Linux 非阻塞 fifo（按需记录）

Question

我喜欢“按需”记录程序输出。例如。输出被记录到终端，但另一个进程可以随时挂接当前输出。

经典的方法是：

myprogram 2>&1 | tee /tmp/mylog

按需

tail /tmp/mylog

但是，即使在驱动器空间不足之前不使用，这也会创建不断增长的日志文件。所以我的尝试是：

mkfifo /tmp/mylog
myprogram 2>&1 | tee /tmp/mylog

并且按需

cat /tmp/mylog

现在我可以随时读取/tmp/mylog。但是，任何输出都会阻止程序，直到读取 /tmp/mylog。我喜欢 fifo 来刷新任何未读回的传入数据。怎么做呢？

Answer 1

受您问题的启发，我编写了一个简单的程序，可以让您执行此操作：

$ myprogram 2>&1 | ftee /tmp/mylog

它的行为与 tee 类似，但将标准输入克隆到标准输出和命名管道（目前的要求），而不会阻塞。这意味着如果您想以这种方式记录，可能会丢失日志数据，但我想这在您的场景中是可以接受的。
技巧是阻止 SIGPIPE 信号并忽略写入损坏的 fifo 时的错误。当然，此示例可以通过多种方式进行优化，但到目前为止，它确实做到了我猜是工作。

/* ftee - clone stdin to stdout and to a named pipe 
(c) racic@stackoverflow
WTFPL Licence */

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

int main(int argc, char *argv[])
{
    int readfd, writefd;
    struct stat status;
    char *fifonam;
    char buffer[BUFSIZ];
    ssize_t bytes;
    
    signal(SIGPIPE, SIG_IGN);

    if(2!=argc)
    {
        printf("Usage:\n someprog 2>&1 | %s FIFO\n FIFO - path to a"
            " named pipe, required argument\n", argv[0]);
        exit(EXIT_FAILURE);
    }
    fifonam = argv[1];

    readfd = open(fifonam, O_RDONLY | O_NONBLOCK);
    if(-1==readfd)
    {
        perror("ftee: readfd: open()");
        exit(EXIT_FAILURE);
    }

    if(-1==fstat(readfd, &status))
    {
        perror("ftee: fstat");
        close(readfd);
        exit(EXIT_FAILURE);
    }

    if(!S_ISFIFO(status.st_mode))
    {
        printf("ftee: %s in not a fifo!\n", fifonam);
        close(readfd);
        exit(EXIT_FAILURE);
    }

    writefd = open(fifonam, O_WRONLY | O_NONBLOCK);
    if(-1==writefd)
    {
        perror("ftee: writefd: open()");
        close(readfd);
        exit(EXIT_FAILURE);
    }

    close(readfd);

    while(1)
    {
        bytes = read(STDIN_FILENO, buffer, sizeof(buffer));
        if (bytes < 0 && errno == EINTR)
            continue;
        if (bytes <= 0)
            break;

        bytes = write(STDOUT_FILENO, buffer, bytes);
        if(-1==bytes)
            perror("ftee: writing to stdout");
        bytes = write(writefd, buffer, bytes);
        if(-1==bytes);//Ignoring the errors
    }
    close(writefd); 
    return(0);
}

您可以使用以下标准命令对其进行编译：

$ gcc ftee.c -o ftee

您可以通过运行以下命令快速验证它：

$ ping www.google.com | ftee /tmp/mylog

$ cat /tmp/mylog

另请注意 - 这不是多路复用器。一次只能让一个进程执行 $ cat /tmp/mylog。

Answer 2

这是一个（非常）古老的线程，但我最近遇到了类似的问题。事实上，我需要的是将标准输入克隆到标准输出，并复制到非阻塞的管道。第一个答案中提出的 ftee 确实有帮助，但（对于我的用例）太不稳定了。这意味着我丢失了如果我及时处理的话本可以处理的数据。

我面临的场景是，我有一个进程（some_process），它聚合一些数据并每三秒将其结果写入标准输出。 （简化的）设置看起来像这样（在实际设置中我使用命名管道）：

some_process | ftee >(onlineAnalysis.pl > results) | gzip > raw_data.gz

现在， raw_data.gz 必须被压缩并且必须是完整的。 ftee 很好地完成了这项工作。但是我在中间使用的管道太慢，无法获取冲出的数据 - 但它足够快，可以处理所有内容（如果它可以到达它），这是用普通三通进行测试的。但是，如果未命名管道发生任何情况，普通的 T 形结构会阻塞，并且由于我希望能够按需挂接，所以 T 形结构不是一个选项。回到主题：当我在中间放置一个缓冲区时，情况会变得更好，结果是：

some_process | ftee >(mbuffer -m 32M| onlineAnalysis.pl > results) | gzip > raw_data.gz

