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A signal handler can be installed for more than one signal. Use the
sigaction
routine with the SA_RESETHAND flag to control this.
4.2.3 Signal-Handling Concepts
A signal is said to be generated for (or sent to) a process when the event that causes the signal first occurs. Examples of such events include detection of hardware faults, timer expiration, and terminal activity, as well as the invocation of kill . In some circumstances, the same event generates signals for multiple processes.
Each process has an action to be taken in response to each signal defined by the system. A signal is said to be delivered to a process when the appropriate action for the process and signal is taken.
During the time between the generation of a signal and its delivery, the signal is said to be pending. Ordinarily, this interval cannot be detected by an application. However, a signal can be blocked from delivery to a process:
Each process has a signal mask that defines the set of signals currently blocked from delivery to it. The signal mask for a process is initialized from that of its parent. The sigaction , sigprocmask , and sigsuspend functions control the manipulation of the signal mask.
The determination of which action is taken in response to a signal is
made at the time the signal is delivered, allowing for any changes
since the time of generation. This determination is independent of the
means by which the signal was originally generated. If a subsequent
occurrence of a pending signal is generated, it is
implementation-dependent as to whether the signal is delivered more
than once. The HP C RTL delivers the signal only once. The
order in which multiple, simultaneously pending signals are delivered
to a process is unspecified.
4.2.4 Signal Actions
This section applies to the sigaction , signal , sigvec , and ssignal functions.
There are three types of action that can be associated with a signal:
SIG_DFL
SIG_IGN
pointer to a function
Initially, all signals are set to SIG_DFL or SIG_IGN prior to entry of the main routine (see the exec functions.) The actions prescribed by these values are:
SIG_DFL --- signal-specific default action
- The default actions for the signals defined in this document are specified under <signal.h> .
- If the default action is to stop the process, the execution of that process is temporarily suspended. When a process stops, a SIGCHLD signal is generated for its parent process, unless the parent process has set the SA_NOCLDSTOP flag. While a process is stopped, any additional signals that are sent to the process are not delivered until the process is continued, except SIGKILL which always terminates the receiving process. A process that is a member of an orphaned process group is not allowed to stop in response to the SIGSTOP, SIGTTIN, or SIGTTOU signals. In cases where delivery of one of these signals would stop such a process, the signal is discarded.
- Setting a signal action to SIG_DFL for a signal that is pending and whose default action is to ignore the signal (for example, SIGCHLD), causes the pending signal to be discarded, whether or not it is blocked.
SIG_IGN --- ignore signal
- Delivery of the signal has no effect on the process. The behavior of a process is undefined after it ignores a SIGFPE, SIGILL, or SIGSEGV signal that was not generated by kill or raise .
- The system does not allow the action for the SIGKILL or SIGSTOP signals to be set to SIG_IGN.
- Setting a signal action to SIG_IGN for a signal that is pending causes the pending signal to be discarded, whether or not it is blocked.
- If a process sets the action for the SIGCHLD signal to SIG_IGN, the behavior is unspecified.
pointer to a function --- catch signal
- On delivery of the signal, the receiving process executes the signal-catching function at the specified address. After returning from the signal-catching function, the receiving process resumes execution at the point at which it was interrupted.
- Specify the signal-catching function as:
void func(int signo);
Here, func is the specified signal-catching function and signo is the signal number of the signal being delivered.- The behavior of a process is undefined after it returns normally from a signal-catching function for a SIGFPE, SIGKILL, or SIGSEGV signal that was not generated by kill or raise .
- The system does not allow a process to catch the signals SIGKILL and SIGSTOP.
- If a process establishes a signal-catching function for the SIGCHLD signal while it has a terminated child process for which it has not waited, it is unspecified whether a SIGCHLD signal is generated to indicate that child process.
This section discusses how HP C RTL signal handling is implemented with and interacts with OpenVMS exception handling. Information in this section allows you to write OpenVMS exception handlers that do not conflict with HP C RTL signal handling. For information on OpenVMS exception handling, see the OpenVMS Procedure Calling and Condition Handling Standard.
The HP C RTL implements signals with OpenVMS exceptions. When gsignal or raise is called, the signal number is translated to a particular OpenVMS exception, which is used in a call to LIB$SIGNAL. This mechanism is necessary to catch an OpenVMS exception resulting from a user error and translate it into a corresponding UNIX signal. For example, an ACCVIO resulting from a write to a NULL pointer is translated to a SIGBUS or SIGSEGV signal.
Tables 4-4 and 4-5 list the HP C RTL signal names, the corresponding OpenVMS Alpha and Integrity server system exceptions, the event that generates the signal, and the optional signal code for use with the gsignal and raise functions.
