char* geraStringAleatoria(){

char *validchars = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
char *novastr;
register int i;
int str_len;

// novo tamanho
str_len = (rand() % MAX_STR_SIZE );

// checa tamanho
str_len += ( str_len < MIN_STR_SIZE ) ? MIN_STR_SIZE : 0;

// aloca memoria
novastr = ( char * ) malloc ( (str_len + 1) * sizeof(char));
if ( !novastr ){
    printf("[*] Erro ao alocar memoria.n" );
    exit ( EXIT_FAILURE );
}

// gera string aleatória
for ( i = 0; i < str_len; i++ ) {
    novastr[i] = validchars[ rand() % strlen(validchars) ];
    novastr[i + 1] = 0x0;
}

   return novastr;
}