Differences

This shows you the differences between two versions of the page.

--- os_cp:threads [2024/04/18 11:32]
jkonczak [Accessing the same data from multiple threads]
+++ os_cp:threads [2025/04/25 13:55] (current)
jkonczak [POSIX condition variables]
@@ Line 69: / Line 69: @@
 \\
 <html><span style="float:right"><small>Needs header:<br><code>pthread.h</code></small></span>
-<a href="https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_create.html"></html>
+<a href="https://pubs.opengroup.org/onlinepubs/9799919799/functions/pthread_create.html"></html>
 ''int **pthread_create**(pthread_t *restrict //threadIdentifier//,''  \\
 ''                   const pthread_attr_t *restrict //attr//,''  \\
@@ Line 132: / Line 132: @@
 \\
 <html><span style="float:right"><small>Needs header: <code>pthread.h</code></small></span>
-<a href="https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_join.html"></html>
+<a href="https://pubs.opengroup.org/onlinepubs/9799919799/functions/pthread_join.html"></html>
 ''int **pthread_join**(pthread_t //thread//, void %%**%%//value_ptr//);''
 <html></a></html>
@@ Line 209: / Line 209: @@
 \\
 <html><span style="float:right"><small>Needs header: <code>pthread.h</code></small></span>
-<a href="https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_detach.html"></html>
+<a href="https://pubs.opengroup.org/onlinepubs/9799919799/functions/pthread_detach.html"></html>
 ''int **pthread_detach**(pthread_t //thread//);''
 <html></a></html>
@@ Line 275: / Line 275: @@
 POSIX offers a mechanism called //cancellation//. That is, one can send to a
 thread a cancellation request using the
-''[[https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_cancel.html|pthread_cancel]]''
+''[[https://pubs.opengroup.org/onlinepubs/9799919799/functions/pthread_cancel.html|pthread_cancel]]''
 function. Each thread can select is behaviour on receiving such request using the
-''[[https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_setcancelstate.html|pthread_setcancelstate]]''
+''[[https://pubs.opengroup.org/onlinepubs/9799919799/functions/pthread_setcancelstate.html|pthread_setcancelstate]]''
 and
-''[[https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_setcancelstate.html|pthread_setcanceltype]]''
+''[[https://pubs.opengroup.org/onlinepubs/9799919799/functions/pthread_setcancelstate.html|pthread_setcanceltype]]''
 functions. One may either allow or disallow cancalling a thread, and one may
 choose whether the cancallation should ouccur immediatly after a request, or
 at the next cancallation point. Cancellation points are the execution points
 of several pthread functions, among which notably
-''[[https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_setcancelstate.html|pthread_testcancel]]''
+''[[https://pubs.opengroup.org/onlinepubs/9799919799/functions/pthread_setcancelstate.html|pthread_testcancel]]''
 is there just to this end.
 Once the thread complies to the cancellation request, it roughly does the same
@@ Line 374: / Line 374: @@
 }
 </code>
+<html></div></html>
+<html><div style="margin-top:-1.4em; line-height: 1em"></html>
+<small>If the code above always returns the right answer, try to run it a million times: \\
+''for X in `seq 1000000`; do RES=$(%%./%%//progname//); test "$RES" -ne 16000 && echo -e "\n$RES" && break || echo -n '.'; done''
+</small>
 <html></div></html>
@@ Line 396: / Line 401: @@
 \\
 <html><span style="float:right"><small>Needs header: <code>pthread.h</code></small></span>
-<a href="https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_mutex_init.html"></html>
+<a href="https://pubs.opengroup.org/onlinepubs/9799919799/functions/pthread_mutex_init.html"></html>
 ''pthread_mutex_t //mutex// = **PTHREAD_MUTEX_INITIALIZER**;''
 <html></a></html>
@@ Line 403: / Line 408: @@
 \\
 <html><span style="float:right"><small>Needs header: <code>pthread.h</code></small></span>
-<a href="https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_mutex_init.html"></html>
+<a href="https://pubs.opengroup.org/onlinepubs/9799919799/functions/pthread_mutex_init.html"></html>
 ''int **pthread_mutex_init**(pthread_mutex_t *restrict //mutex//,'' \\
 ''                       const pthread_mutexattr_t *restrict //attr//)''
@@ Line 415: / Line 420: @@
 \\
 <html><span style="float:right"><small>Needs header: <code>pthread.h</code></small></span>
-<a href="https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_mutexattr_init.html"></html>
+<a href="https://pubs.opengroup.org/onlinepubs/9799919799/functions/pthread_mutexattr_init.html"></html>
