int my_atoi(char* pointer)
{
    int result = 0;
    char* pointer1;
    multiplier = 1;
    char sign = 1;

    if(*pointer == '-')
    sign =- 1;

    pointer1 = pointer;

    while(*pointer != '')
    {
        if(*pointer >= '0' && *pointer <= '9')
            multiplier = multiplier * 10;

        pointer = pointer + 1;
    }

    pointer = pointer1;

    while(*pointer != '')
    {
        if(*pointer >= '0' && *pointer <= '9')
        {
            result = result + ( (*pointer%48)  * multiplier);
            multiplier = multiplier / 10;
        }

        pointer = pointer+1;
    }

    return (result * sign) / 10;
}

while(*pointer != '')
{
    if(*pointer >= '0' && *pointer <= '9')
        multiplier = multiplier * 10;

    pointer = pointer + 1;
}

pointer = pointer1;

while(*pointer != '')
{
    if(*pointer >= '0' && *pointer <= '9')
    {
        result = result + ( (*pointer%48)  * multiplier);
        multiplier = multiplier / 10;
    }

    pointer = pointer+1;
}

while (isdigit(*c))
{
    value *= 10;
    value += (int) (*c - '0');
    c++;
}

result = result + ( (*pointer%48)  * multiplier);

if(*pointer == '-')
    sign =- 1;

if(*pointer == '-') sign = -1;

int my_atoi(const char* pointer)

pointer++;         // same as pointer = pointer+1;
multiplier /= 10;  // same as multiplier = multiplier / 10;
multiplier *= 10;  // same as multiplier = multiplier * 10;

#include <stdio.h>
#include <assert.h>
#include <ctype.h>

long long int my_atoi(const char *c)
{
    long long int value = 0;
    int sign = 1;
    if( *c == '+' || *c == '-' )
    {
        if( *c == '-' ) sign = -1;
        c++;
    }
    while (isdigit(*c))
    {
        value *= 10;
        value += (int) (*c-'0');
        c++;
    }
    return (value * sign);
}

int main(void)
{
    assert(5 == my_atoi("5"));
    assert(-2 == my_atoi("-2"));
    assert(-1098273980709871235 == my_atoi("-1098273980709871235"));
    puts("All good."); // I reach this statement on my system
}

int my_atoi(const char* pointer) {
  while (isspace((unsigned char) *pointer)) {
    pointer++;
  }
  ...

#include <ctype.h>
#include <limits.h>

int my_atoi(const char* pointer) { // good idea to make the `const`
  int result = 0;
  while (isspace((unsigned char) *pointer)) {
    pointer++;
  }
  char sign = *pointer;
  if (*pointer == '-' || *pointer == '+') {  // text could lead with a '+'
    pointer++;
  }
  int ch;
  // isdigit() expects an unsigned char or EOF, not char
  while ((ch = (unsigned char)(*pointer)) != 0) {
    if (!isdigit(ch)) break;
    ch -= '0';
    // Will overflow occur?
    if ((result < INT_MIN/10) ||
        (result == INT_MIN/10  && ch > -(INT_MIN%10))) Handle_Overflow();
    result *= 10;
    result -= ch;  // - , not +
    pointer++;
    }
  if (sign != '-') {
    if (result < -INT_MAX) Handle_Overflow();
    result = -result;
  }
  return result;
}

int my_atoi(const char *s, int *ErrorCode);

int my_atoi(const char *s, const char **endptr);

int my_atoi(const char* pointer) {
  int result = 0;
  while (isspace((unsigned char) *pointer)) {
    pointer++;
  }
  char sign = *pointer;
  if (*pointer == '-' || *pointer == '+') {
    pointer++;
  }
  while (isdigit((unsigned char)*pointer)) {
    result = result*10 - (*pointer++ - '0');
    }
  if (sign != '-') {
    result = -result;
  }
  return result;
}

char sign = *pointer;
  if (*pointer == '-' || *pointer == '+') {
    pointer++;
  }

char sign = *pointer;
  if (sign == '-' || sign == '+') {
    pointer++;
  }

#include <string.h>
#include <math.h>
#include <stdbool.h>

int to_natural_number(const char* string) {
    int index = strlen(string) - 1;
    int number = pow(10, index) * (*string - '0');

    return (index == 0) ? number : number + to_natural_number(string + 1);
}

bool isMinusSign(char symbol) {
    return symbol == '-';
}

int my_atoi(const char* string) {
    int sign = isMinusSign(*string) ? -1 : 1;
    int offset = isMinusSign(*string) ? 1 : 0;

    return sign * to_natural_number(string + offset);
}

/* test cases */
my_atoi("-100") == -100;
my_atoi("0") == 0;
my_atoi("100") == 100;