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

typedef struct {
  char *pointer;
  int size;
} safe_char_pointer;

static safe_char_pointer safe_malloc (int size) {
  safe_char_pointer t;
  t.pointer = malloc(size);
  t.size = size;
  return t;
};

static inline void safe_write(safe_char_pointer x, int i, char c) {
  if (i < 0 || i >= x.size) {
    fprintf(stderr, "Array limits exceeded!\n");
    exit(1);
  };
  x.pointer[i] = c;
};

static inline char safe_read(safe_char_pointer x, int i) {
  if (i < 0 || i >= x.size) {
    fprintf(stderr, "Array limits exceeded!\n");
    exit(1);
  };
  return x.pointer[i];
};

int main () {

  safe_char_pointer a = safe_malloc(10);

  for (int i = 0; i < 10; i++) {
    safe_write(a, i, 65 + i);
  };

  safe_write(a, 9, 0);
  printf("Some shit: %s\n", a.pointer);

  for (int i = 0; i <= 10; i++) {
    int c = safe_read(a, i);
    printf("Byte %d is %d\n", i, c);
  };

};

  0040058d  E8AEFFFFFF        call qword 0x400540 This is the call to malloc()
  00400592  BEA4074000        mov esi,0x4007a4 This is the pointer to the format string used for printf()
  00400597  C60041            mov byte [rax],0x41
  0040059a  C6400142          mov byte [rax+0x1],0x42
  0040059e  C6400243          mov byte [rax+0x2],0x43
  004005a2  C6400344          mov byte [rax+0x3],0x44
  004005a6  4889C2            mov rdx,rax
  004005a9  C6400445          mov byte [rax+0x4],0x45
  004005ad  C6400546          mov byte [rax+0x5],0x46
  004005b1  4889C5            mov rbp,rax
  004005b4  C6400647          mov byte [rax+0x6],0x47
  004005b8  C6400748          mov byte [rax+0x7],0x48
  004005bc  BF01000000        mov edi,0x1
  004005c1  C6400849          mov byte [rax+0x8],0x49
  004005c5  C6400900          mov byte [rax+0x9],0x0
  004005c9  31C0              xor eax,eax
  004005cb  E880FFFFFF        call qword 0x400550 This is the call to printf()

  004005d0  0FBE4C1D00        movsx ecx,byte [rbp+rbx+0x0]
  004005d5  89DA              mov edx,ebx
  004005d7  31C0              xor eax,eax
  004005d9  BECA074000        mov esi,0x4007ca This is the address of the format string for printf().
  004005de  BF01000000        mov edi,0x1
  004005e3  4883C301          add rbx,byte +0x1
  004005e7  E864FFFFFF        call qword 0x400550 This is the call to printf()
  004005ec  4883FB0A          cmp rbx,byte +0xa Compare variable 'i' to 10
  004005f0  75DE              jnz 0x4005d0 main+0x50 If it isn't ten, loop back to the beginning.
  004005f2  488B0D5F0A2000    mov rcx,[rel 0x601058] __TMC_END__
  004005f9  BFB3074000        mov edi,0x4007b3 This is the string that contains the error message.
  004005fe  BA16000000        mov edx,0x16
  00400603  BE01000000        mov esi,0x1
  00400608  E863FFFFFF        call qword 0x400570 This is the call to fprintf() to write the error message.
  0040060d  BF01000000        mov edi,0x1
  00400612  E849FFFFFF        call qword 0x400560 This is the call to exit().

  00000026  0F8EB9000000      jng qword 0xe5 if size is insufficient, jump to error routine
  0000002c  83FE03            cmp esi,byte +0x3 test size against what is needed for the next iteration of this loop
  0000002f  C6470243          mov byte [rdi+0x2],0x43 write the value for this iteration of this loop

void external_function (safe_char_pointer a) {

  safe_write(a, 9, 0);

  for (int i = 0; i < 10; i++) {
    safe_write(a, i, 65 + i);
  };

  safe_write(a, 9, 0);
  printf("Some shit: %s\n", a.pointer);

  for (int i = 0; i <= 10; i++) {
    int c = safe_read(a, i);
    printf("Byte %d is %d\n", i, c);
  };

};

  00000032  83FE09            cmp esi,byte +0x9 compare array size to 9
  00000035  55                push rbp
  00000036  53                push rbx
  00000037  0F8EB5000000      jng qword 0xf2 if size is not greater than 9, jump to error routine
  0000003d  4889FD            mov rbp,rdi
  00000040  4889FA            mov rdx,rdi
  00000043  C60741            mov byte [rdi],0x41
  00000046  C6470142          mov byte [rdi+0x1],0x42
  0000004a  C6470243          mov byte [rdi+0x2],0x43
  0000004e  4189F4            mov r12d,esi
  00000051  C6470344          mov byte [rdi+0x3],0x44
  00000055  C6470445          mov byte [rdi+0x4],0x45
  00000059  BE00000000        mov esi,0x0
  0000005e  C6470546          mov byte [rdi+0x5],0x46
  00000062  C6470647          mov byte [rdi+0x6],0x47
  00000066  31C0              xor eax,eax
  00000068  C6470748          mov byte [rdi+0x7],0x48
  0000006c  C6470849          mov byte [rdi+0x8],0x49
  00000070  4883C501          add rbp,byte +0x1
  00000074  C6470900          mov byte [rdi+0x9],0x0
  00000078  BF01000000        mov edi,0x1