O_ASYNC flag : sample program not working

srimalik · 2008-03-21 13:13:39

Hi,

I was trying a program with async IO.
according to the man page(open 2M) when a IO is done on a file descriptor which was created with flags O_ASYNC, a signal ISGIO should be delivered to the process. But for My program its not happening.

Could somebdy please help? Is there somthing wrong with this program. ?

#define _GNU_SOURCE
#include<stdio.h>
#include<sys/fcntl.h>
#include<signal.h>

void sig_handler(int signum)
{

  printf("Signal received\n");
}

int main()
{
  int fd, rv, flags;
  char buff[100];
  signal(SIGIO,sig_handler);
  fd = open("/dev/pts/4",O_NONBLOCK|O_ASYNC|O_RDWR);
  if(fd < 0)
   {
    printf("open failed\n");
    perror("open");
  }

  rv = fcntl(fd, F_SETOWN, getpid());
  if(rv < 0 )
   {
    perror("F_SETOWN");
  }
  rv = fcntl(fd, F_GETOWN );
  if(rv < 0 )
   {
    perror("F_SETOWN");
  }

  printf("getown retuened %d\n",rv);

  flags = fcntl(fd, F_GETFL) ;
  rv = fcntl(fd,F_SETSIG,SIGIO);
  rv = fcntl(fd, F_SETFL, flags | O_ASYNC|O_RDWR | O_NONBLOCK);

  if(rv < 0 )
   {
    perror("F_SETFL");
  }
   //pause();
 while(1)
 {
  sleep(3);
  read(fd,buff,4);
  printf("read \n");
 }


  return(0);
}

strace output

[root@localhost smalik]# strace ./a.out 
execve("./a.out", ["./a.out"], [/* 27 vars */]) = 0
brk(0)                                  = 0x9f1c000
access("/etc/ld.so.preload", R_OK)      = -1 ENOENT (No such file or directory)
open("/etc/ld.so.cache", O_RDONLY)      = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=102370, ...}) = 0
mmap2(NULL, 102370, PROT_READ, MAP_PRIVATE, 3, 0) = 0xb7f16000
close(3)                                = 0
open("/lib/libc.so.6", O_RDONLY)        = 3
read(3, "\177ELF\1\1\1\0\0\0\0\0\0\0\0\0\3\0\3\0\1\0\0\0\360D\233\0004\0\0\0"..., 512) = 512
fstat64(3, {st_mode=S_IFREG|0755, st_size=1692524, ...}) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb7f15000
mmap2(0x99e000, 1410608, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x99e000
mmap2(0xaf1000, 12288, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x153) = 0xaf1000
mmap2(0xaf4000, 9776, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0xaf4000
close(3)                                = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb7f14000
set_thread_area({entry_number:-1 -> 6, base_addr:0xb7f146c0, limit:1048575, seg_32bit:1, contents:0, read_exec_only:0, limit_in_pages:1, seg_not_present:0, useable:1}) = 0
mprotect(0xaf1000, 8192, PROT_READ)     = 0
mprotect(0x99a000, 4096, PROT_READ)     = 0
munmap(0xb7f16000, 102370)              = 0
rt_sigaction(SIGIO, {0x8048544, [IO], SA_RESTART}, {SIG_DFL}, 8) = 0
open("/dev/pts/4", O_RDWR|O_NONBLOCK|O_ASYNC) = 3
getpid()                                = 6291
fcntl64(3, F_SETOWN, 6291)              = 0
fcntl64(3, F_GETOWN)                    = 6291
fstat64(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 2), ...}) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb7f2e000
write(1, "getown retuened 6291\n", 21getown retuened 6291
)  = 21
fcntl64(3, F_GETFL)                     = 0x2802 (flags O_RDWR|O_NONBLOCK|O_ASYNC)
fcntl64(3, F_SETSIG, 0x1d)              = 0
fcntl64(3, F_SETFL, O_RDWR|O_NONBLOCK|O_ASYNC) = 0
rt_sigprocmask(SIG_BLOCK, [CHLD], [], 8) = 0
rt_sigaction(SIGCHLD, NULL, {SIG_DFL}, 8) = 0
rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0
nanosleep({3, 0}, {3, 0})               = 0
read(3, 0xbfe8e604, 4)                  = -1 EAGAIN (Resource temporarily unavailable)
write(1, "read \n", 6read 
)                  = 6
rt_sigprocmask(SIG_BLOCK, [CHLD], [], 8) = 0
rt_sigaction(SIGCHLD, NULL, {SIG_DFL}, 8) = 0
rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0
nanosleep({3, 0}, {3, 0})               = 0
read(3, 0xbfe8e604, 4)                  = -1 EAGAIN (Resource temporarily unavailable)
write(1, "read \n", 6read 
)                  = 6
rt_sigprocmask(SIG_BLOCK, [CHLD], [], 8) = 0
rt_sigaction(SIGCHLD, NULL, {SIG_DFL}, 8) = 0
rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0
nanosleep({3, 0}, {3, 0})               = 0
read(3, 0xbfe8e604, 4)                  = -1 EAGAIN (Resource temporarily unavailable)
write(1, "read \n", 6read 
)                  = 6
rt_sigprocmask(SIG_BLOCK, [CHLD], [], 8) = 0
rt_sigaction(SIGCHLD, NULL, {SIG_DFL}, 8) = 0
rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0
nanosleep({3, 0}, {3, 0})               = 0
read(3, 0xbfe8e604, 4)                  = -1 EAGAIN (Resource temporarily unavailable)
write(1, "read \n", 6read 
)                  = 6
rt_sigprocmask(SIG_BLOCK, [CHLD], [], 8) = 0
rt_sigaction(SIGCHLD, NULL, {SIG_DFL}, 8) = 0
rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0
nanosleep({3, 0}, {3, 0})               = 0
read(3, 0xbfe8e604, 4)                  = -1 EAGAIN (Resource temporarily unavailable)
write(1, "read \n", 6read 
)                  = 6
rt_sigprocmask(SIG_BLOCK, [CHLD], [], 8) = 0
rt_sigaction(SIGCHLD, NULL, {SIG_DFL}, 8) = 0
rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0
nanosleep({3, 0},  <unfinished ...>
[root@localhost smalik]#