但是仍然丢失了我可以处理的数据。因此，我继续将之前提出的 ftee 扩展为缓冲版本 (bftee)。它仍然具有所有相同的属性，但使用（低效？）内部缓冲区以防写入失败。如果缓冲区已满，它仍然会丢失数据，但对于我的情况来说它工作得很好。一如既往，还有很大的改进空间，但当我从这里复制代码时，我想将其分享给可能使用它的人。

/* bftee - clone stdin to stdout and to a buffered, non-blocking pipe 
    (c) racic@stackoverflow
    (c) fabraxias@stackoverflow
    WTFPL Licence */

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

    // the number of sBuffers that are being held at a maximum
    #define BUFFER_SIZE 4096
    #define BLOCK_SIZE 2048

    typedef struct {
      char data[BLOCK_SIZE];
      int bytes;
    } sBuffer;

    typedef struct {
      sBuffer *data;  //array of buffers
      int bufferSize; // number of buffer in data
      int start;      // index of the current start buffer
      int end;        // index of the current end buffer
      int active;     // number of active buffer (currently in use)
      int maxUse;     // maximum number of buffers ever used
      int drops;      // number of discarded buffer due to overflow
      int sWrites;    // number of buffer written to stdout
      int pWrites;    // number of buffers written to pipe
    } sQueue;

    void InitQueue(sQueue*, int);              // initialized the Queue
    void PushToQueue(sQueue*, sBuffer*, int);  // pushes a buffer into Queue at the end 
    sBuffer *RetrieveFromQueue(sQueue*);       // returns the first entry of the buffer and removes it or NULL is buffer is empty
    sBuffer *PeakAtQueue(sQueue*);             // returns the first entry of the buffer but does not remove it. Returns NULL on an empty buffer
    void ShrinkInQueue(sQueue *queue, int);    // shrinks the first entry of the buffer by n-bytes. Buffer is removed if it is empty
    void DelFromQueue(sQueue *queue);          // removes the first entry of the queue

    static void sigUSR1(int);                  // signal handled for SUGUSR1 - used for stats output to stderr
    static void sigINT(int);                   // signla handler for SIGKILL/SIGTERM - allows for a graceful stop ?

    sQueue queue;                              // Buffer storing the overflow
    volatile int quit;                         // for quiting the main loop

    int main(int argc, char *argv[])
    {   
        int readfd, writefd;
        struct stat status;
        char *fifonam;
        sBuffer buffer;
        ssize_t bytes;
        int bufferSize = BUFFER_SIZE;

        signal(SIGPIPE, SIG_IGN);
        signal(SIGUSR1, sigUSR1);
        signal(SIGTERM, sigINT);
        signal(SIGINT,  sigINT);

        /** Handle commandline args and open the pipe for non blocking writing **/

        if(argc < 2 || argc > 3)
        {   
            printf("Usage:\n someprog 2>&1 | %s FIFO [BufferSize]\n"
                   "FIFO - path to a named pipe, required argument\n"
                   "BufferSize - temporary Internal buffer size in case write to FIFO fails\n", argv[0]);
            exit(EXIT_FAILURE);
        }

        fifonam = argv[1];
        if (argc == 3) {
          bufferSize = atoi(argv[2]);
          if (bufferSize == 0) bufferSize = BUFFER_SIZE;
        }

        readfd = open(fifonam, O_RDONLY | O_NONBLOCK);
        if(-1==readfd)
        {   
            perror("bftee: readfd: open()");
            exit(EXIT_FAILURE);
        }

        if(-1==fstat(readfd, &status))
        {
            perror("bftee: fstat");
            close(readfd);
            exit(EXIT_FAILURE);
        }

        if(!S_ISFIFO(status.st_mode))
        {
            printf("bftee: %s in not a fifo!\n", fifonam);
            close(readfd);
            exit(EXIT_FAILURE);
        }

        writefd = open(fifonam, O_WRONLY | O_NONBLOCK);
        if(-1==writefd)
        {
            perror("bftee: writefd: open()");
            close(readfd);
            exit(EXIT_FAILURE);
        }

        close(readfd);


