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IPC.md

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IPC.md

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# IPC
## **Overview**
Provides functions and structures related to inter-process communication \(IPC\), including signals, semaphores, and message queues.
**Since:**
1.0
**Version:**
1.0
## **Summary**
## Files

File Name	Description
ipcstat.h	Defines the macros related to the IPC functions.
mqueue.h	Provides message queue operation functions and related structures.
semaphore.h	Provides functions and structures related to semaphore operations.
signal.h	Provides signal operation functions and structures, including changing the signal processing mode and the process signal mask set.
ipc.h	Defines IPC-related macros.

## Data Structures

Data Structure Name	Description
mq_attr	Describes message queue attributes.
sem_t	Defines semaphores.
sigaltstack	Describes a signal stack.
__ucontext	Describes the user execution context.
__sigset_t	Defines a signal set.
sigaction	Defines the signal processing actions and related attributes. You can use the structures to change the processing actions or attributes.
sigval	Describes signal values in different cases.
__si_fields	Describes signal information.
sigevent	Describes asynchronous notifications.

## Macros

Macro Name and Value	Description
IPC_STAT 2	Defines the input parameter used to obtain information from the kernel.
SEM_FAILED ((sem_t *)0)	Defines the semaphore failure flag.
SA_NOCLDSTOP 1	Stops a child process without sending the SIGCHLD signal.
SA_NOCLDSTOP 0x00000001	Stops a child process without sending the SIGCHLD signal.
SA_NOCLDWAIT 2	Sets the SIGCHLD flag to prevent zombie processes.
SA_NOCLDWAIT 0x00000002	Sets the SIGCHLD flag to prevent zombie processes.
SA_SIGINFO 4	Transfers the siginfo_t structure with the signal.
SA_SIGINFO 0x00000004	Transfers the siginfo_t structure with the signal.
SA_ONSTACK 0x08000000	Uses the signal stack specified by sigaltstack for signal processing.
SA_ONSTACK 0x08000000	Uses the signal stack specified by sigaltstack for signal processing.
SA_RESTART 0x10000000	Obtains the restart signal.
SA_RESTART 0x10000000	Obtains the restart signal.
SA_NODEFER 0x40000000	The signals being processed are not masked during signal processing.
SA_NODEFER 0x40000000	The signals being processed are not masked during signal processing.
SA_RESETHAND 0x80000000	The default processing mode is restored after the signal processing is complete.
SA_RESETHAND 0x80000000	The default processing mode is restored after the signal processing is complete.
SA_RESTORER 0x04000000	Not intended for application use. This flag is used by C libraries to indicate that the sa_restorer field contains the address of a trampoline.
SA_RESTORER 0x04000000	Not intended for application use. This flag is used by C libraries to indicate that the sa_restorer field contains the address of a trampoline.
MINSIGSTKSZ 2048	Defines the minimum size of the replaceable signal stack.
MINSIGSTKSZ 2048	Defines the minimum size of the replaceable signal stack.
SIGSTKSZ 8192	Defines the size of a signal stack.
SIGSTKSZ 8192	Defines the size of a signal stack.
_NSIG 65	Number of supported signals.
_NSIG_BPW __BITS_PER_LONG	Number of bits occupied by a word.
_NSIG_WORDS (_NSIG / _NSIG_BPW)	Number of words required by all signals.
SIGHUP 1	Hangs up a process.
SIGINT 2	Interrupts a process from keyboard.
SIGQUIT 3	Exits a process from keyboard. A core file is generated.
SIGILL 4	Illegal instruction.
SIGTRAP 5	Breakpoint, used for debugging.
SIGABRT 6	Abnormal termination.
SIGIOT 6	Equivalent to SIGABRT.
SIGBUS 7	Invalid address access.
SIGFPE 8	Floating-point exceptions.
SIGKILL 9	Forcibly stops a process.
SIGUSR1 10	User-defined signal 1.
SIGSEGV 11	Illegal memory access.
SIGUSR2 12	User-defined signal 2.
SIGPIPE 13	The pipe is broken.
SIGALRM 14	Timer signal.
SIGTERM 15	Termination signal.
SIGSTKFLT 16	Coprocessor stack error.
SIGCHLD 17	Child process terminated or stopped.
SIGCONT 18	Resumes a suspended process.
SIGSTOP 19	Suspends a process.
SIGTSTP 20	Stops a process by entering the command on the terminal.
SIGTTIN 21	Input required by the background process.
SIGTTOU 22	Output required by the background process.
SIGURG 23	Urgent socket condition.
SIGXCPU 24	CPU time limit exceeded.
SIGXFSZ 25	File size limit exceeded.
SIGVTALRM 26	Virtual timer, used to calculate the CPU occupation time of a process.
SIGPROF 27	Calculates the CPU time occupied by a process and the system calling time.
SIGWINCH 28	Window size change.
SIGIO 29	Input/Output.
SIGPOLL SIGIO	Equivalent to SIGIO.
SIGPWR 30	Power failure.
SIGSYS 31	Non-existent system call.
SIGUNUSED 31	Equivalent to SIGSYS.
SIGRTMIN 32	Start of a reliable signal.
SIGRTMIN (__libc_current_sigrtmin())	Start of a reliable signal.
SIGRTMAX _NSIG	End of a reliable signal.
SIGRTMAX (__libc_current_sigrtmax())	End of a reliable signal.
SA_THIRTYTWO 0x02000000	Transfers signals in a 32-bit mode even if the task runs in a 26-bit mode.
SA_NOMASK SA_NODEFER	Signals being processed are not masked during signal processing.
SA_ONESHOT SA_RESETHAND	The default processing mode is restored after the signal processing is complete.
SIG_BLOCK 0	Defines the function behaviors such as sigprocmask() and pthread_sigmask(), which are used to add signals to the mask set.
SIG_UNBLOCK 1	Defines the function behaviors such as sigprocmask() and pthread_sigmask(), which are used to remove signals from the mask set.
SIG_SETMASK 2	Defines the function behaviors such as sigprocmask() and pthread_sigmask(), which are used to set the mask set.
sa_handler _u._sa_handler	Defines the simplified writing used to indicate the __sa_handler.sa_handler field in sigaction.
sa_handler __sa_handler.sa_handler	Defines the simplified writing used to indicate the __sa_handler.sa_handler field in sigaction.
sa_sigaction _u._sa_sigaction	Defines the simplified writing used to indicate the __sa_handler.sa_sigaction field in sigaction.
sa_sigaction __sa_handler.sa_sigaction	Defines the simplified writing used to indicate the __sa_handler.sa_sigaction field in sigaction.
SIG_HOLD ((void (*)(int)) 2)	Defines the function behaviors such as sigset() and adds a specified signal to the process signal mask without changing the current processing mode of the signal.
SIGEV_SIGNAL 0	Sets the notification method in sigevent: signal notification.
SIGEV_NONE 1	Sets the notification method in sigevent: no notification.
SIGEV_THREAD 2	Sets the notification method in sigevent: thread notification.
SIG_ERR ((__sighandler_t)-1)	Defines the value returned when a function such as signal() fails.
SIG_DFL ((__sighandler_t)0)	Defines the default signal processing mode.
SIG_IGN ((__sighandler_t)1)	Defines the signal processing mode in which the signal is ignored.
IPC_CREAT 01000	Defines the input parameter used to create an IPC object.
IPC_EXCL 02000	Defines the input parameter used together with IPC_CREAT to prevent duplicate key values during IPC creation.
IPC_NOWAIT 04000	Defines the input parameter that specifies whether the communication is in non-blocking mode.
IPC_RMID 0	Defines the input parameter used to delete an IPC object.
IPC_SET 1	Defines the input parameter used to set information to the kernel.
IPC_INFO 3	Defines the input parameter used to obtain the system-level restriction information of a specified communication type, for example, shminfo.
IPC_PRIVATE ((key_t) 0)	Defines the IPC object as a private object.

