writev (nr 20)

Linux Signature

ssize_t writev(int fd, const struct iovec *iov, int iovcnt);

Where struct iovec is:

struct iovec {
    void  *iov_base;  // Starting address
    size_t iov_len;   // Number of bytes
};

Description

Writes data from multiple buffers (a “scatter/gather” array) to a file descriptor in a single atomic operation. This is what musl’s printf uses internally instead of plain write.

Current Implementation

Looks up fd in the current process’s per-process file descriptor table. Iterates through iovcnt iovec entries (each 16 bytes: iov_base: u64, iov_len: u64). For each non-empty buffer, calls FileHandle::write() on the handle.

• Console fds (stdout/stderr): Each buffer is printed via ConsoleHandle::write() (UTF-8 with ASCII fallback).
• Invalid fds: Returns -EBADF (-9).
• Returns the total number of bytes written across all iovec entries on success.
• Short-circuits on error from any individual write.

Source: osl/src/syscalls/io.rs — sys_writev

Future Work

• Validate that iov and all iov_base pointers are valid user-space addresses.
• Handle partial writes.
• Cap iovcnt at UIO_MAXIOV (1024) per Linux convention.