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| .TH SEM_POST 3 2021-03-22 "Linux" "Linux Programmer's Manual" |
| .SH NAME |
| sem_post \- unlock a semaphore |
| .SH SYNOPSIS |
| .nf |
| .B #include <semaphore.h> |
| .PP |
| .BI "int sem_post(sem_t *" sem ); |
| .fi |
| .PP |
| Link with \fI\-pthread\fP. |
| .SH DESCRIPTION |
| .BR sem_post () |
| increments (unlocks) the semaphore pointed to by |
| .IR sem . |
| If the semaphore's value consequently becomes greater than zero, |
| then another process or thread blocked in a |
| .BR sem_wait (3) |
| call will be woken up and proceed to lock the semaphore. |
| .SH RETURN VALUE |
| .BR sem_post () |
| returns 0 on success; |
| on error, the value of the semaphore is left unchanged, |
| \-1 is returned, and |
| .I errno |
| is set to indicate the error. |
| .SH ERRORS |
| .TP |
| .B EINVAL |
| .I sem |
| is not a valid semaphore. |
| .TP |
| .B EOVERFLOW |
| .\" Added in POSIX.1-2008 TC1 (Austin Interpretation 213) |
| The maximum allowable value for a semaphore would be exceeded. |
| .SH ATTRIBUTES |
| For an explanation of the terms used in this section, see |
| .BR attributes (7). |
| .ad l |
| .nh |
| .TS |
| allbox; |
| lbx lb lb |
| l l l. |
| Interface Attribute Value |
| T{ |
| .BR sem_post () |
| T} Thread safety MT-Safe |
| .TE |
| .hy |
| .ad |
| .sp 1 |
| .SH CONFORMING TO |
| POSIX.1-2001. |
| .SH NOTES |
| .BR sem_post () |
| is async-signal-safe: |
| it may be safely called within a signal handler. |
| .SH EXAMPLES |
| See |
| .BR sem_wait (3) |
| and |
| .BR shm_open (3). |
| .SH SEE ALSO |
| .BR sem_getvalue (3), |
| .BR sem_wait (3), |
| .BR sem_overview (7), |
| .BR signal\-safety (7) |