ocurrencias.c

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

int indiceDeCaracterEnCadena(char *cadena, char caracter)
{
    int indice = 0;
    while (cadena[indice] != '\0')
    {
        char actual = cadena[indice];
        if (actual == caracter)
        {
            return indice;
        }
        indice++;
    }
    return -1;
}

int ultimoIndiceDeCaracterEnCadena(char *cadena, char caracter)
{
    int indice = strlen(cadena) - 1;
    while (indice >= 0)
    {
        char actual = cadena[indice];
        if (actual == caracter)
        {
            return indice;
        }
        indice--;
    }
    return -1;
}

int main()
{
    char cadena[] = "Hola, me llamo Parzibyte";
    char busqueda = 'o';
    printf("%d y %d", indiceDeCaracterEnCadena(cadena, busqueda), ultimoIndiceDeCaracterEnCadena(cadena, busqueda));
}