1PTHREAD_CLEANUP_PUSH(3)    Linux Programmer's Manual   PTHREAD_CLEANUP_PUSH(3)
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NAME

6       pthread_cleanup_push, pthread_cleanup_pop - push and pop thread cancel‐
7       lation clean-up handlers
8

SYNOPSIS

10       #include <pthread.h>
11
12       void pthread_cleanup_push(void (*routine)(void *),
13                                 void *arg);
14       void pthread_cleanup_pop(int execute);
15
16       Compile and link with -pthread.
17

DESCRIPTION

19       These functions manipulate the calling thread's stack of thread-cancel‐
20       lation clean-up handlers.  A clean-up handler is a function that is au‐
21       tomatically executed when a thread is canceled  (or  in  various  other
22       circumstances  described  below); it might, for example, unlock a mutex
23       so that it becomes available to other threads in the process.
24
25       The pthread_cleanup_push() function pushes routine onto the top of  the
26       stack  of clean-up handlers.  When routine is later invoked, it will be
27       given arg as its argument.
28
29       The pthread_cleanup_pop() function removes the routine at  the  top  of
30       the  stack  of clean-up handlers, and optionally executes it if execute
31       is nonzero.
32
33       A cancellation clean-up handler is popped from the stack  and  executed
34       in the following circumstances:
35
36       1. When  a thread is canceled, all of the stacked clean-up handlers are
37          popped and executed in the reverse of the order in which  they  were
38          pushed onto the stack.
39
40       2. When  a  thread  terminates by calling pthread_exit(3), all clean-up
41          handlers are executed as described in the preceding point.   (Clean-
42          up  handlers are not called if the thread terminates by performing a
43          return from the thread start function.)
44
45       3. When a thread calls pthread_cleanup_pop() with a nonzero execute ar‐
46          gument, the top-most clean-up handler is popped and executed.
47
48       POSIX.1  permits pthread_cleanup_push() and pthread_cleanup_pop() to be
49       implemented as macros that expand to text containing '{' and  '}',  re‐
50       spectively.   For  this  reason,  the  caller must ensure that calls to
51       these functions are paired within the same function, and  at  the  same
52       lexical  nesting  level.  (In other words, a clean-up handler is estab‐
53       lished only during the execution of a specified section of code.)
54
55       Calling longjmp(3) (siglongjmp(3)) produces undefined  results  if  any
56       call  has  been made to pthread_cleanup_push() or pthread_cleanup_pop()
57       without the matching call of the pair since the jump buffer was  filled
58       by   setjmp(3)  (sigsetjmp(3)).   Likewise,  calling  longjmp(3)  (sig‐
59       longjmp(3)) from inside a clean-up handler produces  undefined  results
60       unless  the jump buffer was also filled by setjmp(3) (sigsetjmp(3)) in‐
61       side the handler.
62

RETURN VALUE

64       These functions do not return a value.
65

ERRORS

67       There are no errors.
68

ATTRIBUTES

70       For an  explanation  of  the  terms  used  in  this  section,  see  at‐
71       tributes(7).
72
73       ┌────────────────────────┬───────────────┬─────────┐
74Interface               Attribute     Value   
75       ├────────────────────────┼───────────────┼─────────┤
76pthread_cleanup_push(), │ Thread safety │ MT-Safe │
77pthread_cleanup_pop()   │               │         │
78       └────────────────────────┴───────────────┴─────────┘
79

CONFORMING TO

81       POSIX.1-2001, POSIX.1-2008.
82

NOTES

84       On Linux, the pthread_cleanup_push()  and  pthread_cleanup_pop()  func‐
85       tions  are implemented as macros that expand to text containing '{' and
86       '}', respectively.  This means that variables declared within the scope
87       of  paired  calls  to  these functions will be visible within only that
88       scope.
89
90       POSIX.1 says that the effect of using return, break, continue, or  goto
91       to  prematurely  leave  a  block  bracketed  pthread_cleanup_push() and
92       pthread_cleanup_pop() is undefined.  Portable applications should avoid
93       doing this.
94