## Typedefs

Typedef Name	Description
ucontext_t	typedef struct __ucontext Describes the user execution context.
stack_t	typedef struct sigaltstack Describes a signal stack.
sighandler_t ) (int)	typedef void(* Function pointer of signal handler.

Typedef Name

Description

ucontext_t

typedef struct __ucontext

Describes the user execution context.

stack_t

typedef struct sigaltstack

Describes a signal stack.

sighandler_t ) (int)

typedef void(*

Function pointer of signal handler.

## Functions

Function Name	Description
mq_close (mqd_t mqdes)	int Closes a message queue that is no longer used.
mq_getattr (mqd_t mqdes, struct mq_attr *attr)	int Obtains the attributes of the message queue specified by the descriptor. The values of mq_maxmsg, mq_msgsize, and mq_curmsgs are fixed.
mq_open (const char *name, int oflag,...)	mqd_t Creates a message queue or opens an existing message queue.
mq_receive (mqd_t mqdes, char buffer, size_t size, unsigned prioptr)	ssize_t Receives a message from a specified message queue.
mq_send (mqd_t mqdes, const char *buffer, size_t size, unsigned prio)	int Sends a message to a specified message queue.
mq_setattr (mqd_t mqdes, const struct mq_attr __restrict newattr, struct mq_attr __restrict oldattr)	int Sets the attributes of the message queue specified by the descriptor. The mq_maxmsg, mq_msgsize, and mq_curmsgs attributes cannot be modified. mq_flags supports the O_NONBLOCK attribute only.
mq_timedreceive (mqd_t mqdes, char __restrict buffer, size_t size, unsigned __restrict prioptr, const struct timespec *__restrict timeout)	ssize_t Receives a message from the message queue, with a timeout period specified.
mq_timedsend (mqd_t mqdes, const char buffer, size_t size, unsigned prio, const struct timespec timeout)	int Sends a message to a specified message queue, with a timeout period specified.
mq_unlink (const char *name)	int Decreases the reference count of the message queue by 1, or deletes the message queue if the reference count is 0.
sem_close (sem_t *sem)	int Closes a specified semaphore.
sem_destroy (sem_t *sem)	int Destroys a specified anonymous semaphore that is no longer used.
sem_getvalue (sem_t __restrict sem, int __restrict sval)	int Obtains the count value of a specified semaphore.
sem_init (sem_t *sem, int pshared, unsigned int value)	int Creates and initializes an anonymous semaphore.
sem_post (sem_t *sem)	int Increments the semaphore count by 1.
sem_timedwait (sem_t __restrict sem, const struct timespec __restrict timeout)	int Obtains the semaphore, with a timeout period specified.
sem_trywait (sem_t *sem)	int Attempts to obtain the semaphore.
sem_unlink (const char *name)	int Deletes a specified semaphore.
sem_wait (sem_t *sem)	int Obtains the semaphore.
kill (pid_t pid, int sig)	int Sends a signal to a specified process.
sigemptyset (sigset_t *set)	int Clears all signals in a specified signal set.
sigfillset (sigset_t *set)	int Adds all signals to a specified signal set.
sigaddset (sigset_t *set, int signum)	int Adds a signal to a specified signal set.
sigdelset (sigset_t *set, int signum)	int Delete a signal from a specified signal set.
sigismember (const sigset_t *set, int signum)	int Checks whether a signal is in the signal set.
sigprocmask (int how, const sigset_t __restrict set, sigset_t __restrict oldset)	int Changes the signal mask of the calling thread.
sigsuspend (const sigset_t *mask)	int Sets mask as the signal mask set of the current process and suspends the process until a signal processing action is triggered.
sigaction (int signum, const struct sigaction __restrict act, struct sigaction __restrict oldact)	int Checks or changes the processing action associated with a specified signal. Do not use the process creation function or thread creation function in the registered callback processing function. Otherwise, the process is abnormal. For example, do not use the fork or pthread_create function.
sigpending (sigset_t *set)	int Obtains the signal suspended by the current thread.
sigwait (const sigset_t __restrict set, int __restrict sig)	int Suspends the calling thread and waits for the signal.
sigwaitinfo (const sigset_t __restrict set, siginfo_t __restrict info)	int Suspends the calling thread and waits for the signal, with a timeout period specified.
sigtimedwait (const sigset_t __restrict set, siginfo_t __restrict info, const struct timespec *__restrict timeout)	int Suspends the calling thread and waits for the signal, with a timeout period specified.
pthread_sigmask (int how, const sigset_t __restrict set, sigset_t __restrict oldset)	int Checks or changes the signal mask of the calling thread.
psignal (int signum, const char *s)	void Prints signal information.
sigpause (int signum)	int Deletes the signal specified by signum from the signal mask of the calling process and suspends the process until the signal is received.
sighold (int signum)	int Adds the signal specified by signum to the signal mask of the calling process.
sigrelse (int signum)	int Deletes the signal specified by signum from the signal mask of the calling process.
sigisemptyset (const sigset_t *set)	int Checks whether the signal set is empty.
sigorset (sigset_t dest, const sigset_t left, const sigset_t *right)	int Adds the union set of left and right to dest.
sigandset (sigset_t dest, const sigset_t left, const sigset_t *right)	int Adds the intersection of left and right to dest.
signal (int signum, sighandler_t handler)	sighandler_t Changes the processing action of a specified signal. Do not use the process creation function or thread creation function in the registered callback processing function. Otherwise, the process is abnormal. For example, do not use the fork or pthread_create function.
raise (int signum)	int Sends a signal to the calling process.

