my program](https://i.stack.imgur.com/nvyaw.png)
After running my program waiting till there will be a symbol to read, but I want to end the program when the read function cannot read anything
Id tried to set the flag inside the open function and it worked, but my exercise is to set the flag by the fcntl func
First, you should read the current flags and update it like:
int flags = fcntl (a, F_GETFL, 0);
if (flags == -1) {
// handle error
}
flags |= O_NONBLOCK;
fcntl (a, F_SETFL, flags);
Related
I have this peice of code that should read from a pseudo terminal and print to another pseudo terminal (stdin):
// Declare & define a file descriptor.
int fd;
int status;
char string[] = "XXXX";
char buffer[8];
// Establish connection between a file and the file descriptor.
fd = open("/dev/pts/4", O_RDWR);
// Check if connection was established.
if(fd == -1){
printf("Assigning failed: %s\n", strerror(errno));
}
else{
fputs("Assigning successful!\n", stdout);
// Set some flags for the connection.
fcntl(fd, F_SETFL, 0);
}
// Read from the connection.
status = read(fd, buffer, 8);
if (status == -1){
printf("Read failed: %s\n", strerror(errno));
}
else{
printf("I read this: %s\n", buffer);
}
return 0;
I don't know exactly what fcntl(fd, F_SETFL, 0); does, but untill I used it I wasn't able to read from the /dev/pts/4. After I used it for the first time I could read normaly, even if I commented out this line of code.
I tried to explain it to myself reading POSIX...
F_SETFL flag is defined in the POSIX as:
Set the file status flags, defined in <fcntl.h>, for the file
description associated with fildes from the corresponding bits in the
third argument, arg, taken as type int. Bits corresponding to the file
access mode and the file creation flags, as defined in <fcntl.h>, that
are set in arg shall be ignored. If any bits in arg other than those
mentioned here are changed by the application, the result is
unspecified. If fildes does not support non-blocking operations, it is
unspecified whether the O_NONBLOCK flag will be ignored.
So it sets file status flags asociated with the 1st argument (file descriptor) of fnctl() to 0? I found an explanation about the status flags in the POSIX:
The <fcntl.h> header shall define the following symbolic constants for
use as file status flags for open(), openat(), and fcntl(). The values
shall be suitable for use in #if preprocessing directives.
O_APPEND Set append mode. O_DSYNC Write according
to synchronized I/O data integrity completion. O_NONBLOCK
Non-blocking mode. O_RSYNC Synchronized read I/O
operations. O_SYNC Write according to synchronized I/O
file integrity completion.
So does fcntl(fd, F_SETFL, 0); set all those to 0? Are all of those flags a single bit? How do I only set one of them to 1 or 0? And how do I know which bit is assigned to which flag?
It's a bitmask. Each flag is a bit (or several bits) in the integer you pass to fcntl. Each symbolic constant expands to a number in which only the corresponding bit(s) is set, and so you can set several flags by ORing them together. For instance, if bit 2 is the append flag, then O_APPEND would be 0b100 == 4; if bit 3 is the nonblock flag, then O_NONBLOCK would be 0b1000 == 8, and so on. But by using the symbolic constants, you don't need to know which bits they actually are, which is good since they may vary between OSes.
In fcntl(fd, F_SETFL, 0), the number 0 has no bits set, so all the flags are cleared. To set append and clear the rest, you could do fcntl(fd, F_SETFL, O_APPEND);. To set both append and nonblock, OR their constants together: fcntl(fd, F_SETFL, O_APPEND | O_NONBLOCK);
With similar bit operations, you can operate on the flags returned by fl = fcntl(fd, F_GETFL):
int fl = fcntl(fd, F_GETFL);
if (fl & O_APPEND) { ... } // test if in append mode
fcntl(fd, F_SETFL, fl | O_NONBLOCK); // turn on nonblock and keep everything else the same
fcntl(fd, F_SETFL, fl & ~O_DSYNC); // turn off dsync and keep everything else the same
There's some more examples here.
I'm using fcntl() on file descriptor with following call :
Retval = select(
MaxSocketId + 1,
&ReadSocketSet,
(fd_set *)NULL,
(fd_set *)NULL,
(struct timeval *)NULL
);
if (Retval <= 0) {
for (lIndexFD = 3; lIndexFD < (MaxSocketId + 1); lIndexFD++) {
if ((lFlag = fcntl(lIndexFD, F_GETFD)) < 0) {
if (errno == 9) {
FD_CLR(lIndexFD, &ActiveSocketSet);
}
}
else
printf(" \n In fcntl Else cond %d ", lFlag);
}
continue;
}
But my process is going in infinite loop in else condition for fcntl() . It appears that fcntl() is returning 0.
