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COMPAQ
Compaq C

Compaq C
Run-Time Library Reference Manual for OpenVMS Systems


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You can translate the error codes to a message, similar to that found in UNIX systems, by using the perror or strerror function. If errno is set to EVMSERR, perror cannot translate the error code, and prints the following message, followed by the OpenVMS error message associated with the value:


%s:nontranslatable vms error code: xxxxxx vms message: 

In the message, %s is the string you supply to perror ; xxxxxx is the OpenVMS condition value.

If errno is set to EVMSERR, then the OpenVMS condition value is available in the vaxc$errno variable declared in the <errno.h> header file. The vaxc$errno variable is guaranteed to have a valid value only if errno is set to EVMSERR; if errno is set to a value other than EVMSERR, the value of vaxc$errno is undefined.

See also the strerror function in the Reference Section for another way to translate error codes.

4.2 Signal Handling

A signal is a form of software interrupt to the normal execution of a user process. Signals occur as a result of a variety of events, including any of the following:

4.2.1 OpenVMS Versus UNIX Terminology

Both OpenVMS and UNIX systems provide signal-handling mechanisms that behave differently but use similar terminology. With the Compaq C RTL, you can program using either signal-handling mechanism. Before describing the signal-handling routines, some terminology must be established.

The UNIX term for a software interrupt is signal. A routine called by the UNIX system to process a signal is termed a signal handler.

A software interrupt on an OpenVMS system is referred to as a signal, condition, or exception. A routine called by the OpenVMS system to process software interrupts is termed a signal handler, condition handler, or exception handler.

To prevent confusion, the terms signal and signal handler in this manual refer to UNIX interrupts and interrupt processing routines, while the terms exception and exception handler refer to OpenVMS interrupts and interrupt processing routines.

4.2.2 UNIX Signals and the Compaq C RTL

Signals are represented by mnemonics defined in the <signal.h> header file. Table 4-3 lists the supported signal mnemonics and the corresponding event that causes each signal to be generated on the OpenVMS operating system.

Table 4-3 Compaq C RTL Signals
Name Description Generated by
SIGABRT 1 Abort abort ()
SIGALRM Alarm clock Timer AST, alarm routine
SIGBUS Bus error Access violation or change mode user
SIGCHLD Child process stopped Child process terminated or stopped
SIGEMT EMT instruction Compatibility mode trap or opcode reserved to customer
SIGFPE Floating-point
exception
Floating-point overflow/underflow
SIGHUP Hang up Data set hang up
SIGILL 1 Illegal
instruction
Illegal instruction, reserved operand, or reserved address mode
SIGINT 4 Interrupt OpenVMS Ctrl/C interrupt
SIGIOT 1 IOT instruction Opcode reserved to customer
SIGKILL 2, 3 Kill External signal only
SIGQUIT 5 Quit Not implemented.
SIGPIPE Broken pipe Write to a pipe with no readers.
SIGSEGV Segment
violation
Length violation or change mode user
SIGSYS System call
error
Bad argument to system call
SIGTERM Software
terminate
External signal only
SIGTRAP 1 Trace trap TBIT trace trap or breakpoint fault instruction
SIGUSR1 User-defined signal Explicit program call to raise the signal
SIGUSR2 User-defined signal Explicit program call to raise the signal
SIGWINCH 6 window size changed Explicit program call to raise the signal


1Cannot be reset when caught.
2Cannot be caught or ignored.
3Cannot be blocked.
4Setting SIGINT can affect processing of Ctrl/Y interrupts. For example, in response to a caller's request to block or ignore SIGINT, the Compaq C RTL disables the Ctrl/Y interrupt.
5"Not implemented" for SIGQUIT means that there is no external event, including a Ctrl/Y interrupt, that would trigger a SIGQUIT signal, thereby causing a signal handler established for SIGQUIT to be invoked. This signal can be generated only through an appropriate Compaq C RTL function, such as raise.
6Supported on OpenVMS Version 7.3 and higher.

