Possible error in the logging of a function that returns whether an integer is an Armstrong number

Question

Armstrong numbers are numbers that are the sum of their own digits, each raised to the number of digits.
Ex: 371 (3 digits) = 3³ + 7³ + 1³.
Knowing this, my algorithm gives error with the number 153 and only with it (the program gives me the wrong result, that is, the function returns 0), I could not find out why. I compile by gcc through vscode.

Some Armstrong numbers for testing: from 1 to 9, 153, 370, 371, 407, 1654.

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

int eh_numero_de_armstrong(int n);

int main()
{
  int x;

  printf("Digite um numero qualquer: ");
  scanf("%d", &x);

  if (eh_numero_de_armstrong(x))
    printf("%d eh um numero de Armstrong.\n", x);
  else
    printf("%d nao eh um numero de Armstrong.\n", x);

  return 0;
}

int eh_numero_de_armstrong(int n)
{
  char str[100];
  int soma_alg = 0;

  sprintf(str, "%d", n);
  for (int i = 0; i < strlen(str); i++)
    soma_alg += pow(str[i] - '0', strlen(str));

  if (n == soma_alg)
    return 1;
  else
    return 0;
}

Answer 1

The program was working for some, it also worked on an online compiler for me, but in the end I did a function to calculate the power. I just added this function and swapped Pow for calc_pow in the question code.

Thanks for trying to help, @anonimo @user140828

int calc_pow(int base, int exp)
{
  int pow_result = 1;

  for (int i = 1; i <= exp; i++)
    pow_result *= base;

  return pow_result;
}