I would like to know in which condition it returns 0 and what to do to handle this situation.
UPDATE:
if (Retval <= 0) may change to if (Retval < 0).
When Retval is zero, select is running ok.
When Retval is -1 and errno is EBADF, then use fcntl check fd is valid.
You watched fcntl it's always return 0, that's because:
Have not set fd's FD_CLOEXEC flags
select is not fail and fcntl not fail too, because all fd is valid.
fcntl have many kinds of cmd. When using F_GETFD, it means retrive file descriptor flags.
Check fcntl's manual, there is only one flag(FD_CLOEXEC) in this type. So if not set this flag for the fd, then F_GETFD will return value 0.
File descriptor flags
The following commands manipulate the flags associated with a file descriptor. Currently, only one such flag is defined: FD_CLOEXEC, the close-on-exec flag. If the FD_CLOEXEC bit is 0, the file
descriptor will remain open across an execve(2), otherwise it will be closed.
F_GETFD (void)
Read the file descriptor flags; arg is ignored.
F_SETFD (int)
Set the file descriptor flags to the value specified by arg.
When will it not return 0?
When open one file, the flag FD_CLOEXEC is default disabled. You can enable it like this.
fd = open(filepath, O_RDONLY | O_CLOEXEC)
Call fcntl(fd, F_SETFD, FD_CLOEXEC) to enable flag FD_CLOEXEC.
I am starting serial port programming in Linux. After reading several examples on the web, I don't understand exact effect of fcntl(fd, F_SETFL, 0)? It is clearing bits, but what flags does it affect? What does it set and or clear?
Take one by one
1) Function call used
fcntl() - It perform the operation on file descriptor passed in argument.
2) 2nd argument in call
F_SETFL (int)
Set the file status flags to the value specified by arg. File access
mode (O_RDONLY, O_WRONLY, O_RDWR) and file creation flags (i.e.,
O_CREAT, O_EXCL, O_NOCTTY, O_TRUNC) in arg are ignored. On Linux this
command can change only the O_APPEND, O_ASYNC, O_DIRECT, O_NOATIME,
and O_NONBLOCK flags.
3) 3rd argument in call
It is 0 means, It set file status flag to zero.
As Jean-Baptiste Yunès said in comment.
file access mode and file creation flags are ignored. This command
reset every other flags: no append, no async, no direct, no atime, and
no nonblocking
So finally
fcntl(fd, F_SETFL, 0)
This call will set opened file desciptor's file status flag to value 0.
But idealy this way we should not change file status flag.
Best way is to first get the current file status flag using F_GETFL and then just change required bit in that.
See example:
If you want to modify the file status flags, you should get the current flags with F_GETFL and modify the value. Don’t assume that the flags listed here are the only ones that are implemented; your program may be run years from now and more flags may exist then. For example, here is a function to set or clear the flag O_NONBLOCK without altering any other flags:
/* Set the O_NONBLOCK flag of desc if value is nonzero,
or clear the flag if value is 0.
Return 0 on success, or -1 on error with errno set. */
int
set_nonblock_flag (int desc, int value)
{
int oldflags = fcntl (desc, F_GETFL, 0);
/* If reading the flags failed, return error indication now. */
if (oldflags == -1)
return -1;
/* Set just the flag we want to set. */
if (value != 0)
oldflags |= O_NONBLOCK;
else
oldflags &= ~O_NONBLOCK;
/* Store modified flag word in the descriptor. */
return fcntl (desc, F_SETFL, oldflags);
}
per the man page for fcntl()
F_SETFL (int)
Set the file status flags to the value specified by arg. File
access mode (O_RDONLY, O_WRONLY, O_RDWR) and file creation
flags (i.e., O_CREAT, O_EXCL, O_NOCTTY, O_TRUNC) in arg are
ignored. On Linux this command can change only the O_APPEND,
O_ASYNC, O_DIRECT, O_NOATIME, and O_NONBLOCK flags. It is not
possible to change the O_DSYNC and O_SYNC flags; see BUGS,
below.
This is from the man page.
