Question

can any body tell me how to conver const char* to char*?

get_error_from_header(void *ptr, size_t size, size_t nmemb, void *data) {
    ErrorMsg *error = (ErrorMsg *)data;
    char* err = strstr((const char *)ptr,"550");
    //error  cannot convert const char** to char*
    if(err) {
        strncpy(error->data,(char*)ptr,LENGTH_ERROR_MESSAGE-1);
        error->data[LENGTH_ERROR_MESSAGE-1] = '\0';
        error->ret = true;
    }
    return size*nmemb;
}

Answer 1

You don’t appear to use err in the rest of that function, so why bother creating it?

if (NULL != strstr((const char *)ptr, "550"))
{

If you do need it, will you really need to modify whatever it points to? If not, then declare it as const also:

const char* err = strstr((const char *)ptr, "550");

Finally, as casts are such nasty things, it is best to use a specific modern-style cast for the operation you want to perform. In this case:

if (NULL != strstr(reinterpret_cast<const char *>(ptr), "550"))
{