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signal.h File Reference

Standard C Libraries -. More...

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Macros

#define SIG_DFL   ((void (*)(int))0)
 
#define SIG_ERR   ((void (*)(int))-1)
 
#define SIG_IGN   ((void (*)(int))1)
 
#define SIGABRT   1
 
#define SIGFPE   4
 
#define SIGILL   3
 
#define SIGINT   2
 
#define SIGSEGV   5
 
#define SIGTERM   6
 

Typedefs

typedef int sig_atomic_t
 

Functions

int raise (int sig)
 

Variables

void(*)(int) signal (int sig, void(*func)(int))
 

Detailed Description

Standard C Libraries -.

Compiler:
MPLAB XC16 compiler

Description

The header file, signal.h, consists of a type, several macros and two functions that specify how the program handles signals while it is executing. A signal is a condition that may be reported during the program execution. Signals are synchronous, occurring under software control via the raise function. A signal may be handled by:

  • Default handling (SIG_DFL); the signal is treated as a fatal error and execution stops
  • Ignoring the signal (SIG_IGN); the signal is ignored and control is returned to the user application
  • Handling the signal with a function designated via signal.

By default, all signals are handled by the default handler, which is identified by SIG_DFL. The type sig_atomic_t is an integer type that the program access atomically. When this type is used with the keyword volatile, the signal handler can share the data objects with the rest of the program.

Software License Agreement

The documentation in this header file has been copied from the documentation provided with the Microchip MPLAB XC16 compiler. The original license agreement included with the XC16 compiler applies!

Macro Definition Documentation

#define SIG_DFL   ((void (*)(int))0)

Description: Used as the second argument and/or the return value for signal to specify that the default handler should be used for a specific signal.

Include: <signal.h>

#define SIG_ERR   ((void (*)(int))-1)

Description: Used as the return value for signal when it cannot complete a request due to an error.

Include: <signal.h>

#define SIG_IGN   ((void (*)(int))1)

Description: Used as the second argument and/or the return value for signal to specify that the signal should be ignored.

Include: <signal.h>

#define SIGABRT   1

Description: Name for the abnormal termination signal.

Include: <signal.h>

Remarks:
SIGABRT represents an abnormal termination signal and is used in conjunction with raise or signal. The default raise behavior (action identified by SIG_DFL) is to output to the standard error stream: abort - terminating See the example accompanying signal to see general usage of signal names and signal handling.

Example:

#include <signal.h> // for raise, SIGABRT
#include <stdio.h> // for printf
int main(void) {
raise(SIGABRT);
printf("Program never reaches here.");
}

Output:
ABRT

Explanation:
ABRT stands for "abort".

#define SIGFPE   4

Description: Signals floating-point, error such as for division by zero or result out of range.

Include: <signal.h>

Remarks:
SIGFPE is used as an argument for raise and/or signal. When used, the default behavior is to print an arithmetic error message and terminate the calling program. This may be overridden by a user function that defines the signal handler actions. See signal for an example of a user-defined function.

Example:

#include <signal.h> // for raise, SIGFPE
#include <stdio.h> // for printf
int main(void) {
raise(SIGFPE);
printf("Program never reaches here");
}

Output:
FPE

Explanation:
FPE stands for "floating-point error".

#define SIGILL   3

Description: Signals illegal instruction.

Include: <signal.h>

Remarks:
SIGILL is used as an argument for raise and/or signal. When used, the default behavior is to print an invalid executable code message and terminate the calling program. This may be overridden by a user function that defines the signal handler actions. See signal for an example of a user-defined function.

Example:

#include <signal.h> // for raise, SIGILL
#include <stdio.h> // for printf
int main(void) {
raise(SIGILL);
printf("Program never reaches here");
}

Output:
ILL

Explanation:
ILL stands for "illegal instruction".

#define SIGINT   2

Description: Interrupt signal.

Include: <signal.h>

Remarks:
SIGINT is used as an argument for raise and/or signal. When used, the default behavior is to print an interruption message and terminate the calling program. This may be overridden by a user function that defines the signal handler actions. See signal for an example of a user-defined function.

Example:

#include <signal.h> // for raise, SIGINT
#include <stdio.h> // for printf
int main(void) {
raise(SIGINT);
printf("Program never reaches here.");
}

Output:
INT

Explanation:
INT stands for "interruption".

#define SIGSEGV   5

Description: Signals invalid access to storage.

