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Xv6 pipeline

Reference: Xv6-RISCV-Book 1.3 Pipes

pipe

The Xv6 system calls pipe() to create the pipe. Pipes are similar to chan in the Go language. We use | said the pipeline in the Shell, to command: echo “hello world” | wc, can use the following code:

// upipe.c

#include "kernel/types.h"
#include "kernel/stat.h"
#include "user/user.h"

int main(a) {
	int p[2];  // file descriptors for the pipe
	
	char *argv[2];
	argv[0] = "wc";
	argv[1] = 0;  // NULL

	pipe(p);  // creates a new pipe: records the read and write file descriptors in the array p

	if (fork() == 0) {
		// redirection
		close(0);
		dup(p[0]);  // stdin = <- pipe

		close(p[0]);
		close(p[1]);

		exec("wc", argv);
	} else {
		close(p[0]);

		write(p[1]."hello world\n".12);  // pipe <- str

		close(p[1]);
	}

	exit(0);
}
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Compile and run:

$ upipe
1 2 12
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The implementation of pipe handling in Xv6 SH is similar to this code: fork two processes to redirect the standard output or input and run commands on the left and right sides of the pipe. See the user/sh. C: 100.

Pipeline V.S. temporary file

There seems to be little difference between using pipes and temporary files:

# pipeline
echo "hello world" | wc
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# temporary file
echo hello world >/tmp/xyz; wc </tmp/xyz
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But pipes are better:

  • Pipes are cleaned automatically (temporary files should be manually deleted)
  • Pipes can hold streams of any length (sufficient disk space is required for temporary files)
  • Pipes can be run in parallel (temporary files can only be run in one, and then started in the second)
  • Blocking reads and writes on pipes are more convenient than non-blocking temporary files when dealing with interprocess communication problems.

EOF

By CDFMLR 2021-02-20

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