srimalik · 2008-03-21 17:31:43

Similar program with sockets worked? but the man page clearly says that it should work with sockets, FIFOs, terminals and pseudu terminals.

T-Dawg · 2008-03-22 01:49:37

did you see this at the end of the man page?

BUGS
Currently, it is not possible to enable signal-driven I/O by specifying
O_ASYNC when calling open(); use fcntl(2) to enable this flag.

so try leaving out O_ASYNC in your open() command but add and extra line below it for fcntl:

fcntl(fd, F_SETFL, O_ASYNC);

srimalik · 2008-03-22 06:03:34

I tried setting it using fcntl but my program disn't work. BTW follwoing program worked

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/types.h>
#include <signal.h>

int h;

void async(int s)
{
        char c;
        int x = 0;
        printf("signal\n"); fflush(stdout);
        /*while (read(h, &c, 1) == 1) x++; */
         read(h, &c, 1);
        printf("read: %c\n", c); /* fflush(stdout); */
        if (fcntl(h, F_SETFL, O_NONBLOCK) < 0) perror("fcntl"), exit(1);
        if (fcntl(h, F_SETFL, O_ASYNC | O_NONBLOCK) < 0) perror("fcntl"), exit(1);
        sleep(1);
}

int main(void)
{
        h = open("/dev/tty", O_RDONLY | O_NONBLOCK);
        if (h < 0) perror("open"), exit(1);
        signal(SIGIO, async);
        if (fcntl(h, F_SETFL, O_ASYNC | O_NONBLOCK) < 0) perror("fcntl"), exit(1);
        while (1) sleep(1);
}

Arch Linux

#1 2008-03-21 13:13:39

O_ASYNC flag : sample program not working

#2 2008-03-21 17:31:43

Re: O_ASYNC flag : sample program not working

#3 2008-03-22 01:49:37

Re: O_ASYNC flag : sample program not working

#4 2008-03-22 06:03:34

Re: O_ASYNC flag : sample program not working

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