 ''int **pthread_mutexattr_init**(pthread_mutexattr_t *//attr//)''
 <html></a></html>
 \\
 and finally set the attributes using corresponding functions:
-  * <html><a href="https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_mutexattr_settype.html"></html> ''int pthread_mutexattr_set**type**(pthread_mutexattr_t *//attr//, int //type//)''<html></a></html> sets mutex type (see below),
+  * <html><a href="https://pubs.opengroup.org/onlinepubs/9799919799/functions/pthread_mutexattr_settype.html"></html> ''int pthread_mutexattr_set**type**(pthread_mutexattr_t *//attr//, int //type//)''<html></a></html> sets mutex type (see below),
-  * <small><html><a href="https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_mutexattr_setpshared.html"></html>''int pthread_mutexattr_set**pshared**(pthread_mutexattr_t *//attr//, int //pshared//)''<html></a></html> allows sharing mutexes between processes (they need to reside in shared memory), </small>
+  * <small><html><a href="https://pubs.opengroup.org/onlinepubs/9799919799/functions/pthread_mutexattr_setpshared.html"></html>''int pthread_mutexattr_set**pshared**(pthread_mutexattr_t *//attr//, int //pshared//)''<html></a></html> allows sharing mutexes between processes (they need to reside in shared memory), </small>
-  * <small><html><a href="https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_mutexattr_setrobust.html"></html>''int pthread_mutexattr_set**robust**(pthread_mutexattr_t *//attr//, int //robust//)''<html></a></html> when a thread holding a mutex terminated, a robust mutex will return appropriate error instead of waiting,</small>
+  * <small><html><a href="https://pubs.opengroup.org/onlinepubs/9799919799/functions/pthread_mutexattr_setrobust.html"></html>''int pthread_mutexattr_set**robust**(pthread_mutexattr_t *//attr//, int //robust//)''<html></a></html> when a thread holding a mutex terminated, a robust mutex will return appropriate error instead of waiting,</small>
-  * <small>''[[https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_mutexattr_setprotocol.html|pthread_mutexattr_setprotocol]]'' and ''[[https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_mutexattr_setprioceiling.html|setprioceiling]]'' deal with priorities.</small>
+  * <small>''[[https://pubs.opengroup.org/onlinepubs/9799919799/functions/pthread_mutexattr_setprotocol.html|pthread_mutexattr_setprotocol]]'' and ''[[https://pubs.opengroup.org/onlinepubs/9799919799/functions/pthread_mutexattr_setprioceiling.html|setprioceiling]]'' deal with priorities.</small>
 The mutex type impacts what happens when a mutex that is already locked is
@@ Line 447: / Line 452: @@
 \\
 <html><span style="float:right"><small>Needs header: <code>pthread.h</code></small></span>
-<a href="https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_mutex_lock.html"></html>
+<a href="https://pubs.opengroup.org/onlinepubs/9799919799/functions/pthread_mutex_lock.html"></html>
 ''int **pthread_mutex_lock**(pthread_mutex_t *//mutex//)'' \\
 ''int pthread_mutex_trylock(pthread_mutex_t *//mutex//)''
 <html></a></html>
 \\
-<small><html><a href="https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_mutex_timedlock.html"></html>
+<small><html><a href="https://pubs.opengroup.org/onlinepubs/9799919799/functions/pthread_mutex_timedlock.html"></html>
 ''int pthread_mutex_timedlock(pthread_mutex_t *restrict //mutex//, const struct timespec *restrict //abstime//)''
 <html></a></html></small>
@@ Line 467: / Line 472: @@
 \\
 <html><span style="float:right"><small>Needs header: <code>pthread.h</code></small></span>
-<a href="https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_mutex_lock.html"></html>
+<a href="https://pubs.opengroup.org/onlinepubs/9799919799/functions/pthread_mutex_lock.html"></html>
 ''int **pthread_mutex_unlock**(pthread_mutex_t *//mutex//)''
 <html></a></html>
@@ Line 613: / Line 618: @@
 \\
 To create a rwlock one has to create a ''pthread_rwlock_t'' variable and initialize
-it with either ''[[https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_rwlock_init.html|PTHREAD_RWLOCK_INITIALIZER]]'' macro or ''[[https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_rwlock_init.html|pthread_rwlock_init]]'' function.
+it with either ''[[https://pubs.opengroup.org/onlinepubs/9799919799/functions/pthread_rwlock_init.html|PTHREAD_RWLOCK_INITIALIZER]]'' macro or ''[[https://pubs.opengroup.org/onlinepubs/9799919799/functions/pthread_rwlock_init.html|pthread_rwlock_init]]'' function.