To call a signal handler that you have established with signal or sigvec , the HP C RTL intercepts the OpenVMS exceptions that correspond to signals by having an OpenVMS exception handler in the main routine of the program. If your program has a main function, then this exception handler is automatically established. If you do not have a main function, or if your main function is written in a language other than HP C, then you must invoke the VAXC$CRTL_INIT routine to establish this handler.
The HP C RTL uses OpenVMS exceptions to implement the setjmp and longjmp functions. When the longjmp function is called, a C$_LONGJMP OpenVMS exception is signaled. To prevent the C$_LONGJMP exception from being interfered with by user exception handlers, use the VAXC$ESTABLISH routine to establish user OpenVMS exception handlers instead of calling LIB$ESTABLISH. The C$_LONGJMP mnemonic is defined in the <errnodef.h> header file.
If you want to use OpenVMS exception handlers and UNIX signals in your C program, your OpenVMS exception handler must be prepared to accept and resignal the OpenVMS exceptions listed in Table 4-4 (ALPHA ONLY), as well as the C$_LONGJMP exception and any C$ facility exception that might be introduced in future versions of the HP C RTL. This is because UNIX signals are global in context, whereas OpenVMS exceptions are stack-frame based.
Consequently, an OpenVMS exception handler always receives the exception that corresponds to the UNIX signal before the HP C RTL exception handler in the main routine does. By resignaling the OpenVMS exception, you allow the HP C RTL exception handler to receive the exception. You can intercept any of those OpenVMS exceptions yourself, but in doing so you will disable the corresponding UNIX signal.
| Name | OpenVMS Exception | Generated By | Code |
|---|---|---|---|
| SIGABRT | SS$_OPCCUS | The abort function | -- |
| SIGALRM | SS$_ASTFLT | The alarm function | -- |
| SIGBUS | SS$_ACCVIO | Access violation | -- |
| SIGBUS | SS$_CMODUSER | Change mode user | -- |
| SIGCHLD | C$_SIGCHLD | Child process stopped | -- |
| SIGEMT | SS$_COMPAT | Compatibility mode trap | -- |
| SIGFPE | SS$_DECDIV | Decimal divide trap | FPE_DECDIV_TRAP |
| SIGFPE | SS$_DECINV | Decimal invalid operand trap | FPE_DECINV_TRAP |
| SIGFPE | SS$_DECOVF | Decimal overflow trap | FPE_DECOVF_TRAP |
| SIGFPE | SS$_HPARITH | Floating/decimal division by 0 | FPE_FLTDIV_TRAP |
| SIGFPE | SS$_HPARITH | Floating overflow trap | FPE_FLTOVF_TRAP |
| SIGFPE | SS$_HPARITH | Floating underflow trap | FPE_FLTUND_TRAP |
| SIGFPE | SS$_HPARITH | Integer overflow | FPE_INTOVF_TRAP |
| SIGFPE | SS$_HPARITH | Invalid operand | FPE_INVOPR_TRAP |
| SIGFPE | SS$_HPARITH | Inexact result | FPE_INXRES_TRAP |
| SIGFPE | SS$_INTDIV | Integer div by zero | FPE_INTDIV_TRAP |
| SIGFPE | SS$_SUBRNG | Subscript out of range | FPE_SUBRNG_TRAP |
| SIGFPE | SS$_SUBRNG1 | Subscript1 out of range | FPE_SUBRNG1_TRAP |
| SIGFPE | SS$_SUBRNG2 | Subscript2 out of range | FPE_SUBRNG2_TRAP |
| SIGFPE | SS$_SUBRNG3 | Subscript3 out of range | FPE_SUBRNG3_TRAP |
| SIGFPE | SS$_SUBRNG4 | Subscript4 out of range | FPE_SUBRNG4_TRAP |
| SIGFPE | SS$_SUBRNG5 | Subscript5 out of range | FPE_SUBRNG5_TRAP |
| SIGFPE | SS$_SUBRNG6 | Subscript6 out of range | FPE_SUBRNG6_TRAP |
