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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:
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.
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 TerminologyBoth 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.
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.
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 You can have the signal blocked by using one of the following functions: sigblock 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 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:
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.
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.
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 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 4.2.5 Signal Handling and OpenVMS Exception HandlingThis 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.
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.
1Supported on OpenVMS Version 7.3 and higher. 2SS$_BADWINCNT when C$_SIGWINCH not defined (OpenVMS versions before 7.3).
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