pipe (nr 22) / pipe2 (nr 293)

Linux Signature

int pipe(int pipefd[2]);
int pipe2(int pipefd[2], int flags);

Description

Creates a unidirectional data channel (pipe). Returns two file descriptors: pipefd[0] for reading and pipefd[1] for writing.

Both syscalls share the same implementation: pipe(fds) is dispatched as pipe2(fds, 0) (no flags).

Current Implementation

1. Create pipe: Allocates a PipeInner (shared VecDeque<u8> buffer) wrapped in PipeReader and PipeWriter handles.
2. Allocate fds: Allocates two file descriptors in the process’s fd table.
3. Apply flags: If O_CLOEXEC (0o2000000) is set, both fds get FD_CLOEXEC flag.
4. Write to user buffer: Writes [read_fd, write_fd] as two i32 values to the user buffer.

Pipe Semantics

• Read: If the buffer is empty and the writer is still open, the reader blocks via block_current_thread(). When the writer appends data, it wakes the blocked reader. Returns 0 (EOF) if the writer has been closed and the buffer is empty.
• Write: Appends data to the shared buffer and wakes any blocked reader. Currently unbounded (no backpressure).
• Close: Closing the write end sets write_closed = true and wakes any blocked reader (so it gets EOF). Closing the read end drops the reader’s Arc reference.

Source: osl/src/syscalls/fs.rs — sys_pipe2, osl/src/syscalls/mod.rs — pipe(22) dispatch, libkernel/src/file.rs — PipeReader, PipeWriter, make_pipe

Usage from C (musl)

#include <unistd.h>
#include <fcntl.h>

int fds[2];
pipe2(fds, O_CLOEXEC);
write(fds[1], "hello", 5);
close(fds[1]);

char buf[32];
ssize_t n = read(fds[0], buf, sizeof(buf)); // n = 5
close(fds[0]);

Errors

Future Work

• Bounded buffer with write-side blocking (backpressure).
• O_NONBLOCK flag support.