Signals are integral to any Unix® or Linux® application and are part of the POSIX®.1 standard.

A signal is automatically sent to the parent when a child process terminates (SIGCHLD). Signals are also used for many other synchronous and asynchronous notifications, such as: waking a process when a wait call such as pause(), alarm(), or sleep() is performed (SIGALRM); or informing a process that it has issued a memory violation (SIGSEGV).

Signals in POSIX are quite powerful. Each thread can block incoming signals on a per-signal basis, define signal handlers for each signal it might receive, and queue signals (in the case of a real-time signal). Standard Unix POSIX signals do not guarantee delivery. Furthermore, POSIX signals are not queueable nor can they carry any information beyond the signal number itself.

POSIX.1b, supported fully by the LynxOS RTOS, improves this situation. POSIX.1b specifies real-time signals (a minimum of 8 are required) SIGRTMIN through SIGRTMAX to provide these capabilities. Real-time signals can carry data, are queued (delivery to a specific thread is guaranteed), and can be prioritized.

 

POSIX.1 Signals

SIGABRT	Abnormal termination signal caused by the abort() function. A portable program should void catching SIGABRT.
SIGALRM	The timer set by the alarm() function has timed-out.
SIGFPE	Arithmetic exception, such as overflow or division by zero.
SIGHUP	Hangup detected on controlling terminal or death of a controlling process.
SIGILL	Illegal instruction indicating a program error. Applications may wish to catch this signal and attempt to recover from bugs. A portable program should not intentionally generate illegal instructions. After a SIGILL is caught, the only portable thing to do is to siglongjmp() back to a known place in your program (or callexit()).
SIGINT	Interrupt special character typed on controlling keyboard.
SIGKILL	Termination signal. This signal cannot be caught or ignored.
SIGPIPE	Write to a pipe with no readers.
SIGQUIT	Quit special character typed on controlling keyboard.
SIGSEGV	Invalid memory reference. Like SIGILL, portable programs should not intentionally generate invalid memory references.
SIGTERM	Termination signal.
SIGUSR1	Application-defined signal 1.
SIGUSR2	Application-defined signal 2.

 

Job Control Signals (POSIX.1)

SIGCHLD	Child process terminated or stopped. By default, this signal is ignored.
SIGCONT	Continue the process if it is currently stopped; otherwise, ignore the signal.
SIGSTOP	Stop signal. This signal cannot be caught or ignored.
SIGTSTP	Stop special character typed on the controlling keyboard.
SIGTTIN	Read from the controlling terminal attempted by a member of a background process group.
SIGTTOU	Write to controlling terminal attempted by a member of a background process group

 

"But is it POSIX-conformant?"

POSIX for embedded developers What is POSIX? POSIX.1, .1b and .1c POSIX Function Calls Reference Processes and Virtual Memory Parent-Child Processes Threads in POSIX Signals in POSIX LynxOS: POSIX-conformant RTOS POSIX PDF BROCHURE (170 kB) POSIX TRAINING: Real-time system course O'REILLY PROGRAMMING GUIDES for LynuxWorks operating systems POSIX Programmers Guide Pthreads Programming POSIX.4 Programmers Guide A savvy engineer just needs to ask a few quick questions about an operating system in order to determine if it might fully support the POSIX standards. "Does each process in the operating system reside in a different name space and have its own symbols table? "Does the operating system support the fork() call?" "Can the operating system distinguish between threads and processes?" "Does the operating system support signals?"

 

VIEW A LIST OF POSIX FUNCTION CALLS

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