## **Details**
## **Macro Definition Documentation**
## MINSIGSTKSZ \[1/2\]
```
#define MINSIGSTKSZ   2048
```
**Description:**
Defines the minimum size of the replaceable signal stack.
Minimum size of the replaceable signal stack.
## MINSIGSTKSZ \[2/2\]
```
#define MINSIGSTKSZ   2048
```
**Description:**
Defines the minimum size of the replaceable signal stack.
Minimum size of the replaceable signal stack.
## sa\_handler \[1/2\]
```
#define sa_handler   _u._sa_handler
```
**Description:**
Defines the simplified writing used to indicate the **\_\_sa\_handler.sa\_handler** field in [sigaction](sigaction.md).
Indicates the **\_u.\_sa\_handler** field in [sigaction](sigaction.md).
## sa\_handler \[2/2\]
```
#define sa_handler   __sa_handler.sa_handler
```
**Description:**
Defines the simplified writing used to indicate the **\_\_sa\_handler.sa\_handler** field in [sigaction](sigaction.md).
Indicates the **\_u.\_sa\_handler** field in [sigaction](sigaction.md).
## SA\_NOCLDSTOP \[1/2\]
```
#define SA_NOCLDSTOP   1
```
**Description:**
Stops a child process without sending the [SIGCHLD](IPC.md#ga0e63521a64cc8bc2b91d36a87d32c9f8) signal.
[SIGCHLD](IPC.md#ga0e63521a64cc8bc2b91d36a87d32c9f8) is not generated when the child process is stopped.
## SA\_NOCLDSTOP \[2/2\]
```
#define SA_NOCLDSTOP   0x00000001
```
**Description:**
Stops a child process without sending the [SIGCHLD](IPC.md#ga0e63521a64cc8bc2b91d36a87d32c9f8) signal.
[SIGCHLD](IPC.md#ga0e63521a64cc8bc2b91d36a87d32c9f8) is not generated when the child process is stopped.
## SA\_NODEFER \[1/2\]
```
#define SA_NODEFER   0x40000000
```
**Description:**
The signals being processed are not masked during signal processing.
Signals being processed are not masked during signal processing.
## SA\_NODEFER \[2/2\]
```
#define SA_NODEFER   0x40000000
```
**Description:**
The signals being processed are not masked during signal processing.
Signals being processed are not masked during signal processing.
## SA\_ONSTACK \[1/2\]
```
#define SA_ONSTACK   0x08000000
```
**Description:**
Uses the signal stack specified by [sigaltstack](sigaltstack.md) for signal processing.
The signal stack specified by [sigaltstack](sigaltstack.md) is used for signal processing.
## SA\_ONSTACK \[2/2\]
```
#define SA_ONSTACK   0x08000000
```
**Description:**
Uses the signal stack specified by [sigaltstack](sigaltstack.md) for signal processing.
The signal stack specified by [sigaltstack](sigaltstack.md) is used for signal processing.
## sa\_sigaction \[1/2\]
```
#define sa_sigaction   _u._sa_sigaction
```
**Description:**
Defines the simplified writing used to indicate the **\_\_sa\_handler.sa\_sigaction** field in [sigaction](sigaction.md).
Indicates the **\_\_sa\_handler.sa\_sigaction** field in [sigaction](sigaction.md).
## sa\_sigaction \[2/2\]
```
#define sa_sigaction   __sa_handler.sa_sigaction
```
**Description:**
Defines the simplified writing used to indicate the **\_\_sa\_handler.sa\_sigaction** field in [sigaction](sigaction.md).
Indicates the **\_\_sa\_handler.sa\_sigaction** field in [sigaction](sigaction.md).
## SA\_SIGINFO \[1/2\]
```
#define SA_SIGINFO   4
```
**Description:**
Transfers the **siginfo\_t** structure with the signal.
Transfers the **siginfo\_t** with the signal.
## SA\_SIGINFO \[2/2\]
```
#define SA_SIGINFO   0x00000004
```
**Description:**
Transfers the **siginfo\_t** structure with the signal.
Transfers the **siginfo\_t** with the signal.
## SIGRTMAX \[1/2\]
```
#define SIGRTMAX   [_NSIG](IPC.md#ga275020780cb70a8b0cba8db5accc5d19)
```
**Description:**
End of a reliable signal.
Defines the highest available signal number.
## SIGRTMAX \[2/2\]
```
#define SIGRTMAX   (__libc_current_sigrtmax())
```
**Description:**
End of a reliable signal.
Defines the highest available signal number.
## SIGRTMIN \[1/2\]
```
#define SIGRTMIN   32
```
**Description:**
Start of a reliable signal.
Defines the lowest available signal number.
## SIGRTMIN \[2/2\]
```
#define SIGRTMIN   (__libc_current_sigrtmin())
```
**Description:**
Start of a reliable signal.
Defines the lowest available signal number.
## SIGSTKSZ \[1/2\]
```
#define SIGSTKSZ   8192
```
**Description:**
Defines the size of a signal stack.
Size of a signal stack.
## SIGSTKSZ \[2/2\]
```
#define SIGSTKSZ   8192
```
**Description:**
Defines the size of a signal stack.
Size of a signal stack.
## **Typedef Documentation**
## stack\_t
```
typedef struct [sigaltstack](sigaltstack.md) [stack_t](IPC.md#ga395f9ff4025fe05bb535322593abde72)
```
**Description:**
Describes a signal stack.
Describes stack information for signal processing.
## **Function Documentation**
## kill\(\)
```
int kill (pid_t pid, int sig )
```
**Description:**
Sends a signal to a specified process.
**Parameters:**