 \\
 A read-write lock can be locked with:
-  * ''[[https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_rwlock_rdlock.html|pthread_rwlock_rdlock]]'' and ''[[https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_rwlock_rdlock.html|pthread_rwlock_tryrdlock]]'' in shared (read) mode,
+  * ''[[https://pubs.opengroup.org/onlinepubs/9799919799/functions/pthread_rwlock_rdlock.html|pthread_rwlock_rdlock]]'' and ''[[https://pubs.opengroup.org/onlinepubs/9799919799/functions/pthread_rwlock_rdlock.html|pthread_rwlock_tryrdlock]]'' in shared (read) mode,
-  * ''[[https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_rwlock_wrlock.html|pthread_rwlock_wrlock]]'' and ''[[https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_rwlock_wrlock.html|pthread_rwlock_trywrlock]]'' in exclusive (write) mode.
+  * ''[[https://pubs.opengroup.org/onlinepubs/9799919799/functions/pthread_rwlock_wrlock.html|pthread_rwlock_wrlock]]'' and ''[[https://pubs.opengroup.org/onlinepubs/9799919799/functions/pthread_rwlock_wrlock.html|pthread_rwlock_trywrlock]]'' in exclusive (write) mode.
 <small>''pthread_rwlock_timedrdlock'' / ''pthread_rwlock_timedwrlock'' variants are also available.</small>
 \\
-To unlock a rwlock (locked in any mode), ''[[https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_rwlock_unlock.html|pthread_rwlock_unlock]]'' is used.
+To unlock a rwlock (locked in any mode), ''[[https://pubs.opengroup.org/onlinepubs/9799919799/functions/pthread_rwlock_unlock.html|pthread_rwlock_unlock]]'' is used.
@@ Line 699: / 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 815: / Line 801: @@
 \\
 <html><span style="float:right"><small>Need header:<br><code>pthread.h</code></small></span>
-<a href="https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_key_create.html"></html>
+<a href="https://pubs.opengroup.org/onlinepubs/9799919799/functions/pthread_key_create.html"></html>
 ''int **pthread_key_create**(pthread_key_t *//key//, void (*//destructor//)(void*))''
 <html></a></html>
 \\
-<html><a href="https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_setspecific.html"></html>
+<html><a href="https://pubs.opengroup.org/onlinepubs/9799919799/functions/pthread_setspecific.html"></html>
 ''int **pthread_setspecific**(pthread_key_t //key//, const void *//value//)''
 \\
@@ Line 891: / 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,
@@ Line 915: / Line 933: @@
 \\
 <html><span style="float:right"><small>Needs header: <code>pthread.h</code></small></span>
-<a href="https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_cond_init.html"></html>
+<a href="https://pubs.opengroup.org/onlinepubs/9799919799/functions/pthread_cond_init.html"></html>
 ''pthread_cond_t //cond// = **PTHREAD_COND_INITIALIZER**;''
 <html></a></html>
@@ Line 923: / Line 941: @@
 \\
 <html><span style="float:right"><small>Needs header: <code>pthread.h</code></small></span>
-<a href="https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_cond_init.html"></html>
+<a href="https://pubs.opengroup.org/onlinepubs/9799919799/functions/pthread_cond_init.html"></html>
 ''int pthread_cond_init(pthread_cond_t *restrict //cond//, const pthread_condattr_t *restrict //attr//)''
 <html></a></html>
@@ Line 931: / Line 949: @@
 \\
 <html><span style="float:right"><small>Needs header: <code>pthread.h</code></small></span>
-<a href="https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_cond_wait.html"></html>
+<a href="https://pubs.opengroup.org/onlinepubs/9799919799/functions/pthread_cond_wait.html"></html>
 ''int **pthread_cond_wait**(pthread_cond_t *restrict //cond//, pthread_mutex_t *restrict //mutex//)''
 <html></a></html>
@@ Line 940: / Line 958: @@
 \\
 <html><span style="float:right"><small>Needs header: <code>pthread.h</code></small></span>
-<a href="https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_cond_broadcast.html"></html>
+<a href="https://pubs.opengroup.org/onlinepubs/9799919799/functions/pthread_cond_broadcast.html"></html>
 ''int **pthread_cond_signal**(pthread_cond_t *restrict //cond//)''\\
 ''int **pthread_cond_broadcast**(pthread_cond_t *restrict //cond//)''

Jan Kończak

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