CODEWITHSUNDEEP
c++pipe
neuromancer
neuromancer

Reputation: 55509

How to make this pipe work in c++?

I am programming a shell in c++. It needs to be able to pipe the output from one thing to another. For example, in linux, you can pipe a textfile to more by doing cat textfile | more.

My function to pipe one thing to another is declared like this:

void pipeinput(string input, string output);

I send "cat textfile" as the input, and "more" as the output.

In c++ examples that show how to make pipes, fopen() is used. What do I send as my input to fopen()? I have seen c++ examples of pipeing using dup2 and without suing dup2. What's dup2 used for? How do you know if you need to use it or not?

Upvotes: 2

Views: 893

Answers (3)

Jonathan Leffler
Jonathan Leffler

Reputation: 754100

For a simple, two-command pipeline, the function interface you propose may be sufficient. For the general case of an N-stage pipeline, I don't think it is flexible enough.

The pipe() system call is used to create a pipe. In context, you will be creating one pipe before forking. One of the two processes will arrange for the write end of the pipe to become its standard output (probably using dup2()), and will then close both of the file descriptors originally returned by pipe(). It will then execute the command that writes to the pipe (cat textfile in your example). The other process will arrange for the read enc of the pipe to become its standard input (probably using dup2() again), and will then close both of the file descriptor originally returned by pipe(). It will then execute the command that reads from the pipe (more in your example).

Of course, there will be still a third process around - the parent shell process - which forked off a child to run the entire pipeline. You might decide you want to refine the mechanisms a bit if you want to track the statuses of each process in the pipeline; the process organization is then a bit different.

Upvotes: 4

Paul Beckingham
Paul Beckingham

Reputation: 14905

Take a look at popen(3), which is a way to avoid execvp.

Upvotes: 4

wallyk
wallyk

Reputation: 57784

fopen() is not used to create pipes. It can be used to open the file descriptor, but it is not necessary to do so.

Pipes are created with the pipe(2) call, before forking off the process. The subprocess has a little bit of file descriptor management to do before execing the command. See the example in pipe's documentation.

Upvotes: 1

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