Name	Description
pid	Indicates the target process ID. Parameters 0 and -1 are not supported.
sig	Indicates the signal to be sent.

**Attention:**
The default signal behavior does not support **STOP** and **CONTINUE**, and **COREDUMP** is not involved. [SIGSTOP](IPC.md#ga069e358bc9a864b7dc4fed2440eda14c), [SIGKILL](IPC.md#gaddd8dcd406ce514ab3b4f576a5343d42), and [SIGCONT](IPC.md#ga024f43063003e753afc6cca1acd6e104) cannot be masked. If a process is killed but its parent process is not recycled, a zombie process will be generated. After an asynchronous signal is sent to a process, the signal callback is executed only after the process is scheduled. The kernel cannot forcibly kill the process by using the signal. After the process is killed, [SIGCHLD](IPC.md#ga0e63521a64cc8bc2b91d36a87d32c9f8) is sent to its parent process. The sending action cannot be canceled. A sleeping process cannot be woken up by a signal. For example, a process invokes the **sleep** function to enter the sleeping state. Currently, suspended and zombie processes cannot be forcibly killed.
**Returns:**
Returns **0** if the operation is successful; returns **-1** and sets **errno** to a value in the following table if the operation fails.

errno	Description
EINVAL	sig is invalid.
EPERM	The current process does not have permission to send the signal to the target process.

## mq\_close\(\)
```
int mq_close (mqd_t mqdes)
```
**Description:**
Closes a message queue that is no longer used.
**Parameters:**

Name	Description
mqdes	Indicates the descriptor of the message queue to be closed.

**Returns:**
Returns **0** if the message queue is closed; returns **-1** and sets **errno** to a value in the following table if the message queue fails to be closed.

errno	Description
EBADF	mqdes is invalid.
EAGAIN	Failed to delete the message queue in the function.
EFAULT	Failed to release the space allocated with the message queue.
EINVAL	mqdes is invalid when the message queue is deleted in the function.

## mq\_getattr\(\)
```
int mq_getattr (mqd_t mqdes, struct [mq_attr](mq_attr.md) * attr )
```
**Description:**
Obtains the attributes of the message queue specified by the descriptor. The values of **mq\_maxmsg**, **mq\_msgsize**, and **mq\_curmsgs** are fixed.
**Parameters:**

Name	Description
mqdes	Indicates the message queue descriptor.
attr	Indicates the pointer to the obtained message queue attributes.

**Returns:**
Returns **0** if the operation is successful; returns **-1** and sets **errno** to a value in the following table if the operation fails.

errno	Description
EBADF	mqdes is invalid.
EINVAL	attr is NULL.

## mq\_open\(\)
```
mqd_t mq_open (const char * name, int oflag, ... )
```
**Description:**
Creates a message queue or opens an existing message queue.
**Parameters:**

Name	Description
name	Indicates the name of the message queue to be created or opened. The maximum value is 259 bytes.
oflag	Indicates the open flag, including O_RDONLY, O_WRONLY, O_RDWR, O_CREAT, O_EXCL, and O_NONBLOCK.
mode	Used for permission control (not supported currently). Its type is mode_t. This is parameter is required only during the creation of a message queue.
attr	Indicates the pointer to the queue attributes to be set. Its type is struct mq_attr. This parameter is optional and is used only during the creation of a message queue.

**Attention:**
The message consists of 64 bytes by default, and the maximum length of the message is 64 KB. A maximum of 1024 message queues can be created. The mq\_curmsgs field in [mq\_attr](mq_attr.md) is not supported.
**Returns:**
Returns a message queue descriptor if the creation or opening is successful; returns **\(mqd\_t\)-1** and sets **errno** to a value in the following table if the operation fails.

errno	Description
EEXIST	Both O_CREAT and O_EXCL are specified in oflag, and a queue with this name already exists.
EINVAL	O_CREAT is specified in oflag, and attr is invalid. To be specific, the attribute value of the mq_maxmsg field is less than 0 or greater than USHRT_MAX, or the attribute value of mq_msgsize is less than 0 or greater than USHRT_MAX-4. name is NULL, or the length of the string specified by name is 0.
ENAMETOOLONG	name is too long.
ENOENT	O_CREAT is not specified in oflag, and no queue with this name exists.
ENOSPC	Insufficient memory.
ENFILE	The number of opened message queues exceeds the maximum.

## mq\_receive\(\)
```
ssize_t mq_receive (mqd_t mqdes, char * buffer, size_t size, unsigned * prioptr )
```
**Description:**
Receives a message from a specified message queue.
The message queue must have been open by [mq\_open\(\)](IPC.md#gaf5d8bf423701bd1783849119511381a5).
**Parameters:**

Name	Description
mqdes	Indicates the message queue descriptor.
buffer	Indicates the pointer to the buffer for receiving the message.
size	Indicates the size of the buffer for receiving the message.
prioptr	Indicates the priority of the received message if the value is not NULL. This parameter is not used because priority-based message processing is not supported.

