os_cp:semaphores:solutions
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os_cp:semaphores:solutions [2023/05/10 09:38] jkonczak utworzono |
os_cp:semaphores:solutions [2023/09/15 15:18] (current) jkonczak |
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| <html><div style="line-height:1em"></html> | <html><div style="line-height:1em"></html> | ||
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| + | ~~Exercise.#~~ | ||
| + | <code c> | ||
| + | #include <fcntl.h> | ||
| + | #include <semaphore.h> | ||
| + | #include <stdio.h> | ||
| + | |||
| + | int main() { | ||
| + | sem_t *mySemaphore = sem_open("/mySemaphore", O_CREAT | O_EXCL, 0666, 0); | ||
| + | if (mySemaphore == SEM_FAILED) { | ||
| + | perror("Creating the semaphore failed"); | ||
| + | return 1; | ||
| + | } | ||
| + | sem_wait(mySemaphore); | ||
| + | return 0; | ||
| + | } | ||
| + | </code> | ||
| + | |||
| + | <code c> | ||
| + | #include <fcntl.h> | ||
| + | #include <semaphore.h> | ||
| + | #include <stdio.h> | ||
| + | |||
| + | int main() { | ||
| + | sem_t *mySemaphore = sem_open("/mySemaphore", 0, 0666, 0); | ||
| + | if (mySemaphore == SEM_FAILED) { | ||
| + | perror("Opening the semaphore failed"); | ||
| + | return 1; | ||
| + | } | ||
| + | sem_post(mySemaphore); | ||
| + | return 0; | ||
| + | } | ||
| + | </code> | ||
| + | |||
| + | ~~Exercise.#~~ | ||
| + | |||
| <code c> | <code c> | ||
| #include <fcntl.h> | #include <fcntl.h> | ||
| Line 79: | Line 115: | ||
| } | } | ||
| </code> | </code> | ||
| + | |||
| + | ~~Exercise.#~~ | ||
| + | |||
| + | Line: | ||
| + | <code c> if (createSucceeded) sleep(10);</code> | ||
| + | added anywhere after line 13 and before line 22 delays the first process from | ||
| + | initializing the semaphore for 10 seconds. If another process attempts to wait | ||
| + | on uninitialized semaphore, it is likely to get stuck forever. | ||
| + | |||
| + | ~~Exercise.#~~ | ||
| + | |||
| + | Lines between ''sem_wait'' and ''sem_post'' are the critical region. | ||
| + | |||
| + | ~~Exercise.#~~ | ||
| + | |||
| + | When the program is run concurrently with arguments ''0 1'' and ''1 0'', it will | ||
| + | deadlock if both processes manage to acquire the first semaphore (respectively | ||
| + | ''0'' for the first process, and ''1'' for the second) and then try to acquire | ||
| + | the second one (respectively ''1'' and ''0''). | ||
| + | |||
| + | ~~Exercise.#~~ | ||
| + | |||
| + | One of the simpler solutions is to wait on semaphores in a predefined order, | ||
| + | that is instead of: | ||
| + | <code c> | ||
| + | sem_wait(&data->item[argOne].sem); | ||
| + | sem_wait(&data->item[argTwo].sem); | ||
| + | </code> | ||
| + | one may just: | ||
| + | <code c> | ||
| + | sem_wait(&data->item[argOne<argTwo?argOne:argTwo].sem); | ||
| + | sem_wait(&data->item[argOne<argTwo?argTwo:argOne].sem); | ||
| + | </code> | ||
| + | | ||
| <html></div></html> | <html></div></html> | ||
| + | |||
| + | |||
| + | ~~META: | ||
| + | language = en | ||
| + | ~~ | ||
| + | |||
os_cp/semaphores/solutions.1683704319.txt.gz · Last modified: 2023/05/10 09:38 by jkonczak
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