An X/Open specification defines a "signal" as a mechanism by which a process may be notified of, or affected by, an event occurring in the system. The term signal is also used to refer to the event itself.
A signal is said to be generated when the event that causes the signal first occurs. Examples of such events include the following:
A synchronous signal is a signal that is generated by some action attributable to a program running within the thread, such as a system-detected error, raise(), or CEESGL. An asynchronous signal is a signal that is generated for the process by using the kill() function or by an asynchronous event such as terminal activity or an expired timer.
The signal action vector is a list of signal-handling actions for each defined signal. The signal action vector is maintained separately for each process and is inherited from the parent process. The signal action vector specifies the signal-handling actions for both synchronously and asynchronously generated signals.
A signal is said to be delivered to a process when the specified signal-handling action for the signal is taken. A signal is said to be accepted by a process when a signal is selected and returned by one of the sigwait functions.
Signals generated for a process are delivered to or accepted by one thread in the process.
A signal is said to be pending during the interval between the time the signal is generated and the time it is delivered or accepted. Ordinarily, this interval cannot be detected by an application. However, a signal can be blocked from being delivered to a thread. When a signal is blocked, the signal-handling action associated with the signal is not taken. If there are no threads in a call to a sigwait function selecting the signal and if all threads block delivery of the signal, the signal remains pending on the process. The signal remains pending until either a thread calls a sigwait function selecting the signal, a thread unblocks delivery of the signal, or the signal action associated with the signal is set to ignore the signal. The signal blocking mask defines the set of signals that are blocked from delivery to the thread. The signal blocking mask is maintained separately for each thread in the process and is inherited from the thread that created it.
The set of defined signals is determined by the system. The system specifies the attributes for each defined signal. These attributes consist of a signal number, the initial signal action, and the signal default action. The system also specifies an initial signal blocking mask. The set of defined signals, the signal attributes, and signal blocking mask are referred to as signal controls.
A signal can be generated or delivered only to a process that has expressed an interest in signals. An error condition results under the following conditions:
A process can express an interest in signals by calling the Qp0sEnableSignals() API. In addition, calling particular signal APIs implicitly enables the process for signals.
If the process has not been enabled for signals, the process signal controls are set from signal controls established by the system during IPL (the system signal controls). An error condition results if an attempt is made to enable signals for the process before the system signal controls have been initialized.
Once the process signal controls have been initialized, the user is permitted to change the signal controls for the process. For example, the signal blocking mask and the signal action for a signal are commonly changed. Some signal controls, such as the number of defined signals and the signal default action for a signal, cannot be changed at the process level.
The attributes for each defined signal are stored in an object called a signal monitor. The system supports a maximum of 63 signal monitors for each process. The process signal action vector is a list of signal monitors, one for each defined signal. The signal monitor contains, but is not limited to, the following information:
The signal action defines the action to be taken by the system when a process receives an unblocked signal. The user can change the signal action for a process signal monitor. The possible signal actions are:
The handle using signal default action signal action indicates that the system is to take the action specified by the signal default action field when the signal is eligible to be delivered.
The ignore the signal signal action indicates that the user is not interested in handling the signal. When an ignored signal is generated for the process, the system automatically discards the signal, regardless of the blocked or unblocked state of the signal monitor.
The handle the signal by running signal-catching function signal action causes the system to call the signal-catching function when a signal is received for the signal monitor. The signal-catching function is set to point to a procedure within an active activation group.
The signal default action field defines the action to be taken by the system when the signal action is set to handle using signal default action. The signal default action for a signal monitor is set in the system signal controls and cannot be changed for a process signal monitor. The possible signal default actions are:
The terminate the process action puts the process in a phase that ends the process, allowing cancel handlers to be called. If the process is already in the end phase, the terminate the process action is ignored.
The end the request action results in the cancelation of all calls up to the nearest call that has a call status of request processor. If a call with a status of request processor is not present or the job is capable of having multiple threads, the terminate the process action is taken.
The ignore the signal action causes the system to discard the signal. A signal is discarded for a signal monitor in the blocked state when the signal action is handle using signal default action and the default signal action is ignore the signal.
The stop the process action causes the system to place the process in the stopped state. When a process is in the stopped state, it is temporarily suspended until a signal is generated for the process that has continue the process if stopped as its signal default action. When a process is in the stopped state, the normal process control functions remain in effect (the process can be suspended, resumed, or ended). When a signal is generated for a signal monitor that has stop the process as its signal default action, the system removes any pending signals for signal monitors that have continue the process if stopped as their default action.
The continue the process if stopped action causes the system to resume running the process that is in the stopped state, even if the signal monitor with the signal default action of continue the process if stopped is in the blocked state or has a signal action of ignore the signal. When a signal is generated for a signal monitor that has continue the process if stopped as its signal default action, the system removes any pending signals for signal monitors that have stop the process as their signal default action.
The signal exception action causes the system to send the MCH7603 escape message to the process.
