How to see if a pipe is empty

How to see if a pipe is empty

bash check if pipe is empty
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Assuming a pipe,

int pipe_fd[2];
pipe(pipe_fd);

We fork, and expect that one process will write into the pipe at an arbitrary time. In one of the processes, we want to be able to check the contents of the pipe without blocking.

i.e. While a typical read will block if nothing is present and the write end remains open. I want to go do other stuff and potentially even read a bit at a time, do some stuff, and then check back to see if there's more, a la:

close(pipe_fd[1]);

while(1){
    if(/**Check pipe contents**/){
        int present_chars = 0;    

        while( read(pipe_fd[0],&buffer[present_chars],1) != 0)
            ++present_chars;

        //do something

    }
    else
        //do something else
}

Your logic is wrong in that read will not return 0 when it runs out of characters; instead, it will block until it receives more, unless you put the file in non-blocking mode, but then it will return -1 and set errno to EWOULDBLOCK or EAGAIN rather than returning 0. The only time read can ever return 0 is when the size argument was 0 or end-of-file has been reached. And, for pipes, end-of-file means the writing end of the pipe has been closed; end-of-file status does not occur just because there's not any input available yet.

With that said, the simplest way to check is:

if (poll(&(struct pollfd){ .fd = fd, .events = POLLIN }, 1, 0)==1) {
    /* data available */
}

but unless you're using nonblocking mode, you'll need to make this check before every single read operation. Passing a larger buffer to read rather than doing it a byte-at-a-time would eliminate most of the cost of checking.

How to check if a pipe is empty and run a command on the data if it , There's no way to peek at the content of a pipe using commonly available shell utilities, nor is there a way to read a character from the pipe then put it back. How to check if a pipe is empty and run a command on the data 33 14


You can check if there is data to be read with the read() function. From read(3):

When attempting to read from an empty pipe or FIFO:

* If some process has the pipe open for writing and
O_NONBLOCK is set, read() shall return -1 and set
errno to [EAGAIN].

* If some process has the pipe open for writing and
O_NONBLOCK  is  clear,  read() shall block the calling
thread until some data is written or the pipe is
closed by all processes that had the pipe open for
writing.

The read() function shall fail if:

EAGAIN or EWOULDBLOCK

    The file descriptor is for a socket, is marked
    O_NONBLOCK, and no data is waiting to be received.

So if you set O_NONBLOCK, you will be able to tell if something is to be read on the pipe, by simply calling read().

As a reminder, from open(3):

SYNOPSIS
    int open(const char *path, int oflag, ...  );

DESCRIPTION
    Values for oflag are constructed by a
    bitwise-inclusive OR of flags from the following
    list, defined in <fcntl.h>. Applications shall
    specify exactly one  of  the first three values
    (file access modes) below in the value of oflag:

    O_NONBLOCK [...]

I hope it helps.

checking if the read end of the pipe is empty or not · Issue #1647 , In order to prevent the program from stalling, is there a way to check if the read end of the pipe is empty or not before calling read() ? Update: I  In the man page for pipe(7) man 7 pipe, it states that If a process attempts to read from an empty pipe, then read(2) will block until data is available. What exactly does block mean in this contex


R..'s answer is good however poll returns the number of file descriptor structs that have flags set in "revents". This will be 1 if you can read from fd but will also be 1 if any of the error flags are set. This means R..'s answer will say the pipe is readable if it ever enters an error state. A more robust check could be something like this:

bool canReadFromPipe(){
    //file descriptor struct to check if POLLIN bit will be set
    //fd is the file descriptor of the pipe
    struct pollfd fds{ .fd = fd, .events = POLLIN };
    //poll with no wait time
    int res = poll(&fds, 1, 0);

    //if res < 0 then an error occurred with poll
    //POLLERR is set for some other errors
    //POLLNVAL is set if the pipe is closed
    if(res < 0||fds.revents&(POLLERR|POLLNVAL))
    {
        //an error occurred, check errno
    }
    return fds.revents&POLLIN;
}

BASH - Check if unnamed pipe is empty, Use read -t 0 -N 0 to detect if data is available on stdin. Use test -t 0 or tty to try to detect if a pipe is connected to stdin. Problems that develop in the waste pipe that connects your house to the municipal sewer or the septic tank can stop your plumbing drains cold. If the waste pipe breaks, you'll probably need professional help to repair it, and that can be expensive. It's good, therefore, to be able to distinguish between a breakage and a blockage.


Non-blocking I/O with pipes in C, After this function runs successfully, a call to read/write returns -1 if pipe is empty/​full and sets errno to EAGAIN. Example: Child pipe error check. if (pipe(p)  1 point · 12 months ago. Yes, assuming you're piping the same liquid every time add a liquid pipe Element sensor. Set it to that liquid, and run the output of that through a NOT gate. Voila, you can now detect when the water pipe is empty.


pipe(7) - Linux manual page, If a process attempts to read from an empty pipe, then read(2) will block until data is available. If a process attempts to write to a full pipe (see below), then  A complete path of liquid from transmitter to receiver is necessary to get then to detect liquid as the signal will bounce off air and not get from one side to the other. Assuming a horizontal pipe then mounting this on top to bottom may give false negatives if the pipe is not completely full or pumps an air bubble down so a side to side.


Is there any way to check if a liquid pipe is empty or not with , If you know what type of liquid will be in the pipes then a pipe element sensor can detect it - otherwise you'd have to be creative and try to automate it in less  Easy clean the pipe and try and tin it if it will tin its empty (or GAS) or (overflow)if the solder wont run its got water in it.