**Returns:**
Returns the number of bytes in the received message if the message is received; returns **\(mqd\_t\)-1** and sets **errno** to a value in the following table if the message fails to be received.

errno	Description
EINTR	Message receiving is interrupted.
EAGAIN	The message queue is empty, and O_NONBLOCK is set for the message queue associated with mqdes.
EBADF	mqdes is invalid or you do not have the read permission.
EMSGSIZE	The size of the buffer for receiving the message is less than the message size attribute of the message queue.
EINVAL	mqdes is NULL or the value of size is 0.

## mq\_send\(\)
```
int mq_send (mqd_t mqdes, const char * buffer, size_t size, unsigned prio )
```
**Description:**
Sends a message to a specified message queue.
The message queue must have been opened using [mq\_open\(\)](IPC.md#gaf5d8bf423701bd1783849119511381a5).
**Parameters:**

Name	Description
mqdes	Indicates the message queue descriptor.
buffer	Indicates the pointer to the buffer for receiving the message.
size	Indicates the size of the buffer for receiving the message.
prio	Indicates the priority of the message to be sent. This parameter must be 0 because priority-based message processing is not supported.

**Returns:**
Returns **0** if the message is sent; returns **-1** and sets **errno** to a value in the following table if the message fails to be sent.

errno	Description
EINTR	Message sending is interrupted.
EAGAIN	The message queue is full, and O_NONBLOCK is set for the message queue associated with mqdes.
EBADF	mqdes is invalid or you do not have the write permission.
EINVAL	mqdes is NULL or the value of size is 0.
EMSGSIZE	The size of the message to be sent is greater than the message size attribute of the message queue.
EOPNOTSUPP	The operation is not supported.
ENOSPC	Insufficient space.

## mq\_setattr\(\)
```
int mq_setattr (mqd_t mqdes, const struct [mq_attr](mq_attr.md) *__restrict newattr, struct [mq_attr](mq_attr.md) *__restrict oldattr )
```
**Description:**
Sets the attributes of the message queue specified by the descriptor. The **mq\_maxmsg**, **mq\_msgsize**, and **mq\_curmsgs** attributes cannot be modified. **mq\_flags** supports the **O\_NONBLOCK** attribute only.
The message queue must have been opened using [mq\_open\(\)](IPC.md#gaf5d8bf423701bd1783849119511381a5).
**Parameters:**

Name	Description
mqdes	Indicates the message queue descriptor.
newattr	Indicates the pointer to the message queue attribute to be set.
oldattr	Indicates the pointer to the original attribute.

**Returns:**
Returns **0** if the setting is successful; returns **-1** and sets **errno** to a value in the following table if the setting fails.

errno	Description
EBADF	mqdes is invalid.
EINVAL	newattr is NULL.

## mq\_timedreceive\(\)
```
ssize_t mq_timedreceive (mqd_t mqdes, char *__restrict buffer, size_t size, unsigned *__restrict prioptr, const struct [timespec](timespec.md) *__restrict timeout )
```
**Description:**
Receives a message from the message queue, with a timeout period specified.
The message queue must have been open by [mq\_open\(\)](IPC.md#gaf5d8bf423701bd1783849119511381a5).
**Parameters:**

Name	Description
mqdes	Indicates the message queue descriptor.
buffer	Indicates the pointer to the buffer for receiving the message.
size	Indicates the size of the buffer for receiving the message.
prioptr	Indicates the priority of the received message if the value is not NULL. This parameter is not used because priority-based message processing is not supported.
timeout	Indicates the timeout duration for returning a request.

**Returns:**
Returns the number of bytes in the message if the message is received; returns **\(mqd\_t\)-1** and sets **errno** to a value in the following table if the message fails to be received.

errno	Description
EINTR	Message receiving is interrupted.
EAGAIN	The message queue is empty, and O_NONBLOCK is set for the message queue associated with mqdes.
EBADF	mqdes is invalid or you do not have the read permission.
EINVAL	mqdes is NULL or the value of size is 0. timeout is invalid. Specifically, the value of tv_sec is less than 0, and the value of tv_nsec is less than 0 or greater than the ninth power of 10.
EMSGSIZE	The size of the buffer for receiving the message is less than the message size attribute of the message queue.
ETIMEDOUT	No message is received from the queue before the specified timeout period expires.

## mq\_timedsend\(\)
```
int mq_timedsend (mqd_t mqdes, const char * buffer, size_t size, unsigned prio, const struct [timespec](timespec.md) * timeout )
```
**Description:**
Sends a message to a specified message queue, with a timeout period specified.
The message queue must have been open by [mq\_open\(\)](IPC.md#gaf5d8bf423701bd1783849119511381a5).
**Parameters:**

Name	Description
mqdes	Indicates the message queue descriptor.
buffer	Indicates the pointer to the buffer for sending the message.
size	Indicates the size of the message to be sent.
prio	Indicates the priority of the message to be sent. This parameter must be 0 because priority-based message processing is not supported.
timeout	Indicates the timeout duration for returning a request.

**Returns:**
Returns **0** if the message is sent; returns **-1** and sets **errno** to a value in the following table if the message fails to be sent.

errno	Description
EINTR	Message sending is interrupted.
EAGAIN	The message queue is full, and O_NONBLOCK is set for the message queue associated with mqdes.
EBADF	mqdes is invalid or you do not have the write permission.
EINVAL	mqdes is NULL or the value of size is 0. timeout is invalid. Specifically, the value of tv_sec is less than 0, and the value of tv_nsec is less than 0 or greater than the ninth power of 10.
EMSGSIZE	The size of the message to be sent is greater than the message size attribute of the message queue.
EOPNOTSUPP	The operation is not supported.
ETIMEDOUT	No message is sent to the queue before the specified timeout period expires.

## mq\_unlink\(\)
```
int mq_unlink (const char * name)
```
**Description:**
Decreases the reference count of the message queue by 1, or deletes the message queue if the reference count is **0**.
**Parameters:**

Name	Description
name	Indicates the message queue name.