The signal options specify an additional set of attributes for the signal monitor. The primary use of these options is to specify an additional set of actions to be taken by the system when a signal-catching function is called.
A signal is generated by sending a request to a signal monitor. Scheduling of the signal-handling action is controlled separately for each signal monitor through the signal blocking mask. The signal blocking mask is a bit mask that defines the set of signals to be blocked from delivery to the thread. The blocked or unblocked option specified for the nth bit position in the signal blocking mask is applied to the nth signal monitor defined for the process. When signal is unblocked is specified, the signal-handling action is eligible to be scheduled. When signal is blocked is specified, the signal-handling action is blocked from delivery.
The process to receive the signal is identified by a process ID. The process ID is used to indicate whether the signal should be sent to an individual process or to a group of processes (known as a process group). The process ID is a 4-byte binary number that is used to locate an entry in the system-managed process table. A process table entry contains the following information relating to the process:
The parent process is the logical creator of the process. A process group represents a collection of processes that are bound together for some common purpose. An error condition results if the process ID specified when a signal is sent does not represent a valid process or process group.
The process sending a signal must have the appropriate authority to the receiving process. The parent process is allowed to send a signal to a child process (the parent process ID of the receiving process is equal to the process ID of the process sending the signal). A child process is allowed to send a signal to its parent process (the process ID of the receiving process is equal to the parent process ID of the process sending the signal). A process can send a signal to another process if the sending process has *JOBCTL authority defined for the current process user profile or in an adopted user profile. Otherwise, the real or effective user ID of the sending process must match the real or effective user ID of the receiving process. An error condition results if the process does not have authority to send the signal to a receiving process.
The IBM® i support for signals does differ from the usual behavior of signals on UNIX® systems:
On UNIX systems, the standard C signal functions (as defined by American National Standards Institute (ANSI)) and the UNIX signal functions interact. That is, the standard C signal() function operates on the process signal action vector. Likewise, when a signal is generated for a process using the standard C raise() function, the process signal blocking mask and the signal action vector are used to determine the action to be taken.
On IBM i, the behavior of the standard C signal functions depends on a compiler option. When the compiler option SYSIFCOPT(*ASYNCSIGNAL) is specified, the standard C signal() and raise() functions operate like the UNIX signal functions by operating on the process signal action vector and the process signal blocking mask. However, if the SYSIFCOPT(*ASYNCSIGNAL) is not specified the standard C signal functions do not operate like the UNIX signal functions. Although the default C signal model does not interact with the UNIX signal functions, the UNIX signal functions sigaction() and kill() provide the same type of capability as the standard C signal() and raise() functions. For more information, see sigaction()--Examine and Change Signal Action and kill()--Send Signal to Process or Group of Processes.
On most UNIX systems, a process consists of a single thread of control. When the program in control needs to perform a task that is contained in another program, the program uses the fork() and exec() functions to start a child process that runs the other program. The signal controls for the child process are inherited from the parent process. Changes to the signal controls in either the parent or the child process are isolated to the process in which the change is made.
On IBM i, when a program needs to perform a task that is contained in another program, the program calls that program directly. The target program is run using the same process structure. As a result of this call and return mechanism, if a called program changes the process signal controls and does not restore the original signal controls when returning to its caller, the changed process signal controls remain in effect. The called program inherits the signal controls of its caller. However, there are some differences from what would be expected if fork() and exec() were used in a UNIX process:
Programs that use signals and change the signal controls of the process should restore the old actions or signal blocking mask (or both) when they return to their callers. Programs using signals should explicitly enable the process for signals when the program begins. If the process was not enabled for signals when the program was called, the program should also disable signals when it returns to the process. For more information, see Qp0sEnableSignals()--Enable Process for Signals and Qp0sDisableSignals()--Disable Process for Signals.
On UNIX systems, system-detected errors are mapped to signal numbers. For example, a floating point error results in the SIGFPE signal being generated for the process. On IBM i, the default C signal model presents system-detected errors to the user as escape messages which can be handled with C signal handlers established with the C signal() function or with ILE C exception-handling functions, but not with signal handlers established with the UNIX sigaction() function. When the compiler option SYSIFCOPT(*ASYNCSIGNAL) is specified, system-detected errors are mapped to signal numbers and can be handled with signal handlers established either with the C signal() function or the UNIX sigaction() function, but not with ILE C exception-handling functions.
On UNIX systems, an unhandled error condition in a signal-catching function results in ending the process. On IBM i, unhandled error conditions in the signal-catching function are implicitly handled. The signal-catching function is ended and the receiving program resumes running at the point at which it was interrupted. The error condition may be logged in the job log. Aside from the job log entry for the error, no further error notification takes place.
IBM i offers two types of termination actions. The termination action applied to most signals is to end the most recent request. This usually results in ending the current program, which is the expectation of most UNIX programmers. The second termination action is to end the process, which is more severe. The only signal with this action is SIGKILL.
On IBM i, some default actions for signals are different than on typical UNIX systems. For example, the IBM i default action for the SIGPIPE signal is to ignore the signal.