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--- os_cp:threads [2025/04/16 21:57]
jkonczak
+++ os_cp:threads [2025/04/25 13:55] (current)
jkonczak [POSIX condition variables]
@@ Line 704: / Line 704: @@
 ===== Critical sections, deadlocks =====
-~~Exercise.#~~ Which lines of code are the critical section in the code below?
+~~Exercise.#~~ Download and read through the {{os_cp:threads:another_bad_idea.c}} file. Which lines of code are the critical section? \\ <small>The vital parts of the file is presented below.</small>
-~~Exercise.#~~ Spot the deadlock scenario in the code below.
+~~Exercise.#~~ Spot the deadlock scenario in the code from the previous exercise.
 ~~Exercise.#~~ Fix the code so that it no longer is able to deadlock.
-<html><div style="line-height:1.2em"></html>
+<html><div style="line-height:1em"></html>
-<small>
+<code c>
-<code c another_bad_idea.c>
+  …
-#include <pthread.h>
+│ struct myIO {FILE *in; FILE *out;};
-#include <stdint.h>
+│ void* threadFunc(void *num);
-#include <stdio.h>
+  …
-#include <stdlib.h>
+│ struct {
-#include <string.h>
+│   pthread_mutex_t mtx;
+│   char text[256];
-struct {
+│ } item[5];
-  pthread_mutex_t mtx;
+│
-  char text[256];
+│ int main(int argc, char **argv) {
-} item[5];
+  …
+│   for (intptr_t i = 1; i < 5; ++i) {
-const char *arg0;
+│     pthread_t tid;
+│     pthread_create(&tid, NULL, threadFunc, (void *)i);
-void *threadFunc(intptr_t num) {
+│     pthread_detach(tid);
-  char name[1024], cmd[1024];
+│   }
-  sprintf(name, "%s.pipe.%ld", arg0, num);
+│   threadFunc(0);
-  sprintf(cmd, "rm -f %s; mkfifo %s", name, name);
+│ }
-  system(cmd);
+│
-  FILE *myPipe = fopen(name, "r+");
+│ void *threadFunc(void* numRaw) {
+│   intptr_t num = (intptr_t) numRaw;
-  while (1) {
+│   struct myIO win = openWin(num);
-    char line[1024], nl;
+  …
-    fscanf(myPipe, "%1023[^\n]%c", line, &nl);
+│   while (1) {
+│     fprintf(win.out, "> ");
-    int argOne = atoi(line);
+│
-    if (argOne >= 5 || argOne < 0)
+│     char line[1024];
-      continue;
+│     fgets(line, 1024, win.in);
+│     line[strlen(line)-1] = 0;
-    char *argTwoTxt = strchr(line, ' ');
+│
+│     int argOne = atoi(line);
-    if (!argTwoTxt) {
+  …
-      pthread_mutex_lock(&item[argOne].mtx);
+│     char *argTwoTxt = strchr(line, ' ');
-      printf("T%ld reads %d as: %s\n", num, argOne, item[argOne].text);
+│
-      pthread_mutex_unlock(&item[argOne].mtx);
+│     if (!argTwoTxt) {
-      continue;
+│       pthread_mutex_lock(&item[argOne].mtx);
-    }
+│       fprintf(win.out, "T#%ld reads %d as: %s\n", num, argOne, item[argOne].text);
+│       pthread_mutex_unlock(&item[argOne].mtx);
-    argTwoTxt++;
+│       continue;
-    char *e;
+│     }
-    int argTwo = strtol(argTwoTxt, &e, 10);
+│
+│     argTwoTxt++;
-    if (!*e && argTwo < 5 && argTwo >= 0 && argOne != argTwo) {
+│     char *e;
-      pthread_mutex_lock(&item[argOne].mtx);
+│     int argTwo = strtol(argTwoTxt, &e, 10);
-      pthread_mutex_lock(&item[argTwo].mtx);
+│
-      printf("T%ld copies %d to %d\n", num, argTwo, argOne);
+│     if (!*e && argTwo < 5 && argTwo >= 0 && argOne != argTwo) {
-      memcpy(item[argOne].text, item[argTwo].text, sizeof(item[argOne].text));
+│       pthread_mutex_lock(&item[argOne].mtx);
-      pthread_mutex_unlock(&item[argTwo].mtx);
+│       pthread_mutex_lock(&item[argTwo].mtx);
-      pthread_mutex_unlock(&item[argOne].mtx);
+│       fprintf(win.out, "T#%ld copies %d to %d\n", num, argTwo, argOne);
-    } else {
+│       memcpy(item[argOne].text, item[argTwo].text, sizeof(item[argOne].text));
-      pthread_mutex_lock(&item[argOne].mtx);
+│       pthread_mutex_unlock(&item[argTwo].mtx);
-      printf("T%ld assigns to %d the value: %s\n", num, argOne, argTwoTxt);
+│       pthread_mutex_unlock(&item[argOne].mtx);
-      memset(item[argOne].text, 0, sizeof(item[argOne].text));
+│     } else {
-      strncpy(item[argOne].text, argTwoTxt, sizeof(item[argOne].text) - 1);
+│       pthread_mutex_lock(&item[argOne].mtx);