**Returns:**
Returns **0** if the operation is successful; returns **-1** and sets **errno** to a value in the following table if the operation fails.

errno	Description
ENAMETOOLONG	name is too long.
ENOENT	There is no message queue with the given name.
EAGAIN	Failed to delete the message queue.
EBUSY	The message queue to be deleted is being used.
EINVAL	name is NULL, or the length of the string specified by name is 0.

## psignal\(\)
```
void psignal (int signum, const char * s )
```
**Description:**
Prints signal information.
This function is used to print a message on the standard error output. The message consists of the character string **s**, a colon, a space, a string describing the signal information **signum**, and a newline character. If the string **s** is **NULL** or empty, the colon and space are omitted. If **signum** is invalid, the message displayed will indicate an unknown signal.
**Parameters:**

Name	Description
signum	Indicates the signal to be operated.
s	Indicates the customized string s to be printed.

## pthread\_sigmask\(\)
```
int pthread_sigmask (int how, const sigset_t *__restrict set, sigset_t *__restrict oldset )
```
**Description:**
Checks or changes the signal mask of the calling thread.
**Parameters:**

Name	Description
how	Indicates the function behavior.
set	Indicates the signal set to be changed.
oldset	Used for returning the original mask set of the calling thread.

flags	Description
SIG_BLOCK	Adds the signals in set to the mask set of the thread.
SIG_UNBLOCK	Removes the signals in set from the mask set of the thread.
SIG_SETMASK	Sets set of blocked signals as the mask set of the thread.

**Returns:**
Returns **0** if the change is successful; returns **errno** as described below if the change fails.

errno	Description
EINVAL	how is invalid.
EFAULT	The set or oldset points outside the process's allocated address space.

## raise\(\)
```
int raise (int signum)
```
**Description:**
Sends a signal to the calling process.
**Parameters:**

Name	Description
signum	Indicates the signal to be sent.

**Returns:**
Returns 0 if the operation is successful; returns nonzero if the operation fails.
## sem\_close\(\)
```
int sem_close ([sem_t](sem_t.md) * sem)
```
**Description:**
Closes a specified semaphore.
**Parameters:**

Name	Description
sem	Indicates the pointer to the semaphore to close.

**Returns:**
Returns **0** if the semaphore is closed. errno | Description
## sem\_destroy\(\)
```
int sem_destroy ([sem_t](sem_t.md) * sem)
```
**Description:**
Destroys a specified anonymous semaphore that is no longer used.
**Parameters:**

Name	Description
sem	Indicates the pointer to the semaphore to destroy.

**Returns:**
Returns **0** if the destruction is successful.
## sem\_getvalue\(\)
```
int sem_getvalue ([sem_t](sem_t.md) *__restrict sem, int *__restrict sval )
```
**Description:**
Obtains the count value of a specified semaphore.
**Parameters:**

Name	Description
sem	Indicates the pointer to the semaphore.
sval	Indicates the pointer to the obtained count value.

**Returns:**
Returns **0** if the operation is successful.
## sem\_init\(\)
```
int sem_init ([sem_t](sem_t.md) * sem, int pshared, unsigned int value )
```
**Description:**
Creates and initializes an anonymous semaphore.
**Parameters:**

Name	Description
sem	Indicates the pointer to the semaphore to be created or initialized.
pshared	Specifies whether the semaphore can be shared between the threads of a process. 0 indicates that the semaphore is shared between the threads of a process. A non-zero value indicates that the semaphore can be shared between processes.
value	Indicates the initial count value of the new semaphore.

**Returns:**
Returns **0** if the initialization is successful; returns **-1** and sets **errno** to a value in the following table if the initialization fails.

errno	Description
EINVAL	The input parameters are invalid and the initial count value exceeds the limit.

## sem\_post\(\)
```
int sem_post ([sem_t](sem_t.md) * sem)
```
**Description:**
Increments the semaphore count by 1.
Generally, this function is used to increment the count of available shared resources when the process is to leave its critical section. This function is used together with [sem\_wait\(\)](IPC.md#gaad70020dca2241a2b78e272ca033271b) to protect shared resources.
**Parameters:**

Name	Description
sem	Indicates the pointer to the semaphore to be operated.

**Returns:**
Returns **0** if the operation is successful; returns **-1** and sets **errno** to a value in the following table if the operation fails.

errno	Description
EOVERFLOW	The number of semaphores exceeds the maximum number.

## sem\_timedwait\(\)
```
int sem_timedwait ([sem_t](sem_t.md) *__restrict sem, const struct [timespec](timespec.md) *__restrict timeout )
```
**Description:**
Obtains the semaphore, with a timeout period specified.
If the semaphore count is greater than **0**, the shared resources can be obtained and the semaphore count decreases by 1. After the function returns **0**, the code execution enters the resource critical area. Otherwise, the shared resource is unavailable, the function keeps waiting, and code execution cannot enter the resource critical area. If no semaphore is obtained within the specified timeout period, **-1** is returned. This function is used together with [sem\_post\(\)](IPC.md#ga015dce85cab8477c679cc47968958247) to protect shared resources.
**Parameters:**

Name	Description
sem	Indicates the pointer to the semaphore to be operated.
timeout	Indicates the timeout period to be set.

**Returns:**
Returns **0** if the semaphore is obtained; returns **-1** and sets **errno** to a value in the following table if the semaphore fails to be obtained.

errno	Description
EINVAL	sem or timeout is invalid.
EINTR	The waiting for the semaphore is interrupted.
EAGAIN	The semaphore is set to non-blocking mode, and the semaphore count is 0.
ETIMEDOUT	Waiting for the semaphore timed out.

## sem\_trywait\(\)
```
int sem_trywait ([sem_t](sem_t.md) * sem)
```
**Description:**
Attempts to obtain the semaphore.
If the semaphore count is greater than **0**, the shared resources can be obtained and the semaphore count decreases by 1. After the function returns **0**, the code execution enters the resource critical area. Otherwise, the shared resource is unavailable, and the function returns **-1**. This function is used together with [sem\_post\(\)](IPC.md#ga015dce85cab8477c679cc47968958247) to protect shared resources.
**Parameters:**

Name	Description
sem	Indicates the pointer to the semaphore to be operated.

