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Pipe

Lower IO​

Beyond the pipe, we only learn how to use functions like fopen, fclose, fgets, fprintf, fwrite ... to operate on streams using the FILE objects. Those kinds of function helps us to easily to do IO operation we want, but hide some of complexity of the actual IO system calls.

Firstly, before we open or do any operation for any files. There are three files automatically opened: stdin, stdout,and stderr. That is, whenever we open a new file the file descriptor always >=3.

  • stdin stands the standard input(something like scanf, or the input from the command line) usually with descriptor 0,
  • stdout stands the standard output(something like printf) with descriptor 1,
  • stderr stands the standard errors(we somehow use often call when we want to warning errors when open,read or close files) with descriptor 2.

Notice, the descriptor is one field in a FILE struct, also is the index of the FILE, and children inherits all open files descriptors.

Pipe​

pipe(int * fd): fd is a length 2 int array.

In the process model module, we talk about how a process call another process so that they can work simultaneously to enhance our efficiency. We can use fork to have a children process or use excel to change program to run.

Pipe is a kind of communication mechanism we can use to transfer data between related processes. And it takes an array of two file descriptors. One for reading(index 0), one for writing(index 1). Here, the reading is for the other process to read from pipe, and writing is this process write stuff into pipe. And also pipe is a data structure.

Notice:

  • The maximum bytes of pipe is 64kb,
  • If the pipe is close, the read from pipe will be 0 to indicate no more data to read.
  • pipe should call before fork since child inherits open file descriptors from the parent.
  • read for pipe will holding 'till all writing descriptors are close.
  • We have to call fork after pipe for the pipe to have the right file descriptors open in both processes.
  • If we have multiple children and multiple pipe for those children, notice if one of those process holding pipe, the other pipe will wait for the work.

More about pipe​

As mention in process model module, the process will running in order designed by operation system, similarly, pipe will also face the question:

  • The process's write speed is slower than the other process's read speed. Specifically, the process write data into pipe slower than the process read data. (This time, the operation system will help to stop reading)
  • The process's write speed much faster than the other process's read speed so that the pipe always full no where for writing anymore. Similarly, operation system will stop writing 'till finishing reading.

dup2 system call​

int dup2(int oldfd, int newfd): return the new file descriptor or -1 on error

The dup2 will make a copy of an open file descriptor so that we can use it to reset the stdout descriptor so that the writing to stdout can go to our own output file. (We can also read from stdout by dup2)

The dup2 needs 2 parameters, one is the old fd (file descriptor), the other one is the new fd. New one will be the copy of old one after calling this function