A thread ends in one of three ways: natural causes, an early exit, or sudden death. Yes, it’s exciting, but no more than any aspect of programming.
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\nI suppose the normal way for a thread to die is along with its parent program. Because the thread is part of the same overall process, when the main program terminates so do any spawned threads.<\/p>\n
A thread can also terminate early, as was demonstrated in last week’s Lesson<\/a>. This trick is used to capture a thread’s return value.<\/p>\n Another way to rub out a thread is to kill it off. The pthread_cancel()<\/em> function handles the assassination. Its sole argument is the pthread_t<\/em> value of a launched and running thread. Poof! The thread is gone.<\/p>\n The following code launches a thread that runs an endless loop. The main program pauses twice: once just because and a second time to quit.<\/p>\n Here’s a sample run, where I press the Enter key (twice), but wait until the thread first spews out its text:<\/p>\n Press Enter to quit The following update to the code adds the pthread_cancel()<\/em> function to terminate the thread after the first Enter key prompt:<\/p>\n The first prompt’s text (Line 29) is updated to direct the user to press Enter to kill the thread. The pthread_cancel()<\/em> function removes it, followed by output confirming that the thread is dead. Here’s a sample run:<\/p>\n Thread terminated It’s important that you not use pthread_cancel()<\/em> haphazardly. You don’t want to suddenly cancel a thread in the middle of something, which may create an unstable or undesirable condition.<\/p>\n To protect a thread from cancelation, refer to the pthread_setcancelstate()<\/em> and pthread_setcanceltype()<\/em> functions. You can also use the pthread_testcancel()<\/em> function to insert a safe cancellation point within a thread’s code.<\/p>\n A good web page for further research on the topic of multithreaded programming in C is found here:Multithreaded Programming Guide<\/a> from Oracle.<\/p>\n","protected":false},"excerpt":{"rendered":" Like any code, it’s possible for a thread to run endlessly. Continue reading 2022_06_18-Lesson-a.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n#include <stdlib.h>\r\n#include <unistd.h>\r\n#include <pthread.h>\r\n\r\nvoid *eternal(void *p)\r\n{\r\n for( ;; )\r\n {\r\n printf(\"On I go...\\n\");\r\n sleep(1);\r\n }\r\n}\r\n\r\nint main()\r\n{\r\n pthread_t thd;\r\n int r;\r\n\r\n puts(\"Launching endless thread...\");\r\n \/* spawn the new thread *\/<\/span>\r\n r = pthread_create( &thd, NULL, eternal, NULL);\r\n if( r!=0 )\r\n {\r\n perror(\"Thread\");\r\n exit(1);\r\n }\r\n \r\n puts(\"Press Enter just because\");\r\n getchar();\r\n\r\n puts(\"Press Enter to quit\");\r\n getchar();\r\n\r\n return(0);\r\n}<\/pre>\nLaunching endless thread...
\nPress Enter just because
\nOn I go...
\nOn I go...
\nOn I go...<\/p>\n
\nOn I go...
\nOn I go...<\/code><\/p>\n2022_06_18-Lesson-b.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n#include <stdlib.h>\r\n#include <unistd.h>\r\n#include <pthread.h>\r\n\r\nvoid *eternal(void *p)\r\n{\r\n for( ;; )\r\n {\r\n printf(\"On I go...\\n\");\r\n sleep(1);\r\n }\r\n}\r\n\r\nint main()\r\n{\r\n pthread_t thd;\r\n int r;\r\n\r\n puts(\"Launching endless thread...\");\r\n \/* spawn the new thread *\/<\/span>\r\n r = pthread_create( &thd, NULL, eternal, NULL);\r\n if( r!=0 )\r\n {\r\n perror(\"Thread\");\r\n exit(1);\r\n }\r\n \r\n puts(\"Press Enter to kill the thread\");\r\n getchar();\r\n pthread_cancel(thd);\r\n puts(\"Thread terminated\");\r\n\r\n puts(\"Press Enter to quit\");\r\n getchar();\r\n\r\n return(0);\r\n}<\/pre>\nLaunching endless thread...
\nPress Enter to kill the thread
\nOn I go...
\nOn I go...
\nOn I go...
\nOn I go...<\/p>\n
\nPress Enter to quit<\/code><\/p>\n