Interrupting blocked read

Question

My program goes through a loop like this:

...
while(1){
  read(sockfd,buf,sizeof(buf));
  ...
}

The read function blocks when it is waiting for

Answer 1

When your process receives a signal, read() will return and the value of errno will be set to EINTR.

Answer 2

From read(2):

   EINTR  The call was interrupted by a signal before any data
          was read; see signal(7).

If you amend your code to look more like:

cont = 1;
while (1 && cont) {
    ret = read(sockfd, buf, sizeof(buf));
    if (ret < 0 && errno == EINTR)
        cont = arbitrary_function();
}

This lets arbitrary_function() decide if the read(2) should be re-tried or not.

Update

You need to handle the signal in order to get the EINTR behavior from read(2):

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

int interrupted;

void handle_int(num) {
  interrupted = 1;
}

int main(void){
  char buf[9001];
  struct sigaction int_handler = {.sa_handler=handle_int};
  sigaction(SIGINT,&int_handler,0);
  while(!interrupted){
    printf("interrupted: %d\n", interrupted);
    if(read(0,buf,sizeof(buf))<0){
      if(errno==EINTR){
        puts("eintr");
      }else{
        printf("%d\n",errno);
      }
      puts(".");
    }
  }
  puts("end");
  return 0;
}

Gives output:

$ ./foo
interrupted: 0
hello
interrupted: 0
^Ceintr
.
end