I have a sample program:
int main()
{
const char* fn = "/tmp/tmpfifo";
int i = mkfifo(fn, 0666);
int fd = open(fn, O_RDONLY | O_NONBLOCK);
int flags = fcntl(fd, F_GETFL);
flags &= ~O_NONBLOCK;
fcntl(fd, F_SETFL, flags);
char buf[1024];
int rd= read(fd, buf, 100);
cout << rd << endl;
remove(fn);
return 0;
}
It seems that after removing the non-blocking flag from the file descriptor, the read call should block until something is written into the FIFO, but my program always runs without blocking and rd=0 result. Can you please explain this behaviour? Thanks!
The behavior you are seeing is expected. You've done the following:
Opened the read end of the FIFO using O_NONBLOCK, so that a writer need not be present on the FIFO. This guarantees that the open() will immediately succeed.
Disabled O_NONBLOCK before subsequent reads. You've now taken yourself back to a position that is equivalent to the standard (blocking) case where a FIFO had a reader and writer, but the writer closed the FIFO. At that point, the reader should see end-of-file, which is what you are seeing.
It's strange! I tried a code which opens the file without O_NONBLOCK and then procedes in 3 stages. The 3rd stage doesn'act correctly althoug the O_NONBLOCK flag results reset!
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
int main()
{
char buf[1024];
int rd;
const char* fn = "prova.txt";
int i = mkfifo(fn, 0666);
int fd = open(fn, O_RDONLY); // | O_NONBLOCK);
int flags = fcntl(fd, F_GETFL);
//flags &= ~O_NONBLOCK;
printf("1) Waits!\t\tflags=0%08o\n",flags);
rd= read(fd, buf, 100);
printf("%d %o\n",rd,flags);
flags |= O_NONBLOCK;
printf("2) Doesn't wait!\tflags=0%08o\n",flags);
fcntl(fd, F_SETFL, flags);
rd= read(fd, buf, 100);
printf("%d %o\n",rd,flags);
//This doen't act the flag ????
flags &= ~O_NONBLOCK;
fcntl(fd, F_SETFL, flags);
flags=fcntl(fd, F_GETFL);
printf("3) Waits!\t\tflags=0%08o\n",flags);
rd= read(fd, buf, 100);
printf("%d %o\n",rd,flags);
puts("End!");
return 0;
}
Here is the command sequence and the output:
sergio#zarathustra:~$ ./a.out &
[2] 6555
sergio#zarathustra:~$ echo xxx >> prova.txt
1) Waits! flags=000100000
4 100000
2) Doesn't wait! flags=000104000
0 104000
3) Waits! flags=000100000
0 100000
End!
sergio#zarathustra:~$
I looked at your code and at first glance it seems like it should work. There are no errors returned, you don't seem to be breaking any rules, but it's just not blocking.
So I went ahead and traced the read call to see what it was doing:
And it goes all the way to the pipe_read function without any attempt to block. Once it's there it realizes there's nobody on the other side of the pipe and returns EOF.
So this is apparently by design, but with a pipe only the open call will try to block if there's no writer, once open returns it's just assumed that there must be a writer at the other end of that pipe or that you're nonblocking and ready to handle that. And it sort of makes sense. If you're trying to read from a pipe but the writer is gone (or was never there in the first place), you don't want to keep waiting there forever.
If you want to wait until a writer opens the pipe, don't use O_NONBLOCK in the open call. If you do use O_NONBLOCK in open, then there might not be anyone at the other end of the pipe and the read calls may just return EOF without blocking.
So in short make sure there's someone at the other end of the pipe when you're reading from it.
Given an arbitrary file descriptor, can I make it blocking if it is non-blocking? If so, how?
Its been a while since I played with C, but you could use the fcntl() function to change the flags of a file descriptor:
#include <unistd.h>
#include <fcntl.h>
// Save the existing flags
saved_flags = fcntl(fd, F_GETFL);
// Set the new flags with O_NONBLOCK masked out
fcntl(fd, F_SETFL, saved_flags & ~O_NONBLOCK);
I would expect simply non-setting the O_NONBLOCK flag should revert the file descriptor to the default mode, which is blocking:
/* Makes the given file descriptor non-blocking.
* Returns 1 on success, 0 on failure.
*/
int make_blocking(int fd)
{
int flags;
flags = fcntl(fd, F_GETFL, 0);
if(flags == -1) /* Failed? */
return 0;
/* Clear the blocking flag. */
flags &= ~O_NONBLOCK;
return fcntl(fd, F_SETFL, flags) != -1;
}