-      pthread_mutex_unlock(&item[argOne].mtx);
+│       fprintf(win.out, "T#%ld assigns to %d the value: %s\n", num, argOne, argTwoTxt);
-    }
+│       memset(item[argOne].text, 0, sizeof(item[argOne].text));
-  }
+│       strncpy(item[argOne].text, argTwoTxt, sizeof(item[argOne].text) - 1);
-}
+│       pthread_mutex_unlock(&item[argOne].mtx);
+│     }
-int main(int argc, char **argv) {
+│   }
-  arg0 = argv[0];
+│ }
+  …
-  printf("To use this program, write to one of the %s.pipe.<thread_id> the "
-         "following:\n"
-         "  <num>             prints <text> from item <num>\n"
-         "  <num> <text>      puts <text> to item <num>\n"
-         "  <num1> <num2>     copies to item <num1> the text from item <num2>\n"
-         "Valid pipe numbers are 0-4, valid item numbers are 0-4.",
-         arg0);
-  for (int i = 0; i < 5; ++i)
-    pthread_mutex_init(&item[i].mtx, NULL);
-  for (intptr_t i = 1; i < 5; ++i) {
-    pthread_t tid;
-    pthread_create(&tid, NULL, (void *(*)(void *))threadFunc, (void *)i);
-    pthread_detach(tid);
-  }
-  threadFunc(0);
-}
 </code>
-</small>
 <html></div></html>
@@ Line 896: / Line 877: @@
 When one thread needs to execute some logic once a specific condition is fulfilled,
 it should:
+<html><div style="margin-top:-1.2em"></div></html>
+<WRAP group>
+<WRAP half column>
   - lock a mutex,
   - check the condition,
-  - if the condition is false:
+  - while the condition is false:
     - wait on a condition variable,
     - go back to step 2,
   - do the logic,
   - unlock the mutex.
+</WRAP>
+<WRAP half column>
+<html>
+<pre class="code c" style="margin-top:-1.4em;margin-bottom:-1.4em;">
+<span style="opacity:0.66">pthread_mutex_</span>lock<span class="br0">(</span><span class="sy0">&amp;</span>mutex<span class="br0">)</span><span class="sy0">;</span>
+<span class="kw1">while</span><span class="br0">(</span><span class="sy0">!</span><i>condition</i><span class="br0">)</span>
+  <span style="opacity:0.66">pthread_cond_</span>wait<span class="br0">(</span><span class="sy0">&amp;</span>condvar<span class="sy0">,</span> <span class="sy0">&amp;</span>mutex<span class="br0">)</span><span class="sy0">;</span>
+<i>logic<span class="br0">(</span><span class="br0">)</span><span class="sy0">;</span></i>
+<span style="opacity:0.66">pthread_mutex_</span>unlock<span class="br0">(</span><span class="sy0">&amp;</span>mutex<span class="br0">)</span><span class="sy0">;</span>
+</pre>
+</html>
+</WRAP>
+</WRAP>
 A thread that may change the state and thus affect the condition should:
+<html><div style="margin-top:-1.2em"></div></html>
+<WRAP group>
+<WRAP half column>
   - lock the mutex,
   - do its logic,
   - signal the condition variable,
   - unlock the mutex((One can also signal the condition variable after unlocking the mutex.)).
+</WRAP>
+<WRAP half column>
+<html>
+<pre class="code c" style="padding-top:0; padding-bottom:0; margin-top:-1.4em;margin-bottom:-1.4em;">
+<span style="opacity:0.66">pthread_mutex_</span>lock<span class="br0">(</span><span class="sy0">&amp;</span>mutex<span class="br0">)</span><span class="sy0">;</span>
+<i>logic_that_changes_condition<span class="br0">(</span><span class="br0">)</span><span class="sy0">;</span></i>
+<span style="opacity:0.66">pthread_cond_</span>signal<span class="br0">(</span><span class="sy0">&amp;</span>condvar<span class="br0">)</span><span class="sy0">;</span>
+<span style="opacity:0.66">pthread_mutex_</span>unlock<span class="br0">(</span><span class="sy0">&amp;</span>mutex<span class="br0">)</span><span class="sy0">;</span>
+</pre>
+</html>
+</WRAP>
+</WRAP>
 <small>
 The example conditions include:
+<html><div style="margin-top:-1.2em"></div></html>
   * a boolean flag is set; the flag indicates that another thread finished a part of the computation and the results can used,
   * a list of items is not empty; the list contains tasks to be done by this thread,

Jan Kończak

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