How to call the interrupted system in Linux

#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;
} 

#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;
} 

#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;
} 

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}
}