**Returns:**
Returns **0** if the semaphore is obtained; returns **-1** and sets **errno** to a value in the following table if the semaphore fails to be obtained.

errno	Description
EINVAL	sem is invalid.
EAGAIN	The semaphore is set to non-blocking mode, and the semaphore count is 0.

## sem\_unlink\(\)
```
int sem_unlink (const char * name)
```
**Description:**
Deletes a specified semaphore.
This function is used to subtract 1 from the open count of the semaphore. If the open count is **0**, this function deletes the semaphore.
**Parameters:**

Name	Description
name	Indicates the name of the semaphore to be deleted.

**Returns:**
Returns **0** if the operation is successful; returns **-1** and sets **errno** to a value in the following table if the operation fails.

errno	Description
ENAMETOOLONG	name is too long.
ENOENT	There is no semaphore with the given name.

## sem\_wait\(\)
```
int sem_wait ([sem_t](sem_t.md) * sem)
```
**Description:**
Obtains the semaphore.
If the semaphore count is greater than **0**, the shared resources can be obtained and the semaphore count decreases by 1. After the function returns **0**, the code execution enters the resource critical area. Otherwise, the shared resource is unavailable, the function keeps waiting, and code execution cannot enter the resource critical area. This function is used together with [sem\_post\(\)](IPC.md#ga015dce85cab8477c679cc47968958247) to protect shared resources.
**Parameters:**

Name	Description
sem	Indicates the pointer to the semaphore to be obtained.

**Returns:**
Returns **0** if the semaphore is obtained; returns **-1** and sets **errno** to a value in the following table if the semaphore fails to be obtained.

errno	Description
EINVAL	sem is invalid.
EINTR	The waiting for the semaphore is interrupted.
EAGAIN	The semaphore is set to non-blocking mode, and the semaphore count is 0.

## sigaction\(\)
```
int [sigaction](sigaction.md) (int signum, const struct [sigaction](sigaction.md) *__restrict act, struct [sigaction](sigaction.md) *__restrict oldact )
```
**Description:**
Checks or changes the processing action associated with a specified signal. Do not use the process creation function or thread creation function in the registered callback processing function. Otherwise, the process is abnormal. For example, do not use the **fork** or **pthread\_create** function.
**Parameters:**

Name	Description
signum	Indicates the signal to be operated.
act	Indicates the new signal processing action.
oldact	Indicates the original signal processing action if the value is not NULL.

**Attention:**
The **sa\_flag** field in **act** supports the [SA\_SIGINFO](IPC.md#ga59b4c0774aace526b10b6d737075a790) flag only.
**Returns:**
Returns **0** if the operation is successful; returns **-1** and sets **errno** to a value in the following table if the operation fails.

errno	Description
EINVAL	signum is invalid or cannot be captured.
EINVAL	act is NULL.
ENOMEM	Insufficient memory.

## sigaddset\(\)
```
int sigaddset (sigset_t * set, int signum )
```
**Description:**
Adds a signal to a specified signal set.
**Parameters:**

Name	Description
set	Indicates the pointer to the signal set to hold signals.
signum	Indicates the signal to be added.

**Returns:**
Returns **0** if the operation is successful. If **set** or **signum** is invalid, the process is abnormal.
## sigandset\(\)
```
int sigandset (sigset_t * dest, const sigset_t * left, const sigset_t * right )
```
**Description:**
Adds the intersection of **left** and **right** to **dest**.
**Parameters:**

Name	Description
dest	Indicates the pointer to the target signal set.
left	Indicates the left set to be added.
right	Indicates the right set to be added.

**Returns:**
Returns **0** if the operation is successful.
## sigdelset\(\)
```
int sigdelset (sigset_t * set, int signum )
```
**Description:**
Delete a signal from a specified signal set.
**Parameters:**

Name	Description
set	Indicates the pointer to the signal set to hold signals.
signum	Indicates the signal to be deleted.

**Returns:**
Returns **0** if the operation is successful. If **set** or **signum** is invalid, the process is abnormal.
## sigemptyset\(\)
```
int sigemptyset (sigset_t * set)
```
**Description:**
Clears all signals in a specified signal set.
**Parameters:**

Name	Description
set	Indicates the pointer to the signal set to be cleared.

**Returns:**
Returns **0** if the operation is successful. If **set** is invalid, the process is abnormal.
## sigfillset\(\)
```
int sigfillset (sigset_t * set)
```
**Description:**
Adds all signals to a specified signal set.
**Parameters:**

Name	Description
set	Indicates the pointer to the signal set to hold signals.

**Returns:**
Returns **0** if the operation is successful. If **set** is invalid, the process is abnormal.
## sighold\(\)
```
int sighold (int signum)
```
**Description:**
Adds the signal specified by **signum** to the signal mask of the calling process.
**Parameters:**

Name	Description
signum	Indicates the signal to be added.

**Returns:**
Returns **0** if the operation is successful; returns **-1** if the operation fails.
## sigisemptyset\(\)
```
int sigisemptyset (const sigset_t * set)
```
**Description:**
Checks whether the signal set is empty.
**Parameters:**

Name	Description
set	Indicates the pointer to the signal set to be checked.

**Returns:**
Returns **0** if the set is not empty; returns **1** if the set is empty; returns **-1** if the operation fails.
## sigismember\(\)
```
int sigismember (const sigset_t * set, int signum )
```
**Description:**
Checks whether a signal is in the signal set.
**Parameters:**

Name	Description
set	Indicates the pointer to the signal set to be checked.
signum	Indicates the signal to be checked.

