL)    // Проверка ошибки сбоя функции malloc()
  {
    fprintf(stderr, "Error: could not allocate heap memory.\n");
    exit(-1);
  }

  strcpy(char_ptr, "Thes is memory is located in the heap.");
  printf("char_ptr (%p) --> '%s'\n", char_ptr, char_ptr);

  printf("\t[+] allocating 12 bytes of memory on the heap for Int_ptr\n");
  int_ptr = (int *) malloc(12);  // Еще раз выделяем память в куче

  if(int_ptr == NULL)
  {
    fprintf(stderr, "Error: could not alllocate heap memory.\n");
    exit(-1);
  }

  strcpy(char_ptr, "new memory");
  printf("char_ptr (%p) --> '%s'\n", char_ptr, char_ptr);

  printf("\t[-] freeing int_ptr's heap memory...\n");
  free(int_ptr);  // Освободить память в куче
  printf("\t[-] freeing char_ptr's heap memory..\n");
  free(char_ptr); // Освободить еще один блок памяти в куче
}

PS I tried with 1 and 0 to run the program. Works this way and that. But if I start typing something after calling the program from 0 like ./tree.out 123 , then the program crashes

[email protected]:~$ ./tree.out 0913
  [+] allocating 0 bytes of memory on the heap for char_ptr
char_ptr (0x1702420) --> 'Thes is memory is located in the heap.'
  [+] allocating 12 bytes of memory on the heap for Int_ptr
char_ptr (0x1702420) --> 'new memory'
  [-] freeing int_ptr's heap memory...
*** Error in `./tree.out': free(): invalid next size (fast): 0x0000000001702440 ***
======= Backtrace: =========
/lib/x86_64-linux-gnu/libc.so.6(+0x777e5)[0x7ffa970a17e5]
/lib/x86_64-linux-gnu/libc.so.6(+0x8037a)[0x7ffa970aa37a]
/lib/x86_64-linux-gnu/libc.so.6(cfree+0x4c)[0x7ffa970ae53c]
./tree.out[0x400892]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf0)[0x7ffa9704a830]
./tree.out[0x400649]
======= Memory map: ========
00400000-00401000 r-xp 00000000 08:01 20578604                           /home/king/tree.out
00600000-00601000 r--p 00000000 08:01 20578604                           /home/king/tree.out
00601000-00602000 rw-p 00001000 08:01 20578604                           /home/king/tree.out
01702000-01723000 rw-p 00000000 00:00 0                                  [heap]
7ffa90000000-7ffa90021000 rw-p 00000000 00:00 0 
7ffa90021000-7ffa94000000 ---p 00000000 00:00 0 
7ffa96e14000-7ffa96e2a000 r-xp 00000000 08:01 25301471                   /lib/x86_64-linux-gnu/libgcc_s.so.1
7ffa96e2a000-7ffa97029000 ---p 00016000 08:01 25301471                   /lib/x86_64-linux-gnu/libgcc_s.so.1
7ffa97029000-7ffa9702a000 rw-p 00015000 08:01 25301471                   /lib/x86_64-linux-gnu/libgcc_s.so.1
7ffa9702a000-7ffa971ea000 r-xp 00000000 08:01 25301433                   /lib/x86_64-linux-gnu/libc-2.23.so
7ffa971ea000-7ffa973ea000 ---p 001c0000 08:01 25301433                   /lib/x86_64-linux-gnu/libc-2.23.so
7ffa973ea000-7ffa973ee000 r--p 001c0000 08:01 25301433                   /lib/x86_64-linux-gnu/libc-2.23.so
7ffa973ee000-7ffa973f0000 rw-p 001c4000 08:01 25301433                   /lib/x86_64-linux-gnu/libc-2.23.so
7ffa973f0000-7ffa973f4000 rw-p 00000000 00:00 0 
7ffa973f4000-7ffa9741a000 r-xp 00000000 08:01 25301405                   /lib/x86_64-linux-gnu/ld-2.23.so
7ffa975fa000-7ffa975fd000 rw-p 00000000 00:00 0 
7ffa97616000-7ffa97619000 rw-p 00000000 00:00 0 
7ffa97619000-7ffa9761a000 r--p 00025000 08:01 25301405                   /lib/x86_64-linux-gnu/ld-2.23.so
7ffa9761a000-7ffa9761b000 rw-p 00026000 08:01 25301405                   /lib/x86_64-linux-gnu/ld-2.23.so
7ffa9761b000-7ffa9761c000 rw-p 00000000 00:00 0 
7fff870f0000-7fff87111000 rw-p 00000000 00:00 0                          [stack]
7fff871c3000-7fff871c5000 r--p 00000000 00:00 0                          [vvar]
7fff871c5000-7fff871c7000 r-xp 00000000 00:00 0                          [vdso]
ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0                  [vsyscall]
Аварийный останов (сделан дамп памяти)

With the 1st program, it only works with large numbers (about more than 70), and it also fails with small numbers.

Answer 1

https://code-live.ru/post/cpp-command-line-arguments/

Answer 2

C 1 or 0 starts indexing the array of command line argument pointers

The null element usually stores the path to your program's executable file itself.
And starting from the 1st element - command line arguments.

Answer 3

https://msdn.microsoft.com/en-us/library/88w63h9k.aspx
And the program crashes because you do not check the size of the allocated buffer before using the function

strcpy(char_ptr, "Thes is memory is located in the heap.");

If less than the length of your string +1 is allocated, then the rest of the string is written to an arbitrary location, causing an error.

Answer 4

use strncpy() so it's more reliable and more..
"If the string contains a valid sequence of digits representing the number 0, then 0 is also returned, and it is impossible to determine from the returned number whether the string contains a valid number or not. The newer strtol function does not have this disadvantage, so in cases where it is critical, it should be used.
replace your line
else
mem_size = atoi(argv[0]);
on else
mem_size = atoi(argv[1]);
so it doesn't crash (program name ./atoi)
output
[email protected]:~/tmp$ ./atoi 0123
[+] allocating 123 bytes of memory on the heap for char_ptr
char_ptr (0xbd3010) --> 'Thes is memory is located in the heap.'
[+] allocating 12 bytes of memory on the heap for Int_ptr
char_ptr (0xbd3010) --> 'new memory'
[-] freeing int_ptr's heap memory...
[-] freeing char_ptr's heap memory..
[email protected] :~/tmp$
Well, it works without parameters. Then the memory is 50 by default.
And it crashes if the memory is passed zero, then $ ./atoi 0
then the output is
*** Error in `./atoi': free(): invalid next size (fast): 0x0000000000973030 ***
Crash (memory dumped)
like this that also enter the check of the passed arguments into the code.