ejemplo-hack-threads.ipynb

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 "cells": [
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "En este ejemplo vamos a ver cómo se puede implementar el problema de los Lectores/Escritores con mutex.\n",
    "\n",
    "Tenemos dos cerrojos: El primero es un cerrojo global para bloquear la escritura de dato. El segundo es un cerrojo sobre la variable que cuenta el número de lectores."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "7\n",
      "9\n",
      "11\n",
      "13\n",
      "15\n",
      "17\n",
      "19\n",
      "21\n",
      "23\n",
      "25\n",
      "27\n",
      "29\n",
      "31\n",
      "33\n",
      "35\n",
      "37\n",
      "39\n",
      "41\n",
      "43\n",
      "45\n",
      "47\n",
      "49\n"
     ]
    }
   ],
   "source": [
    "//%cflags:-lpthread\n",
    "\n",
    "#include <stdio.h>\n",
    "#include <stdlib.h>\n",
    "#include <unistd.h>\n",
    "#include <pthread.h>\n",
    "\n",
    "int dato = 5;           /* recurso */\n",
    "int n_lect = 0;         /* numero de lectores*/\n",
    "pthread_mutex_t mutex;  /*controlar el acceso a dato*/\n",
    "pthread_mutex_t m_lect; /*controla la variable n_lect*/\n",
    "\n",
    "void* Lector(void* ptr);\n",
    "void* Escritor(void* ptr);\n",
    "\n",
    "void main(int argc, char *argv[]) {\n",
    "  pthread_t th1, th2, th3, th4;\n",
    "  pthread_mutex_init(&mutex, NULL);\n",
    "  pthread_mutex_init(&m_lect, NULL);\n",
    "  pthread_create(&th1, NULL, Lector, NULL);\n",
    "  pthread_create(&th2, NULL, Escritor, NULL);\n",
    "  pthread_create(&th3, NULL, Lector, NULL);\n",
    "  pthread_create(&th4, NULL, Escritor, NULL);\n",
    "  pthread_join(th1, NULL);\n",
    "  pthread_join(th2, NULL);\n",
    "  pthread_join(th3, NULL);\n",
    "  pthread_join(th4, NULL);\n",
    "  pthread_mutex_destroy(&mutex);\n",
    "  pthread_mutex_destroy(&m_lect);\n",
    "  exit(0);\n",
    "}\n",
    "\n",
    "/*codigo del lector */\n",
    "void* Lector(void* ptr) {\n",
    "  while(dato<50) { // <- INCORRECTO, PARA QUE SEA FINITO\n",
    "    pthread_mutex_lock(&m_lect);\n",
    "    n_lect ++;\n",
    "    if (n_lect == 1) {\n",
    "      pthread_mutex_lock(&mutex);\n",
    "    }\n",
    "    pthread_mutex_unlock(&m_lect);\n",
    "    printf(\"%d\\n\", dato); /*leer*/\n",
    "    fflush(stdout);\n",
    "    pthread_mutex_lock(&m_lect);\n",
    "    n_lect--;\n",
    "    if (n_lect == 0) {\n",
    "      pthread_mutex_unlock(&mutex);\n",
    "    }\n",
    "    pthread_mutex_unlock(&m_lect);\n",
    "    sleep(1);\n",
    "  }\n",
    "}\n",
    "\n",
    "/*codigo del escritor */\n",
    "void* Escritor(void* ptr) {\n",
    "  while(dato<50) { // <- INCORRECTO, PARA QUE SEA FINITO\n",
    "    pthread_mutex_lock(&mutex);\n",
    "    dato = dato + 2; /*modificar el recurso */\n",
    "    pthread_mutex_unlock(&mutex);\n",
    "    sleep(1);\n",
    "  }\n",
    "}"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Ahora vamos a ver si una hebra puede abrir un cerrojo echado por otra hebra:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "//%cflags:-lpthread\n",
    "\n",
    "#include <stdio.h>\n",
    "#include <stdlib.h>\n",
    "#include <pthread.h>\n",
    "\n",
    "pthread_mutex_t cerrojo;  \n",
    "\n",
    "void* Hebra1(void* ptr);\n",
    "void* Hebra2(void* ptr);\n",
    "\n",
    "void main(int argc, char *argv[]) {\n",
    "  pthread_t th1, th2;\n",
    "  pthread_mutex_init(&cerrojo, NULL);\n",
    "  pthread_create(&th1, NULL, Hebra1, NULL);\n",
    "  pthread_create(&th2, NULL, Hebra2, NULL);\n",
    "  pthread_join(th1, NULL);\n",
    "  pthread_join(th2, NULL);\n",
    "  pthread_mutex_destroy(&cerrojo);\n",
    "  exit(0);\n",
    "}\n",
    "\n",
    "/*codigo de la hebra */\n",
    "void* Hebra1(void* ptr) {\n",
    "  int i;\n",
    "  printf(\"H1: Hola, soy la hebra1. Voy a echar el cerrojo.\\n\");\n",
    "  pthread_mutex_lock(&cerrojo);\n",
    "  printf(\"H1: ... ya está. Se supone que nadie debe pasar el lock.\\n\");\n",
    "  printf(\"H1: ... me meto en un bucle un poco largo.\\n\");\n",
    "  for(i=0; i<1000000; i++) {\n",
    "  }\n",
    "  printf(\"H1: ... y ahora abro el cerrojo.\\n\");\n",
    "  pthread_mutex_unlock(&cerrojo);\n",
    "  printf(\"H1: ... ya está.\\n\");\n",
    "}\n",
    "\n",
    "/*codigo de la hebra 2 */\n",
    "void* Hebra2(void* ptr) {\n",
    "  int i;\n",
    "  printf(\"H2: Hola, soy la hebra2. Me meto en un bucle corto.\\n\");\n",
    "  for(i=0; i<10000; i++) {\n",
    "  }\n",
    "  printf(\"H2: ... y ahora abro el cerrojo.\\n\");\n",
    "  pthread_mutex_unlock(&cerrojo);\n",
    "  printf(\"H2: ... lo vuelvo a cerrar para ver si puedo pasar.\\n\");\n",
    "  pthread_mutex_lock(&cerrojo);\n",
    "  printf(\"H2: ... y me meto en un bucle un poco largo.\\n\");\n",
    "  for(i=0; i<1000000; i++) {\n",
    "  }\n",
    "  printf(\"H2: ... ya está.\\n\");\n",
    "}"
   ]
  }
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