How to call the interrupted system in Linux

Preface

Slow system calls refer to system calls that may never return, thus blocking the process forever. For example, accept when there is no client connection and read when there is no input are both slow system calls.

In Linux, when a process blocked in a slow system call captures a signal, the system call will be interrupted and the signal processing function will be executed instead. This is the interrupted system call.

However, when the signal processing function returns, the following situations may occur:

If the signal processing function is registered with signal, the system call will be automatically restarted and the function will not return.
If the signal processing function is registered with sigaction
- By default, the system call will not be automatically restarted, the function will return failure, and errno will be set to EINTR
- The system call will automatically restart only when the SA_RESTART flag of the interrupt signal is valid.

Next, we write code to verify the above situations respectively, where the system call selects read, the interrupt signal selects SIGALRM, and the interrupt signal is generated by alarm.

Using signal

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

void handler(int s)
{
  printf("read is interrupted by signal handler\n");
  return;
}

int main()
{
  char buf[10];
  int nread = 0;

  signal(SIGALRM, handler);
  alarm(2);

  printf("read start\n");
  nread = read(STDIN_FILENO, buf, sizeof(buf));
  printf("read return\n");

  if ((nread < 0) && (errno == EINTR))
  {
    printf("read return failed, errno is EINTR\n");
  }

  return 0;
}

Use sigaction + default

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

void handler(int s)
{
  printf("read is interrupted by signal handler\n");
  return;
}

int main()
{
  char buf[10];
  int nread = 0;
  struct sigaction act;

  sigemptyset(&act.sa_mask);
  act.sa_handler = handler;
  act.sa_flags = 0; //Do not set the SA_RESTART flag for the SIGALRM signal, and use the default processing method of sigaction //act.sa_flag |= SA_INTERRUPT; //SA_INTERRUPT is the default processing method of sigaction, that is, it does not automatically restart the interrupted system call. //In fact, no matter what the value of act.sa_flags is, as long as SA_RESTART is not set, sigaction is processed according to SA_INTERRUPT sigaction(SIGALRM, &act, NULL);
  alarm(2);

  printf("read start\n");
  nread = read(STDIN_FILENO, buf, sizeof(buf));
  printf("read return\n");

  if ((nread < 0) && (errno == EINTR))
  {
    printf("read return failed, errno is EINTR\n");
  }

  return 0;
}

Use sigaction + specify the SA_RESTART flag

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

void handler(int s)
{
  printf("read is interrupted by signal handler\n");
  return;
}

int main()
{
  char buf[10];
  int nread = 0;
  struct sigaction act;

  sigemptyset(&act.sa_mask);
  act.sa_handler = handler;
  act.sa_flags = 0;
  act.sa_flags |= SA_RESTART; //Set the SA_RESTART flag for the SIGALRM signal sigaction(SIGALRM, &act, NULL);
  alarm(2);

  printf("read start\n");
  nread = read(STDIN_FILENO, buf, sizeof(buf));
  printf("read return\n");

  if ((nread < 0) && (errno == EINTR))
  {
    printf("read return failed, errno is EINTR\n");
  }

  return 0;
}

Due to the differences in how interrupted system calls are handled, the issues related to interrupted system calls for applications are:

The application cannot always know how the signal handler was registered and whether the SA_RESTART flag is set.
Portable code must explicitly handle error returns from key functions. When a function fails and errno equals EINTR, you can handle it accordingly, such as restarting the function.

int nread = read(fd, buf, 1024);

if (nread < 0)
{
  if (errno == EINTR)
  {
    //The read is interrupted, which should not be considered a failure. You can handle it according to actual needs, such as rewriting the call to read, or ignoring it.}
  else
  {
    //read the real read error}
}

Summarize

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