        InitQueue(&queue, bufferSize);
        quit = 0;

        while(!quit)
        {
            // read from STDIN
            bytes = read(STDIN_FILENO, buffer.data, sizeof(buffer.data));

            // if read failed due to interrupt, then retry, otherwise STDIN has closed and we should stop reading
            if (bytes < 0 && errno == EINTR) continue;
            if (bytes <= 0) break;

            // save the number if read bytes in the current buffer to be processed
            buffer.bytes = bytes;

            // this is a blocking write. As long as buffer is smaller than 4096 Bytes, the write is atomic to a pipe in Linux
            // thus, this cannot be interrupted. however, to be save this should handle the error cases of partial or interrupted write none the less.
            bytes = write(STDOUT_FILENO, buffer.data, buffer.bytes);
            queue.sWrites++;

            if(-1==bytes) {
                perror("ftee: writing to stdout");
                break;
            }

            sBuffer *tmpBuffer = NULL;

            // if the queue is empty (tmpBuffer gets set to NULL) the this does nothing - otherwise it tries to write
            // the buffered data to the pipe. This continues until the Buffer is empty or the write fails.
            // NOTE: bytes cannot be -1  (that would have failed just before) when the loop is entered. 
            while ((bytes != -1) && (tmpBuffer = PeakAtQueue(&queue)) != NULL) {
               // write the oldest buffer to the pipe
               bytes = write(writefd, tmpBuffer->data, tmpBuffer->bytes);

               // the  written bytes are equal to the buffer size, the write is successful - remove the buffer and continue
               if (bytes == tmpBuffer->bytes) {
                 DelFromQueue(&queue);
                 queue.pWrites++;
               } else if (bytes > 0) {
                 // on a positive bytes value there was a partial write. we shrink the current buffer
                 //  and handle this as a write failure
                 ShrinkInQueue(&queue, bytes);
                 bytes = -1;
               }
            }
            // There are several cases here:
            // 1.) The Queue is empty -> bytes is still set from the write to STDOUT. in this case, we try to write the read data directly to the pipe
            // 2.) The Queue was not empty but is now -> bytes is set from the last write (which was successful) and is bigger 0. also try to write the data
            // 3.) The Queue was not empty and still is not -> there was a write error before (even partial), and bytes is -1. Thus this line is skipped.
            if (bytes != -1) bytes = write(writefd, buffer.data, buffer.bytes);

            // again, there are several cases what can happen here
            // 1.) the write before was successful -> in this case bytes is equal to buffer.bytes and nothing happens
            // 2.) the write just before is partial or failed all together - bytes is either -1 or smaller than buffer.bytes -> add the remaining data to the queue
            // 3.) the write before did not happen as the buffer flush already had an error. In this case bytes is -1 -> add the remaining data to the queue
            if (bytes != buffer.bytes)
              PushToQueue(&queue, &buffer, bytes);
            else 
              queue.pWrites++;
        }

        // once we are done with STDIN, try to flush the buffer to the named pipe
        if (queue.active > 0) {
           //set output buffer to block - here we wait until we can write everything to the named pipe
           // --> this does not seem to work - just in case there is a busy loop that waits for buffer flush aswell. 
           int saved_flags = fcntl(writefd, F_GETFL);
           int new_flags = saved_flags & ~O_NONBLOCK;
           int res = fcntl(writefd, F_SETFL, new_flags);

           sBuffer *tmpBuffer = NULL;
           //TODO: this does not handle partial writes yet
           while ((tmpBuffer = PeakAtQueue(&queue)) != NULL) {
             int bytes = write(writefd, tmpBuffer->data, tmpBuffer->bytes);
             if (bytes != -1) DelFromQueue(&queue);
           }
        }

        close(writefd);

    }


    /** init a given Queue **/
    void InitQueue (sQueue *queue, int bufferSize) {
      queue->data = calloc(bufferSize, sizeof(sBuffer));
      queue->bufferSize = bufferSize;
      queue->start = 0;
      queue->end = 0;
      queue->active = 0;
      queue->maxUse = 0;
      queue->drops = 0;
      queue->sWrites = 0;
      queue->pWrites = 0;
    }

    /** push a buffer into the Queue**/
    void PushToQueue(sQueue *queue, sBuffer *p, int offset)
    {

        if (offset < 0) offset = 0;      // offset cannot be smaller than 0 - if that is the case, we were given an error code. Set it to 0 instead
        if (offset == p->bytes) return;  // in this case there are 0 bytes to add to the queue. Nothing to write

