View Issue Details
| ID | Project | Category | View Status | Date Submitted | Last Update |
|---|---|---|---|---|---|
| 0000335 | 1003.1(2008)/Issue 7 | System Interfaces | public | 2010-10-16 17:52 | 2013-04-16 13:06 |
| Reporter | weeks | Assigned To | ajosey | ||
| Priority | normal | Severity | Editorial | Type | Error |
| Status | Closed | Resolution | Accepted As Marked | ||
| Name | Nathan Weeks | ||||
| Organization | |||||
| User Reference | |||||
| Section | semget, semop | ||||
| Page Number | 1837,1842 | ||||
| Line Number | 58680,58875 | ||||
| Interp Status | --- | ||||
| Final Accepted Text | 0000335:0000594 | ||||
| Summary | 0000335: semget() and semop() examples do not initialize semval | ||||
| Description | The examples for semget() and semop() do not initialize the semaphore value with semctl() after creating the semaphore set with the second call to semget() and before operating on it with the first call to semop(). The description for semget() contains the bullet: "The data structure associated with each semaphore in the set need not be initialized. The semctl() function with the command SETVAL or SETALL can be used to initialize each semaphore." | ||||
| Desired Action | A couple possibilities: 1. Replace the first call to semop() with "semctl(semid, 0, SETVAL, arg)", where "arg" is of type "union semun" as described in semctl(), and arg.val = 2. 2. Before the first call to semop(), call "semctl(semid, 0, SETVAL, arg)", where arg.val = 0. By keeping the call to semop(), the sem_otime member of the semid_ds data structure associated with semid will be updated, which, while not explicitly referenced in the examples, may be referenced in applications that wish to avoid the race condition described in "Unix Network Programming Volume 2 (2nd edition)" on pages 284-285. The sentence "After creating the semaphore, the program uses a call to semop() to initialize it to the values in the sbuf array" may need to be reworded depending on the solution. | ||||
| Tags | tc1-2008 | ||||
| related to | 0000345 | Closed | ajosey | 1003.1(2008)/Issue 7 | semop must update sem_otime |
| related to | 0000377 | Closed | ajosey | 1003.1(2008)/Issue 7 | Semaphore values should be initialized to zero when a semaphore set is created. |
| related to | 0000439 | Closed | ajosey | 2008-TC1 | XSH/TC1/D1/0262 redundant |
| related to | 0000575 | Closed | ajosey | 2008-TC1 | semget() Future Directions should be updated |
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Rather than maintain two similar examples, it is easier to follow the lead of dlopen() and just refer to the longer example of the two. There are some additional cleanups to be made while improving the example, such as removing unused variables, and removing headers that are not necessary for compilation to succeed (for example, <sys/ipc.h> is required to already be included by <sys/sem.h>). The following changes implement desired action 2, along with enhancing the example to check sem_otime to detect the race. Replace line 58574 [XSH semctl EXAMPLES] with: Refer to semop() (on page 1841). Replace the entire example on lines 58642-58701 [XSH semget EXAMPLES] with: Refer to semop() (on page 1841). After line 58825 [XSH semop EXAMPLES], insert: /* Code to initialize semid. */ ... At line 58841, change "uses a call to semop() to initialize it" to "uses calls to semctl() and semop() to initialize it" After line 58844, add a paragraph: Processes that obtain semid without creating it check that sem_otime is non-zero, to ensure that the creating process has completed the semop() initialization. Delete lines 58848 (<sys/types.h>), 58850 (<sys/ipc.h>), 58854 (<unistd.h>), and 58856-58858 (<pwd.h>, <fcntl.h>, and <limits.h>). At line 58861, delete ", pfd, fv", leaving just "int semid;". Delete lines 58863-58866 (lgn, filename, outstat, pw), and replace them with: union semun { int val; struct semid_ds *buf; unsigned short *array; } arg; struct semid_ds ds; After line 58878, insert: arg.val = 0; Replace line 58882 with: if (semctl(semid, 0, SETVAL, arg) == -1 || semop(semid, &sbuf, 1) == -1) { After line 58889, insert: goto check_init; After line 58894, insert: else { /* Check that semid has completed initialization. */ /* An application may use a retry loop at this point rather than exiting. */ check_init: arg.buf = &ds; if (semctl(semid, 0, IPC_STAT, arg) < 0) { perror("IPC error 3: semctl"); exit(1); } if (ds.sem_otime == 0) { perror("IPC error 4: semctl"); exit(1); } } |
| Date Modified | Username | Field | Change |
|---|---|---|---|
| 2010-10-16 17:52 | weeks | New Issue | |
| 2010-10-16 17:52 | weeks | Status | New => Under Review |
| 2010-10-16 17:52 | weeks | Assigned To | => ajosey |
| 2010-10-16 17:52 | weeks | Name | => Nathan Weeks |
| 2010-10-16 17:52 | weeks | Section | => semget, semop |
| 2010-10-16 17:52 | weeks | Page Number | => 0 |
| 2010-10-16 17:52 | weeks | Line Number | => 0 |
| 2010-10-28 23:07 | eblake | Note Added: 0000594 | |
| 2010-10-28 23:41 | eblake | Relationship added | related to 0000345 |
| 2010-10-29 18:55 | Don Cragun | Page Number | 0 => 1837,1842 |
| 2010-10-29 18:55 | Don Cragun | Line Number | 0 => 58680,58875 |
| 2010-10-29 18:55 | Don Cragun | Interp Status | => --- |
| 2010-11-04 15:25 | eblake | Note Edited: 0000594 | |
| 2010-11-04 15:29 | eblake | Final Accepted Text | => 0000335:0000594 |
| 2010-11-04 15:29 | eblake | Status | Under Review => Resolved |
| 2010-11-04 15:29 | eblake | Resolution | Open => Accepted As Marked |
| 2010-11-04 15:30 | eblake | Tag Attached: tc1-2008 | |
| 2011-02-09 07:49 | Don Cragun | Relationship added | related to 0000377 |
| 2011-05-13 15:34 | eblake | Relationship added | related to 0000439 |
| 2012-08-22 16:01 | eblake | Relationship added | related to 0000575 |
| 2013-04-16 13:06 | ajosey | Status | Resolved => Closed |