**Returns:**
Returns **0** if the signal is not in the set; returns **1** if the signal is in the set. If **set** or **signum** is invalid, the process is abnormal.
## signal\(\)
```
[sighandler_t](IPC.md#ga754cdc0bcfffe07baa426dc252c9101a) signal (int signum, [sighandler_t](IPC.md#ga754cdc0bcfffe07baa426dc252c9101a) handler )
```
**Description:**
Changes the processing action of a specified signal. Do not use the process creation function or thread creation function in the registered callback processing function. Otherwise, the process is abnormal. For example, do not use the **fork** or **pthread\_create** function.
**Parameters:**

Name	Description
signum	Indicates the signal to be operated.
handler	Indicates the pointer to the specified processing action (SIG_IGN or SIG_DFL, or to a custom processing action.

**Returns:**
Returns the pointer to the original processing action if the operation is successful; returns [SIG\_ERR](IPC.md#ga3c330fbddd84bf34e65af370b11998d6) and sets **errno** to a value in the following table if the operation fails.

errno	Description
EINVAL	signum is invalid.
ENOMEM	Insufficient memory.

## sigorset\(\)
```
int sigorset (sigset_t * dest, const sigset_t * left, const sigset_t * right )
```
**Description:**
Adds the union set of **left** and **right** to **dest**.
**Parameters:**

Name	Description
dest	Indicates the pointer to the target signal set.
left	Indicates the left set to be added.
right	Indicates the right set to be added.

**Returns:**
Returns **0** if the operation is successful.
## sigpause\(\)
```
int sigpause (int signum)
```
**Description:**
Deletes the signal specified by **signum** from the signal mask of the calling process and suspends the process until the signal is received.
**Parameters:**

Name	Description
signum	Indicates the signal to be operated.

**Attention:**
After the signal is received, the original signal mask of the process is restored.
**Returns:**
Returns **-1** and sets **errno** to a value in the following table.

errno	Description
EINTR	The suspension wait is interrupted by the signal.

## sigpending\(\)
```
int sigpending (sigset_t * set)
```
**Description:**
Obtains the signal suspended by the current thread.
**Parameters:**

Name	Description
set	Indicates the pointer to the signal set used to store the signal suspended by the current thread.

**Returns:**
Returns **0** if the operation is successful; returns **-1** and sets **errno** to a value in the following table if the operation fails.

errno	Description
EFAULT	set points to an invalid space.
EINVAL	set is NULL.
EINTR	The suspension wait is interrupted by the signal.

## sigprocmask\(\)
```
int sigprocmask (int how, const sigset_t *__restrict set, sigset_t *__restrict oldset )
```
**Description:**
Changes the signal mask of the calling thread.
**Parameters:**

Name	Description
how	Indicates the function behavior.
set	Indicates the pointer to the signal set to be changed.
oldset	Indicates the pointer to the original mask set of the calling thread.

flags	Description
SIG_BLOCK	Adds the signals in set to the mask set of the thread.
SIG_UNBLOCK	Removes the signals in set from the mask set of the thread.
SIG_SETMASK	Sets set of blocked signals as the mask set of the thread.

**Returns:**
Returns **0** if the change is successful; returns **-1** and sets **errno** to a value in the following table if the change fails.

errno	Description
EINVAL	how is invalid.
EFAULT	The set or oldset points outside the process's allocated address space.

## sigrelse\(\)
```
int sigrelse (int signum)
```
**Description:**
Deletes the signal specified by **signum** from the signal mask of the calling process.
**Parameters:**

Name	Description
signum	Indicates the signal to be deleted.

**Returns:**
Returns **0** if the deletion is successful; returns **-1** if the deletion fails.
## sigsuspend\(\)
```
int sigsuspend (const sigset_t * mask)
```
**Description:**
Sets **mask** as the signal mask set of the current process and suspends the process until a signal processing action is triggered.
**Parameters:**

Name	Description
mask	Indicates the pointer to the signal set to be set.

**Returns:**
Returns **-1** and sets **errno** to a value in the following table.

errno	Description
EFAULT	mask points to an invalid space.
EINTR	The suspension wait is interrupted by the signal.
EINVAL	mask is NULL.

## sigtimedwait\(\)
```
int sigtimedwait (const sigset_t *__restrict set, siginfo_t *__restrict info, const struct [timespec](timespec.md) *__restrict timeout )
```
**Description:**
Suspends the calling thread and waits for the signal, with a timeout period specified.
This function is used to suspend execution of the calling thread until one of the signals specified in the signal set becomes pending. This function specifies the timeout behavior.
**Parameters:**

Name	Description
set	Indicates the pointer to the signal set to be waited.
info	Used to save the information about the waited signal if the value is not NULL.
timeout	Indicates the timeout duration.

**Attention:**
The signal to be waited must be in the mask set of the current thread. Once the signal is received, it is deleted from the pending list. The waiting for the signal cannot be interrupted by other signals, that is, **EINTR** is not supported.
**Returns:**
Returns the value of the waited signal if the operation is successful; returns **-1** and sets **errno** to a value in the following table if the operation fails.

errno	Description
EAGAIN	No signal specified by set is generated within the specified timeout period.
EINVAL	timeout is invalid.

## sigwait\(\)
```
int sigwait (const sigset_t *__restrict set, int *__restrict sig )
```
**Description:**
Suspends the calling thread and waits for the signal.
This function is used to suspend execution of the calling thread until one of the signals specified in the signal set becomes pending.
**Parameters:**

Name	Description
set	Indicates the pointer to the signal set to be waited.
sig	Used for saving the waited signal.

**Attention:**
The signal to be waited must be in the mask set of the current thread. Once the signal is received, it is deleted from the pending list.
**Returns:**
Returns **0** if the signal is received; returns **-1** and sets **errno** to a value in the following table if the signal fails to be received.

errno	Description
EINTR	The suspension wait is interrupted by the signal.

## sigwaitinfo\(\)
```
int sigwaitinfo (const sigset_t *__restrict set, siginfo_t *__restrict info )
```
**Description:**
Suspends the calling thread and waits for the signal, with a timeout period specified.
This function is used to suspend execution of the calling thread until one of the signals specified in the signal set becomes pending. This function does not specify the timeout behavior.
**Parameters:**

Name	Description
set	Indicates the pointer to the signal set to be waited.
info	Used to save the information about the waited signal if the value is not NULL.

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