捕获SIGINT信号以终止自定义shell

Question

Hope you can help me to resolve this problem.

For school I have to transform Ctrl+C to a command which doesn't shut down the shell, but he reminds through printf() that I must type exit to close the shell. I don't even know where to start.

Thank a lot.

Answer 1

Here's a trivial implementation of handling SIGINT using sigaction which will work on posix systems. Left out error checking for brevity. The linked manual should explain about sigaction .

Basically the program loops through an infinite loop and break if user types exit. Using write as you can't use printf in signal handler. See signal manual for a list of functions that can be used safely in a signal handler.

#include<stdio.h>
#include<signal.h>
#include<string.h>
#include<stdlib.h>

char s[]="Type 'exit' to terminate\n";

void int_handler (int signum)
{
  write(fileno(stdin), s, sizeof s - 1);
}

int main (void)
{
  char str[256];
  struct sigaction sh;

  sh.sa_handler = int_handler;
  sigemptyset (&sh.sa_mask);
  sh.sa_flags = 0;
  sigaction (SIGINT, &sh, NULL);
  printf("%s", s);

  while(1) {
    fgets(str, sizeof str, stdin);
    char *p = strchr(str, '\n');
    if(p) *p = 0;
    if(!strcmp(str, "exit")) {
      printf("Exiting on request...");
      break;
    }
  }
  return 0;
}

Answer 2

Ctrl+C sends an interrupt signal (SIGINT) to the running process.You can use signal() to catch SIGINT like this:

 #include<stdio.h>
 #include<signal.h>

 void sigint_handler(int sig)
 {
   printf("Type exit to close the shell!\n");
 }


  int main()
  {
    signal(SIGINT, sigint_handler);

    /*Your code should replace the while loop.*/
    while(1)
    {
        printf("Running!\n");
        getchar();
    }

    return 0 ;
  }

Answer 3

As you're talking about doing it from the shell, you probably want:

$ trap "echo Please type \'exit\' to close the shell." SIGINT
<Ctrl-C>
Please type 'exit' to close the shell.
$

This specifies a command to execute when the listed signal is trapped (the trap command can also trap other signals; SIGINT is the one generated by Ctrl-C ). The \\' protects the quote from being interpreted by the shell.