Linux and real-time signals

Question

I am trying to write simple program that uses real-time signals in Linux. But I encountered strange behaviour, first the code:

#include<signal.h>
#include<sys/types.h>
#include<stdlib.h>
#include<stdio.h>
#include"errhandling.h"
#include<string.h>
#include<errno.h>
#include<unistd.h>

void baz(int sig,siginfo_t* info,void *context)
{
  if (sig==SIGUSR1)
      printf("SIGUSR1 %d\n",info->si_value.sival_int);
  else if(sig==SIGRTMIN)
      printf("SIGRTMIN %d\n",info->si_value.sival_int);
  else 
      printf("SIGRTMIN+1 %d\n",info->si_value.sival_int);
  return ;
}

void sig_output()
{
  sigset_t set;
  sigprocmask(0,NULL,&set);
  printf("blokowane sa: ");
  if (sigismember(&set,SIGUSR1))
      printf("SIGUSR1 ");
  if(sigismember(&set,SIGUSR2))
      printf(" SIGUSR2");
  printf("\n");
  return ;
}


int received=0;
int usr2=0;

void foo(int sig)
{
  return ;
}

void usr1_handler(int sig)
{
  printf("usr1_handler\n");
  //++received;
}



void usr2_handler(int sig)
{
  usr2=1;
}

int main(int argc,char **argv)
{
  int i=0;
  pid_t pid=getppid();
  struct sigaction a;
  struct sigaction a2;
  a.sa_flags=SA_SIGINFO;
  sigset_t set;
  sigemptyset(&set);
  //sigaddset(&set,SIGRTMAX);
  sigemptyset(&(a.sa_mask));
  sigemptyset(&(a2.sa_mask));
  a.sa_sigaction=baz;
  sigaction(SIGRTMIN,&a,NULL);
  a2.sa_handler=usr1_handler;
  sigaction(SIGRTMIN+1,&a2,NULL);
  //sigprocmask(SIG_SET,&(a.sa_mask),NULL);
  while(!usr2)
  {
      printf("while\n"); 
      sigsuspend(&set);

  }
      //pause()
  printf("after while\n");
  return EXIT_SUCCESS;
}

when I run this program and it enters this loop with sigsuspend and I send to this program SIGRTMIN everything goes ok - handler executes and it waits for another signal, but when I send it SIGRTMIN+1 I get segmentation fault.

It seems that for real-time signals I need to use this extended handler with 3 arguments, but why? Is it specified somewhere? I run this program on my friend OpenSUSE 12.1 and I don't get segmentation fault for SIGRTMIN+1, but on my Xubuntu 11.10, when I send SIGRTMIN+1 I am getting segmentation fault. Is it problem with my system? Or is it implementation dependent?

Answer 1

Don't use printf() and friends from inside a signal handler. Don't be stubborn... Here's a substitute.

#include <stdarg.h>

int myprintf(const char *fmt, ...);
int myprintf(const char *fmt, ...)
{
    va_list args;
    char buff[512];
    int rc;

    va_start( args, fmt );

    rc = vsnprintf ( buff, sizeof buff, fmt, args );
    va_end (args);

    if (rc >0 && rc < sizeof buff) write(1, buff, rc);

    return rc;
}

UPDATE: this appears to work:

#include <signal.h>
#include <sys/types.h>
#include <stdlib.h>
#include <stdio.h>
#pragma #include"errhandling.h"
#include <string.h>
#include <errno.h>
#include <unistd.h>
#include <stdarg.h>

int myprintf(const char *fmt, ...);
int myprintf(const char *fmt, ...)
{
  va_list args;
  char buff[512];
  int rc;

  va_start( args, fmt );
  rc = vsnprintf ( buff, sizeof buff, fmt, args );
  va_end (args);

  if (rc > 0 && rc < sizeof buff) write(1, buff, rc);

  return rc;
}

void baz(int sig, siginfo_t *info, void *context)
{
#define INFO_VAL(p) p?p->si_value.sival_int:0
  if (sig==SIGUSR1)
      myprintf("SIGUSR1 %d\n", INFO_VAL(info) );
  else if(sig==SIGRTMIN)
      myprintf("SIGRTMIN %d\n", INFO_VAL(info) );
  else
      myprintf("SIGRTMIN%+d %p\n", sig-SIGRTMIN, INFO_VAL(info) );
  return ;
#undef INFO_VAL
}

void sig_output()
{
  sigset_t set;
  sigprocmask(0,NULL,&set);
  myprintf("blokowane sa: ");
  if (sigismember(&set,SIGUSR1))
      myprintf("SIGUSR1 ");
  if(sigismember(&set,SIGUSR2))
      myprintf(" SIGUSR2");
  myprintf("\n");
  return ;
}
volatile int received=0;
volatile int usr2=0;

void foo(int sig)
{
  return ;
}

void usr1_handler(int sig)
{
  myprintf("usr1_handler\n");
  //++received;
}



void usr2_handler(int sig)
{
  usr2=1;
}

int main(int argc,char **argv)
{
  int i=0;
  pid_t pid=getppid();
  struct sigaction a, a2;
  sigset_t set;

  sigemptyset(&set);
  //sigaddset(&set,SIGRTMAX);
  sigemptyset(&a.sa_mask);
  sigemptyset(&a2.sa_mask);
  a.sa_flags = 0;
  a2.sa_flags = SA_SIGINFO;
  a2.sa_sigaction = baz;
  a.sa_handler = usr1_handler;
  sigaction(SIGUSR1,&a,NULL);

  a.sa_handler = usr2_handler;
  sigaction(SIGUSR2,&a,NULL);

  sigaction(SIGRTMIN+1,&a2,NULL);
  sigaction(SIGRTMIN+2,&a2,NULL);
  sigaction(SIGRTMIN+3,&a2,NULL);
  //sigprocmask(SIG_SET,&(a.sa_mask),NULL);
  while(!usr2)
  {
      myprintf("while(!usr2)\n");
      sigsuspend(&set);

  }
      //pause()
  myprintf("after while\n");
  return EXIT_SUCCESS;
}

Answer 2

It seems you are missing to assign a handler to a2.sa_sigaction.

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In general it is not good idea to refer to signals by a raw integer values, as the definition for the various signals might be platform specific.

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Update: Make sure the struct sigaction structures are initialised properly by for example memset()ing them to 0.