EXAMPLES

96       The program below provides a simple example of the use of the functions
97       described in this page.  The program creates a thread that  executes  a
98       loop  bracketed  by  pthread_cleanup_push()  and pthread_cleanup_pop().
99       This loop increments a global variable, cnt, once each second.  Depend‐
100       ing  on what command-line arguments are supplied, the main thread sends
101       the other thread a cancellation request, or sets a global variable that
102       causes the other thread to exit its loop and terminate normally (by do‐
103       ing a return).
104
105       In the following shell session, the main thread  sends  a  cancellation
106       request to the other thread:
107
108           $ ./a.out
109           New thread started
110           cnt = 0
111           cnt = 1
112           Canceling thread
113           Called clean-up handler
114           Thread was canceled; cnt = 0
115
116       From  the above, we see that the thread was canceled, and that the can‐
117       cellation clean-up handler was called and it reset  the  value  of  the
118       global variable cnt to 0.
119
120       In  the  next  run, the main program sets a global variable that causes
121       other thread to terminate normally:
122
123           $ ./a.out x
124           New thread started
125           cnt = 0
126           cnt = 1
127           Thread terminated normally; cnt = 2
128
129       From the above, we see that the clean-up handler was not executed  (be‐
130       cause  cleanup_pop_arg  was  0), and therefore the value of cnt was not
131       reset.
132
133       In the next run, the main program sets a global  variable  that  causes
134       the  other  thread  to terminate normally, and supplies a nonzero value
135       for cleanup_pop_arg:
136
137           $ ./a.out x 1
138           New thread started
139           cnt = 0
140           cnt = 1
141           Called clean-up handler
142           Thread terminated normally; cnt = 0
143
144       In the above, we see that although the thread  was  not  canceled,  the
145       clean-up   handler   was   executed,  because  the  argument  given  to
146       pthread_cleanup_pop() was nonzero.
147
148   Program source
149
150       #include <pthread.h>
151       #include <sys/types.h>
152       #include <stdio.h>
153       #include <stdlib.h>
154       #include <unistd.h>
155       #include <errno.h>
156
157       #define handle_error_en(en, msg) \
158               do { errno = en; perror(msg); exit(EXIT_FAILURE); } while (0)
159
160       static int done = 0;
161       static int cleanup_pop_arg = 0;
162       static int cnt = 0;
163
164       static void
165       cleanup_handler(void *arg)
166       {
167           printf("Called clean-up handler\n");
168           cnt = 0;
169       }
170
171       static void *
172       thread_start(void *arg)
173       {
174           time_t start, curr;
175
176           printf("New thread started\n");
177
178           pthread_cleanup_push(cleanup_handler, NULL);
179
180           curr = start = time(NULL);
181
182           while (!done) {
183               pthread_testcancel();           /* A cancellation point */
184               if (curr < time(NULL)) {
185                   curr = time(NULL);
186                   printf("cnt = %d\n", cnt);  /* A cancellation point */
187                   cnt++;
188               }
189           }
190
191           pthread_cleanup_pop(cleanup_pop_arg);
192           return NULL;
193       }
194
195       int
196       main(int argc, char *argv[])
197       {
198           pthread_t thr;
199           int s;
200           void *res;
201
202           s = pthread_create(&thr, NULL, thread_start, NULL);
203           if (s != 0)
204               handle_error_en(s, "pthread_create");
205
206           sleep(2);           /* Allow new thread to run a while */
207
208           if (argc > 1) {
209               if (argc > 2)
210                   cleanup_pop_arg = atoi(argv[2]);
211               done = 1;
212
213           } else {
214               printf("Canceling thread\n");
215               s = pthread_cancel(thr);
216               if (s != 0)
217                   handle_error_en(s, "pthread_cancel");
218           }
219
220           s = pthread_join(thr, &res);
221           if (s != 0)
222               handle_error_en(s, "pthread_join");
223
224           if (res == PTHREAD_CANCELED)
225               printf("Thread was canceled; cnt = %d\n", cnt);
226           else
227               printf("Thread terminated normally; cnt = %d\n", cnt);
228           exit(EXIT_SUCCESS);
229       }
230

SEE ALSO

232       pthread_cancel(3), pthread_cleanup_push_defer_np(3), pthread_setcancel‐
233       state(3), pthread_testcancel(3), pthreads(7)
234

COLOPHON

236       This  page  is  part of release 5.10 of the Linux man-pages project.  A
237       description of the project, information about reporting bugs,  and  the
238       latest     version     of     this    page,    can    be    found    at
239       https://www.kernel.org/doc/man-pages/.
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243Linux                             2020-06-09           PTHREAD_CLEANUP_PUSH(3)
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