Include: <signal.h>

Remarks:
SIGSEGV is used as an argument for raise and/or signal. When used, the default behavior is to print an invalid storage request message and terminate the calling program. This may be overridden by a user function that defines the signal handler actions. See signal for an example of a user-defined function.

Example:

#include <signal.h> // for raise, SIGSEGV
#include <stdio.h> // for printf
int main(void) {
raise(SIGSEGV);
printf("Program never reaches here.");
}

Output:
SEGV

Explanation:
SEGV stands for "invalid storage access".

#define SIGTERM   6

Description: Signals a termination request.

Include: <signal.h>

Remarks:
SIGTERM is used as an argument for raise and/or signal. When used, the default behavior is to print a termination request message and terminate the calling program. This may be overridden by a user function that defines the signal handler actions. See signal for an example of a user-defined function.

Example:

#include <signal.h> // for raise, SIGTERM
#include <stdio.h> // for printf
int main(void) {
raise(SIGTERM);
printf("Program never reaches here.");
}

Output:
TERM

Explanation:
TERM stands for "termination request".

Typedef Documentation

typedef int sig_atomic_t

Description: A type used by a signal handler.

Include: <signal.h>

Function Documentation

int raise ( int  sig)

Description: Reports a synchronous signal.

Include: <signal.h>

Parameters
sigsignal name
Returns
Returns a 0 if successful; otherwise, returns a non-zero value.

Remarks:
raise sends the signal identified by sig to the executing program.

Example:

#include <signal.h> // for raise, signal,
// SIGILL, SIG_DFL
#include <stdlib.h> // for div, div_t
#include <stdio.h> // for printf
#include <p30f6014.h> // for INTCON1bits
void __attribute__((__interrupt__))
_MathError(void) {
raise(SIGILL);
INTCON1bits.MATHERR = 0;
}
void illegalinsn(int idsig) {
printf("Illegal instruction executed\n");
exit(1);
}
int main(void) {
int x, y;
div_t z;
signal(SIGILL, illegalinsn);
x = 7;
y = 0;
z = div(x, y);
printf("Program never reaches here");
}

Output:
Illegal instruction executed

Explanation:
This example requires the linker script, p30f6014.gld. There are three parts to this example.

  • First, an interrupt handler is written for the interrupt vector, _MathError, to handle a math error by sending an illegal instruction, signal (SIGILL), to the executing program. The last statement in the interrupt handler clears, the exception flag.
  • Second, the function, illegalinsn, will print an error message and call exit.
  • Third, in main, signal (SIGILL, illegalinsn) sets the handler for SIGILL to the function, illegalinsn.

When a math error occurs, due to a divide by zero, the _MathError interrupt vector is called, which in turn, will raise a signal that will call the handler function for SIGILL, which is the function, illegalinsn. Thus, error messages are printed and the program is terminated.

Variable Documentation

void(*)(int) signal(int sig, void(*func)(int))

Description: Controls interrupt signal handling.

Include: <signal.h>

Parameters
sigsignal name
funcfunction to be executed
Returns
Returns the previous value of func.

Example:

#include <signal.h> // for signal, raise,
// SIGINT, SIGILL,
// SIG_IGN, and SIGFPE
#include <stdio.h> // for printf
// Signal handler function
void mysigint(int id) {
printf("SIGINT received\n");
}
int main(void) {
// Override default with user defined function
signal(SIGINT, mysigint);
raise(SIGINT);
// Ignore signal handler
raise(SIGILL);
printf("SIGILL was ignored\n");
// Use default signal handler
raise(SIGFPE);
printf("Program never reaches here.");
}

Output:
SIGINT received
SIGILL was ignored
FPE

Explanation:
The function, mysigint, is the user-defined signal handler for SIGINT. Inside the main program, the function, signal, is called to set up the signal handler (mysigint) for the signal SIGINT that will override the default actions. The function, raise, is called to report the signal SIGINT. This causes the signal handler for SIGINT to use the user-defined function (mysigint) as the signal handler so it prints the "SIGINT received" message.

Next, the function, signal, is called to set up the signal handler SIG_IGN for the signal SIGILL. The constant SIG_IGN is used to indicate the signal should be ignored. The function, raise, is called to report the signal, SIGILL that is ignored.

The function, raise, is called again to report the signal, SIGFPE. Since there is no user-defined function for SIGFPE, the default signal handler is used so the message "FPE" is printed (which stands for "arithmetic error - terminating"). Then, the calling program is terminated. The printf statement is never reached.