| SIGFPE | SS$_SUBRNG7 | Subscript7 out of range | FPE_SUBRNG7_TRAP |
| SIGHUP | SS$_HANGUP | Data set hangup | -- |
| SIGILL | SS$_OPCDEC | Reserved instruction | ILL_PRIVIN_FAULT |
| SIGILL | SS$_ROPRAND | Reserved operand | ILL_RESOP_FAULT |
| SIGINT | SS$_CONTROLC | OpenVMS Ctrl/C interrupt | -- |
| SIGIOT | SS$_OPCCUS | Customer-reserved opcode | -- |
| SIGKILL | SS$_ABORT | External signal only | -- |
| SIGQUIT | SS$_CONTROLY | The raise function | -- |
| SIGPIPE | SS$_NOMBX | No mailbox | -- |
| SIGPIPE | C$_SIGPIPE | Broken pipe | -- |
| SIGSEGV | SS$_ACCVIO | Length violation | -- |
| SIGSEGV | SS$_CMODSUPR | Change mode supervisor | -- |
| SIGSYS | SS$_BADPARAM | Bad argument to system call | -- |
| SIGTERM | Not implemented | -- | -- |
| SIGTRAP | SS$_BREAK | Breakpoint fault instruction | -- |
| SIGUSR1 | C$_SIGUSR1 | The raise function | -- |
| SIGUSR2 | C$_SIGUSR2 | The raise function | -- |
| SIGWINCH 1 | C$_SIGWINCH 2 | The raise function | -- |
|
| Name | OpenVMS Exception | Generated By | Code |
|---|---|---|---|
| SIGABRT | SS$_OPCCUS | The abort function | -- |
| SIGALRM | SS$_ASTFLT | The alarm function | -- |
| SIGBUS | SS$_ACCVIO | Access violation | -- |
| SIGBUS | SS$_CMODUSER | Change mode user | -- |
| SIGCHLD | C$_SIGCHLD | Child process stopped | -- |
| SIGEMT | SS$_COMPAT | Compatibility mode trap | -- |
| SIGFPE | SS$_DECOVF | Decimal overflow trap | FPE_DECOVF_TRAP |
| SIGFPE | SS$_DECDIV | Decimal divide trap | FPE_DECDIV_TRAP |
| SIGFPE | SS$_DECINV | Decimal invalid operand trap | FPE_DECINV_TRAP |
| SIGFPE | SS$_FLTDENORMAL | Denormal operand fault | FPE_FLTDENORMAL_FAULT |
| SIGFPE | SS$_FLTDIV | Floating/decimal division by 0 | FPE_FLTDIV_TRAP |
| SIGFPE | SS$_FLTDIV_F | Floating divide by 0 fault | FPE_FLTDIV_FAULT |
| SIGFPE | SS$_FLTINE | Inexact operation trap | FPE_FLTINE_TRAP |
| SIGFPE | SS$_FLTINV | Invalid operation trap | FPE_FLTINV_TRAP |
| SIGFPE | SS$_FLTINV_F | Invalid operation fault | FPE_FLTINV_FAULT |
| SIGFPE | SS$_FLTOVF | Floating overflow trap | FPE_FLTOVF_TRAP |
| SIGFPE | SS$_FLTUND | Floating underflow trap | FPE_FLTUND_TRAP |
| SIGFPE | SS$_INTDIV | Integer division by 0 | FPE_INTDIV_TRAP |
| SIGFPE | SS$_INTOVF | Integer overflow | FPE_INTOVF_TRAP |
| SIGFPE | SS$_SUBRNG | Subscript-range | FPE_SUBRNG_TRAP |
| SIGHUP | SS$_HANGUP | Data set hangup | -- |
| SIGILL | SS$_OPCDEC | Reserved instruction | ILL_PRIVIN_FAULT |
| SIGILL | SS$_ROPRAND | Reserved operand | ILL_RESOP_FAULT |
| SIGINT | SS$_CONTROLC | OpenVMS Ctrl/C interrupt | -- |
| SIGIOT | SS$_OPCCUS | Customer-reserved opcode | -- |
| SIGKILL | SS$_ABORT | External signal only | -- |
| SIGQUIT | SS$_CONTROLY | The raise function | -- |
| SIGPIPE | SS$_NOMBX | No mailbox | -- |
| SIGPIPE | C$_SIGPIPE | Broken pipe | -- |
| SIGSEGV | SS$_ACCVIO | Length violation | -- |
| SIGSEGV | SS$_CMODSUPR | Change mode supervisor | -- |
| SIGSYS | SS$_BADPARAM | Bad argument to system call | -- |
| SIGTERM | Not implemented | -- | -- |
| SIGTRAP | SS$_TBIT | TBIT trace trap | -- |
| SIGTRAP | SS$_BREAK | Breakpoint fault instruction | -- |
| SIGUSR1 | C$_SIGUSR1 | The raise function | -- |
| SIGUSR2 | C$_SIGUSR2 | The raise function | -- |
| SIGWINCH | C$_SIGWINCH | The raise function | -- |
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