        // this should never happen - offset cannot be bigger than the buffer itself. Panic action
        if (offset > p->bytes) {perror("got more bytes to buffer than we read\n"); exit(EXIT_FAILURE);}

        // debug output on a partial write. TODO: remove this line
        // if (offset > 0 ) fprintf(stderr, "partial write to buffer\n");

        // copy the data from the buffer into the queue and remember its size
        memcpy(queue->data[queue->end].data, p->data + offset , p->bytes-offset);
        queue->data[queue->end].bytes = p->bytes - offset;

        // move the buffer forward
        queue->end = (queue->end + 1) % queue->bufferSize;

        // there is still space in the buffer
        if (queue->active < queue->bufferSize)
        {
            queue->active++;
            if (queue->active > queue->maxUse) queue->maxUse = queue->active;
        } else {
            // Overwriting the oldest. Move start to next-oldest
            queue->start = (queue->start + 1) % queue->bufferSize;
            queue->drops++;
        }
    }

    /** return the oldest entry in the Queue and remove it or return NULL in case the Queue is empty **/
    sBuffer *RetrieveFromQueue(sQueue *queue)
    {
        if (!queue->active) { return NULL; }

        queue->start = (queue->start + 1) % queue->bufferSize;
        queue->active--;
        return &(queue->data[queue->start]);
    }

    /** return the oldest entry in the Queue or NULL if the Queue is empty. Does not remove the entry **/
    sBuffer *PeakAtQueue(sQueue *queue)
    {
        if (!queue->active) { return NULL; }
        return &(queue->data[queue->start]);
    }

    /*** Shrinks the oldest entry i the Queue by bytes. Removes the entry if buffer of the oldest entry runs empty*/
    void ShrinkInQueue(sQueue *queue, int bytes) {

      // cannot remove negative amount of bytes - this is an error case. Ignore it
      if (bytes <= 0) return;

      // remove the entry if the offset is equal to the buffer size
      if (queue->data[queue->start].bytes == bytes) {
        DelFromQueue(queue);
        return;
      };

      // this is a partial delete
      if (queue->data[queue->start].bytes > bytes) {
        //shift the memory by the offset
        memmove(queue->data[queue->start].data, queue->data[queue->start].data + bytes, queue->data[queue->start].bytes - bytes);
        queue->data[queue->start].bytes = queue->data[queue->start].bytes - bytes;
        return;
      }

      // panic is the are to remove more than we have the buffer
      if (queue->data[queue->start].bytes < bytes) {
        perror("we wrote more than we had - this should never happen\n");
        exit(EXIT_FAILURE);
        return;
      }
    }

    /** delete the oldest entry from the queue. Do nothing if the Queue is empty **/
    void DelFromQueue(sQueue *queue)
    {
        if (queue->active > 0) {
          queue->start = (queue->start + 1) % queue->bufferSize;
          queue->active--;
        }
    }

    /** Stats output on SIGUSR1 **/
    static void sigUSR1(int signo) {
      fprintf(stderr, "Buffer use: %i (%i/%i), STDOUT: %i PIPE: %i:%i\n", queue.active, queue.maxUse, queue.bufferSize, queue.sWrites, queue.pWrites, queue.drops);
    }

    /** handle signal for terminating **/
    static void sigINT(int signo) {
      quit++;
      if (quit > 1) exit(EXIT_FAILURE);
    }

此版本还需要一个（可选）参数，该参数指定要为管道缓冲的块的数量。我的示例调用现在如下所示：

some_process | bftee >(onlineAnalysis.pl > results) 16384 | gzip > raw_data.gz

导致在丢弃发生之前缓冲 16384 个块。这会多使用大约 32 MB 的内存，但是...谁在乎呢？

当然，在真实环境中，我使用命名管道，以便我可以根据需要附加和分离。看起来像这样：

mkfifo named_pipe
some_process | bftee named_pipe 16384 | gzip > raw_data.gz &
cat named_pipe | onlineAnalysis.pl > results

此外，该进程对信号的反应如下：
SIGUSR1->将计数器打印到 STDERR
SIGTERM、SIGINT ->第一个退出主循环并将缓冲区刷新到管道，第二个立即终止程序。

