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| .TH SEM_OPEN 3 2021-03-22 "Linux" "Linux Programmer's Manual" |
| .SH NAME |
| sem_open \- initialize and open a named semaphore |
| .SH SYNOPSIS |
| .nf |
| .BR "#include <fcntl.h>" " /* For O_* constants */" |
| .BR "#include <sys/stat.h>" " /* For mode constants */" |
| .B #include <semaphore.h> |
| .PP |
| .BI "sem_t *sem_open(const char *" name ", int " oflag ); |
| .BI "sem_t *sem_open(const char *" name ", int " oflag , |
| .BI " mode_t " mode ", unsigned int " value ); |
| .fi |
| .PP |
| Link with \fI\-pthread\fP. |
| .SH DESCRIPTION |
| .BR sem_open () |
| creates a new POSIX semaphore or opens an existing semaphore. |
| The semaphore is identified by |
| .IR name . |
| For details of the construction of |
| .IR name , |
| see |
| .BR sem_overview (7). |
| .PP |
| The |
| .I oflag |
| argument specifies flags that control the operation of the call. |
| (Definitions of the flags values can be obtained by including |
| .IR <fcntl.h> .) |
| If |
| .B O_CREAT |
| is specified in |
| .IR oflag , |
| then the semaphore is created if |
| it does not already exist. |
| The owner (user ID) of the semaphore is set to the effective |
| user ID of the calling process. |
| The group ownership (group ID) is set to the effective group ID |
| of the calling process. |
| .\" In reality the filesystem IDs are used on Linux. |
| If both |
| .B O_CREAT |
| and |
| .B O_EXCL |
| are specified in |
| .IR oflag , |
| then an error is returned if a semaphore with the given |
| .I name |
| already exists. |
| .PP |
| If |
| .B O_CREAT |
| is specified in |
| .IR oflag , |
| then two additional arguments must be supplied. |
| The |
| .I mode |
| argument specifies the permissions to be placed on the new semaphore, |
| as for |
| .BR open (2). |
| (Symbolic definitions for the permissions bits can be obtained by including |
| .IR <sys/stat.h> .) |
| The permissions settings are masked against the process umask. |
| Both read and write permission should be granted to each class of |
| user that will access the semaphore. |
| The |
| .I value |
| argument specifies the initial value for the new semaphore. |
| If |
| .B O_CREAT |
| is specified, and a semaphore with the given |
| .I name |
| already exists, then |
| .I mode |
| and |
| .I value |
| are ignored. |
| .SH RETURN VALUE |
| On success, |
| .BR sem_open () |
| returns the address of the new semaphore; |
| this address is used when calling other semaphore-related functions. |
| On error, |
| .BR sem_open () |
| returns |
| .BR SEM_FAILED , |
| with |
| .I errno |
| set to indicate the error. |
| .SH ERRORS |
| .TP |
| .B EACCES |
| The semaphore exists, but the caller does not have permission to |
| open it. |
| .TP |
| .B EEXIST |
| Both |
| .B O_CREAT |
| and |
| .B O_EXCL |
| were specified in |
| .IR oflag , |
| but a semaphore with this |
| .I name |
| already exists. |
| .TP |
| .B EINVAL |
| .I value |
| was greater than |
| .BR SEM_VALUE_MAX . |
| .TP |
| .B EINVAL |
| .I name |
| consists of just "/", followed by no other characters. |
| .TP |
| .B EMFILE |
| The per-process limit on the number of open file descriptors has been reached. |
| .TP |
| .B ENAMETOOLONG |
| .I name |
| was too long. |
| .TP |
| .B ENFILE |
| The system-wide limit on the total number of open files has been reached. |
| .TP |
| .B ENOENT |
| The |
| .B O_CREAT |
| flag was not specified in |
| .IR oflag |
| and no semaphore with this |
| .I name |
| exists; |
| or, |
| .\" this error can occur if we have a name of the (nonportable) form |
| .\" /dir/name, and the directory /dev/shm/dir does not exist. |
| .B O_CREAT |
| was specified, but |
| .I name |
| wasn't well formed. |
| .TP |
| .B ENOMEM |
| Insufficient memory. |
| .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_open () |
| T} Thread safety MT-Safe |
| .TE |
| .hy |
| .ad |
| .sp 1 |
| .SH CONFORMING TO |
| POSIX.1-2001, POSIX.1-2008. |
| .SH SEE ALSO |
| .BR sem_close (3), |
| .BR sem_getvalue (3), |
| .BR sem_post (3), |
| .BR sem_unlink (3), |
| .BR sem_wait (3), |
| .BR sem_overview (7) |