O_ASYNC停止生成SIGIO(O_ASYNC stops generating SIGIO)

 这有点长...对于初学者我在Linux 2.6.33，gcc 4.4.4。 
 
 我写了一个小程序，它创建一个命名管道并读取它，直到它看到某个字符串，然后它取消了FIFO，并重新执行自己。 
 
#include<unistd.h>
#include<fcntl.h>
#include<signal.h>
#include<sys/types.h>
#include<sys/stat.h>
int fifo;
#define put(x) write(1, x, (sizeof x)-1)
void reader(int a)
{
      char buf[26];
      int n;
      while((n=read(fifo, buf, 25))>0){
            buf[25] = '\0';
            if(!strncmp(buf, "moo", 3)){
                    put("exec()-ing\n");
                    close(fifo);
                    unlink("lefifo");
                    execl("/home/dave/a.out", "a.out", 0);
            }
            write(1, buf, n);
      }
}

main()
{
      signal(SIGIO, reader);
      mknod("lefifo", 0600|S_IFIFO,0);
      fifo = open("lefifo", O_RDONLY|O_NONBLOCK );
      fcntl(fifo, F_SETOWN, getpid());
      fcntl(fifo, F_SETFL, O_ASYNC);

      for(;;)
            pause();
}
 
 编译后，在后台运行时，我可以lefifo并按预期工作，直到我输入以“moo”开头的字符串。 以下示例会话： 
 
$ gcc fifo.c 
$ ./a.out&
$ echo klar > lefifo
klar
$ echo moo > lefifo
exec()-ing
$ echo klar2 > lefifo
$ echo where did you go > lefifo
$ echo moo > lefifo
$ pkill a.out
 
 生成此痕迹（一些脂肪修剪）： 
 
execve("./a.out", ["./a.out"], [/* 36 vars */]) = 0
mknod("lefifo", S_IFIFO|0600)           = 0
open("lefifo", O_RDONLY|O_NONBLOCK)     = 3
getpid()                                = 3945
fcntl(3, F_SETOWN, 3945)                = 0
fcntl(3, F_SETFL, O_RDONLY|O_ASYNC)     = 0
pause()                                 = ? ERESTARTNOHAND (To be restarted)
--- SIGIO (I/O possible) @ 0 (0) ---
read(3, "klar\n"..., 25)                = 5
write(1, "klar\n"..., 5)                = 5
read(3, ""..., 25)                      = 0
sigreturn()                             = ? (mask now [])
pause()                                 = ? ERESTARTNOHAND (To be restarted)
--- SIGIO (I/O possible) @ 0 (0) ---
read(3, "moo\n"..., 25)                 = 4
write(1, "exec()-ing\n"..., 13)         = 13
close(3)                                = 0
unlink("lefifo")                        = 0
execve("/home/dave/a.out", ["a.out"], [/* 36 vars */]) = 0
mknod("lefifo", S_IFIFO|0600)           = 0
open("lefifo", O_RDONLY|O_NONBLOCK)     = 3
getpid()                                = 3945
fcntl(3, F_SETOWN, 3945)                = 0
fcntl(3, F_SETFL, O_RDONLY|O_ASYNC)     = 0
pause()                                 = ? ERESTARTNOHAND (To be restarted)
--- SIGTERM (Terminated) @ 0 (0) ---
 
 正如您所看到的，第一次，创建FIFO没有问题，SIGIO生成就好了; 但是在exec() ，新的FIFO将不会产生任何信号。 旧的显示成功关闭，似乎成功删除。 
 
 我很难过为什么它会这样做。 有任何想法吗？ 

This is a little long... For starters I'm on Linux 2.6.33, gcc 4.4.4.
 
I've written a small program, which creates a named pipe and reads it, until it sees a certain string, whereupon it gets rid of the FIFO, and re-executes itself.
 
#include<unistd.h>
#include<fcntl.h>
#include<signal.h>
#include<sys/types.h>
#include<sys/stat.h>
int fifo;
#define put(x) write(1, x, (sizeof x)-1)
void reader(int a)
{
      char buf[26];
      int n;
      while((n=read(fifo, buf, 25))>0){
            buf[25] = '\0';
            if(!strncmp(buf, "moo", 3)){
                    put("exec()-ing\n");
                    close(fifo);
                    unlink("lefifo");
                    execl("/home/dave/a.out", "a.out", 0);
            }
            write(1, buf, n);
      }
}

main()
{
      signal(SIGIO, reader);
      mknod("lefifo", 0600|S_IFIFO,0);
      fifo = open("lefifo", O_RDONLY|O_NONBLOCK );
      fcntl(fifo, F_SETOWN, getpid());
      fcntl(fifo, F_SETFL, O_ASYNC);

      for(;;)
            pause();
}
 
When compiled, and run in the background, I can echo to lefifo and it works as expected, until I enter a string beginning with "moo". The following example session:
 
$ gcc fifo.c 
$ ./a.out&
$ echo klar > lefifo
klar
$ echo moo > lefifo
exec()-ing
$ echo klar2 > lefifo
$ echo where did you go > lefifo
$ echo moo > lefifo
$ pkill a.out
 
Generates this trace (some fat trimmed):
 
execve("./a.out", ["./a.out"], [/* 36 vars */]) = 0
mknod("lefifo", S_IFIFO|0600)           = 0
open("lefifo", O_RDONLY|O_NONBLOCK)     = 3
getpid()                                = 3945
fcntl(3, F_SETOWN, 3945)                = 0
fcntl(3, F_SETFL, O_RDONLY|O_ASYNC)     = 0
pause()                                 = ? ERESTARTNOHAND (To be restarted)
--- SIGIO (I/O possible) @ 0 (0) ---
read(3, "klar\n"..., 25)                = 5
write(1, "klar\n"..., 5)                = 5
read(3, ""..., 25)                      = 0
sigreturn()                             = ? (mask now [])
pause()                                 = ? ERESTARTNOHAND (To be restarted)
--- SIGIO (I/O possible) @ 0 (0) ---
read(3, "moo\n"..., 25)                 = 4
write(1, "exec()-ing\n"..., 13)         = 13
close(3)                                = 0
unlink("lefifo")                        = 0
execve("/home/dave/a.out", ["a.out"], [/* 36 vars */]) = 0
mknod("lefifo", S_IFIFO|0600)           = 0
open("lefifo", O_RDONLY|O_NONBLOCK)     = 3
getpid()                                = 3945
fcntl(3, F_SETOWN, 3945)                = 0
fcntl(3, F_SETFL, O_RDONLY|O_ASYNC)     = 0
pause()                                 = ? ERESTARTNOHAND (To be restarted)
--- SIGTERM (Terminated) @ 0 (0) ---
 
As you can see, the first time around, there is no trouble making the FIFO, and SIGIO is generated just fine; but after the exec() the new FIFO won't generate any signals. The old one shows a successful close and seems to get deleted successfully.
 
I'm quite stumped as to why it might behave this way. Any ideas?

原文：https://stackoverflow.com/questions/7034175