也许这对将来的人有帮助......
享受

Answer 3

但是，即使在驱动器空间不足之前不使用，这也会创建一个不断增长的日志文件。

为什么不定期轮换日志呢？甚至还有一个程序可以为您执行此操作 logrotate。

还有一个用于生成日志消息并根据类型对其执行不同操作的系统。它称为syslog。

您甚至可以将两者结合起来。让您的程序生成 syslog 消息，配置 syslog 将它们放入文件中，并使用 logrotate 确保它们不会填满磁盘。

---

如果事实证明您正在为小型嵌入式系统编写并且程序的输出量很大，那么您可能会考虑多种技术。

• 远程系统日志：将系统日志消息发送到网络上的系统日志服务器。
• 使用系统日志中可用的严重性级别对消息执行不同的操作。例如，丢弃“INFO”，但记录并转发“ERR”或更大的值。例如到控制台
• 在程序中使用信号处理程序来重新读取 HUP 上的配置，并以这种方式“按需”改变日志生成。
• 让您的程序侦听 unix 套接字并在打开时将消息写入其中。您甚至可以通过这种方式在您的程序中实现交互式控制台。
• 使用配置文件，提供对日志记录输出的精细控制。

Answer 4

看起来像 bash <> 重定向运算符（3.6.10 打开文件描述符进行读取和写入参见) 使得写入用它打开的文件/fifo 是非阻塞的。
这应该可行：

$ mkfifo /tmp/mylog
$ exec 4<>/tmp/mylog
$ myprogram 2>&1 | tee >&4
$ cat /tmp/mylog # on demend

#bash IRC 频道上 gniourf_gniourf 给出的解决方案。

Answer 5

嵌入式设备上经常使用的 BusyBox 可以创建一个内存缓冲日志

syslogd -C

填充

logger

和读取该日志，但只提供一个全局日志。

logread

，可以很好地

Answer 6

日志记录可以定向到 UDP 套接字。由于UDP是无连接的，因此不会阻塞发送程序。当然，如果接收器或网络无法跟上，日志将会丢失。

myprogram 2>&1 | socat - udp-datagram:localhost:3333

然后，当您想要观察日志记录时：

socat udp-recv:3333 -

还有一些其他很酷的好处，例如能够同时附加多个侦听器或广播到多个设备。

Answer 7

如果您可以在嵌入式设备上安装屏幕，那么您可以在其中运行“myprogram”并将其分离，然后在您想查看日志时随时重新连接。例如：

$ screen -t sometitle myprogram
Hit Ctrl+A, then d to detach it.

每当您想查看输出时，重新附加它：

$ screen -DR sometitle
Hit Ctrl-A, then d to detach it again.

这样您就不必担心程序输出使用磁盘空间。

Answer 8

给定的fifo方法的问题是，当管道缓冲区被填满并且没有读取过程发生时，整个事情将会挂起。

对于 fifo 方法的工作，我认为您必须实现一个类似于 BASH：从两个输入流读取的最佳架构（请参阅下面稍微修改的代码，示例代码 2）。

对于解决方法，您还可以使用 while ... read 构造，而不是通过在 while .. .read 循环将定期覆盖日志文件指定的行数。这将防止日志文件不断增长（示例代码 1）。

# sample code 1

# terminal window 1
rm -f /tmp/mylog
touch /tmp/mylog
while sleep 2; do date '+%Y-%m-%d_%H.%M.%S'; done 2>&1 | while IFS="" read -r line; do 
  lno=$((lno+1))
  #echo $lno
  array[${lno}]="${line}"
  if [[ $lno -eq 10 ]]; then
    lno=$((lno+1))
    array[${lno}]="-------------"
    printf '%s\n' "${array[@]}" > /tmp/mylog
    unset lno array
  fi
  printf '%s\n' "${line}"
done

# terminal window 2
tail -f /tmp/mylog


#------------------------


# sample code 2

# code taken from: 
# https://stackoverflow.com/questions/6702474/bash-best-architecture-for-reading-from-two-input-streams
# terminal window 1

# server
(
rm -f /tmp/to /tmp/from
mkfifo /tmp/to /tmp/from
while true; do 
  while IFS="" read -r -d 
\n' line; do 
    printf '%s\n' "${line}"
  done </tmp/to >/tmp/from &
  bgpid=$!
  exec 3>/tmp/to
  exec 4</tmp/from
  trap "kill -TERM $bgpid; exit" 0 1 2 3 13 15
  wait "$bgpid"
  echo "restarting..."
done
) &
serverpid=$!
#kill -TERM $serverpid

# client
(
exec 3>/tmp/to;
exec 4</tmp/from;
while IFS="" read -r -d 
\n' <&4 line; do
  if [[ "${line:0:1}" == 
\177' ]]; then 
    printf 'line from stdin: %s\n' "${line:1}"  > /dev/null
  else       
    printf 'line from fifo: %s\n' "$line"       > /dev/null
  fi
done &
trap "kill -TERM $"'!; exit' 1 2 3 13 15
while IFS="" read -r -d 
\n' line; do
  # can we make it atomic?
  # sleep 0.5
  # dd if=/tmp/to iflag=nonblock of=/dev/null  # flush fifo
  printf '\177%s\n' "${line}"
done >&3
) &
# kill -TERM $!