By default, when a signal (except for SIGCHLD) occurs, the process is terminated. However, you can choose to have the signal ignored by using one of the following functions:

sigaction
signal
sigvec
ssignal

You can have the signal blocked by using one of the following functions:

sigblock
sigsetmask
sigprocmask
sigsuspend
sigpause

Table 4-3 indicates those signals that cannot be ignored or blocked.

You can also establish a signal handler to catch and process a signal by using one of the following functions:

sigaction
signal
sigvec
ssignal

Unless noted in Table 4-3, each signal can be reset. A signal is reset if the signal handler function calls signal or ssignal to re-establish itself to catch the signal. Example 4-1 shows how to establish a signal handler and reset the signal.

The calling interface to a signal handler is:

void handler (int sigint);

Where sigint is the signal number of the raised signal that caused this handler to be called.

A signal handler installed with sigvec remains installed until it is changed.

A signal handler installed with signal or signal remains installed until the signal is generated.

A signal handler can be installed for more than one signal. Use the sigaction routine with the flag SA_RESETHAND 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 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 function 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 Compaq 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

SIG_IGN --- ignore signal

pointer to a function --- catch signal

4.2.5 Signal Handling and OpenVMS Exception Handling

This section discusses how Compaq 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 Compaq C RTL signal handling. For information on OpenVMS exception handling, see the VAX Procedure Calling and Condition Handling Standard.

The Compaq 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 Compaq C RTL signal names, the corresponding OpenVMS VAX and OpenVMS Alpha exceptions, the event that generates the signal, and the optional signal code for use with the gsignal and raise functions.

Table 4-4 Compaq C RTL Signals and Corresponding OpenVMS VAX Exceptions (VAX ONLY)
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$_DECOVF Decimal overflow trap FPE_DECOVF_TRAP
SIGFPE SS$_FLTDIV Floating/decimal division by 0 FPE_FLTDIV_TRAP
SIGFPE SS$_FLTDIV_F Floating divide by 0 fault FPE_FLTDIV_FAULT
SIGFPE SS$_FLTOVF Floating overflow trap FPE_FLTOVF_TRAP
SIGFPE SS$_FLTOVF_F Floating overflow fault FPE_FLTOVF_FAULT
SIGFPE SS$_FLTUND Floating undeflow trap FPE_FLTUND_TRAP
SIGFPE SS$_FLTUND_F Floating undeflow fault FPE_FLTUND_FAULT
SIGFPE SS$_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$_RADRMOD Reserved addressing ILL_RESAD_FAULT
SIGILL SS$_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$_NOMBX No mailbox --
SIGSEGV SS$_ACCVIO Length violation --
SIGSEGV SS$_CMODUSER Change mode user --
SIGSYS SS$_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 1 C$_SIGWINCH 2 The raise function --


1Supported on OpenVMS Version 7.3 and higher.
2SS$_BADWINCNT when C$_SIGWINCH not defined (OpenVMS versions before 7.3).

To call a signal handler that you have established with signal or sigvec , the Compaq 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 Compaq C, then you must invoke the VAXC$CRTL_INIT routine to establish this handler.

The Compaq 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 Tables 4_4 (VAX ONLY) and 4-5 (ALPHA ONLY) , as well as the C$_LONGJMP exception and any C$ facility exception that might be introduced in future versions of the Compaq 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 Compaq C RTL exception handler in the main routine does. By resignaling the OpenVMS exception, you allow the Compaq 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.

Table 4-5 Compaq C RTL Signals and Corresponding OpenVMS Alpha Exceptions (ALPHA ONLY)
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 --
SIGFP 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 undeflow 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 --
SIGSEGV SS$_ACCVIO Length violation --
SIGSEGV SS$_CMODUSER Change mode user --
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 --


1Supported on OpenVMS Version 7.3 and higher.
2SS$_BADWINCNT when C$_SIGWINCH not defined (OpenVMS versions before 7.3).


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