# terminal window 2
# tests
echo hello > /tmp/to
yes 1 | nl > /tmp/to
yes 1 | nl | tee /tmp/to
while sleep 2; do date '+%Y-%m-%d_%H.%M.%S'; done 2>&1 | tee -a /tmp/to


# terminal window 3
cat /tmp/to | head -n 10

Answer 9

如果您的进程写入任何日志文件，然后擦除该文件并时不时地重新启动，那么它不会变得太大，或者使用logrotate。

tail --follow=name --retry my.log

这就是您所需要的。您将获得与终端一样多的向后滚动。

不需要任何非标准的东西。我没有尝试过使用小日志文件，但我们所有的日志都像这样旋转，而且我从未注意到丢失的行。

Answer 10

为了追随 Fabraxias 的脚步，我将分享我对 racic 代码的小修改。在我的一个用例中，我需要抑制对 STDOUT 的写入，因此我添加了另一个参数：swallow_stdout。如果不是0，则将关闭到STDOUT的输出。

由于我不是 C 编码员，因此我在阅读代码时添加了注释，也许它们对其他人有用。

/* ftee - clone stdin to stdout and to a named pipe 
(c) racic@stackoverflow
WTFPL Licence */

// gcc /tmp/ftee.c -o /usr/local/bin/ftee

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

int main(int argc, char *argv[])
{
    int readfd, writefd;        // read & write file descriptors
    struct stat status;         // read file descriptor status
    char *fifonam;              // name of the pipe
    int swallow_stdout;         // 0 = write to STDOUT
    char buffer[BUFSIZ];        // read/write buffer
    ssize_t bytes;              // bytes read/written

    signal(SIGPIPE, SIG_IGN);   

    if(3!=argc)
    {
        printf("Usage:\n someprog 2>&1 | %s [FIFO] [swallow_stdout] \n" 
            "FIFO           - path to a named pipe (created beforehand with mkfifo), required argument\n"
            "swallow_stdout - 0 = output to PIPE and STDOUT, 1 = output to PIPE only, required argument\n", argv[0]);
        exit(EXIT_FAILURE);
    }
    fifonam = argv[1];
    swallow_stdout = atoi(argv[2]);

    readfd = open(fifonam, O_RDONLY | O_NONBLOCK);  // open read file descriptor in non-blocking mode

    if(-1==readfd)  // read descriptor error!
    {
        perror("ftee: readfd: open()");
        exit(EXIT_FAILURE);
    }

    if(-1==fstat(readfd, &status)) // read descriptor status error! (?)
    {
        perror("ftee: fstat");
        close(readfd);
        exit(EXIT_FAILURE);
    }

    if(!S_ISFIFO(status.st_mode)) // read descriptor is not a FIFO error!
    {
        printf("ftee: %s in not a fifo!\n", fifonam);
        close(readfd);
        exit(EXIT_FAILURE);
    }

    writefd = open(fifonam, O_WRONLY | O_NONBLOCK); // open write file descriptor non-blocking
    if(-1==writefd) // write file descriptor error!
    {
        perror("ftee: writefd: open()");
        close(readfd);
        exit(EXIT_FAILURE);
    }

    close(readfd); // reading complete, close read file descriptor

    while(1) // infinite loop
    {
        bytes = read(STDIN_FILENO, buffer, sizeof(buffer)); // read STDIN into buffer
        if (bytes < 0 && errno == EINTR)
            continue;   // skip over errors

        if (bytes <= 0) 
            break; // no more data coming in or uncaught error, let's quit since we can't write anything

        if (swallow_stdout == 0)
            bytes = write(STDOUT_FILENO, buffer, bytes); // write buffer to STDOUT
        if(-1==bytes) // write error!
            perror("ftee: writing to stdout");
        bytes = write(writefd, buffer, bytes); // write a copy of the buffer to the write file descriptor
        if(-1==bytes);// ignore errors
    }
    close(writefd); // close write file descriptor
    return(0); // return exit code 0
}