feat(uio): readv/writev

include/sys/uio.h

@@ -0,0 +1,75 @@
+#ifndef VLIBC_SYS_UIO_H
+#define VLIBC_SYS_UIO_H
+
+/*
+ * vlibc — <sys/uio.h>.
+ *
+ * Scatter/gather I/O: readv() and writev() transfer data between a file
+ * descriptor and a vector of memory buffers (POSIX.1-2008 base). Both are
+ * unbuffered pass-throughs to the kernel: the iovec array is handed to
+ * SYS_readv/SYS_writev as-is, and the kernel applies its own validation
+ * (iovcnt in 0..IOV_MAX, iov NULL with iovcnt > 0 faults as EFAULT). Failures
+ * are reported as -1 with errno set by the syscall layer.
+ *
+ *   Level 1 (onlyposix): readv, writev.
+ *
+ * The Linux extensions that build on this ABI (preadv/pwritev,
+ * process_vm_readv/process_vm_writev, ...) are deliberately absent — they are
+ * not POSIX and belong to a later compatibility profile.
+ *
+ * struct iovec matches the kernel's struct iovec (x86_64: a pointer and a
+ * size_t, no padding). IOV_MAX is 1024, the Linux UIO_MAXIOV; the kernel
+ * rejects any iovcnt above it with EINVAL.
+ *
+ * None of these declarations carry an intent attribute: every function
+ * performs I/O with side effects and reports failures through errno, so
+ * const/pure would be unsound.
+ */
+
+#include <vlibc/features.h>
+
+#include <stddef.h>
+
+#include <sys/types.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/* Maximum number of iovec entries a single readv()/writev() call accepts. */
+#define IOV_MAX 1024
+
+/*
+ * A scatter/gather segment: iov_base points at the buffer (or, for readv,
+ * the buffer to fill) and iov_len is its size in bytes. Kernel layout, as
+ * above.
+ */
+struct iovec
+{
+    void *iov_base; /* starting address of the segment */
+    size_t iov_len; /* number of bytes in the segment */
+};
+
+/*
+ * Read up to the sum of iov_len bytes from fd into the iovcnt buffers of
+ * iov, in order; return the number of bytes read, 0 at end of file, or -1
+ * with errno set on error. Buffers are filled completely before the next
+ * one is touched.
+ */
+ssize_t
+readv(int fd, const struct iovec *iov, int iovcnt);
+
+/*
+ * Write up to the sum of iov_len bytes from the iovcnt buffers of iov to
+ * fd, in order; return the number of bytes written, or -1 with errno set
+ * on error. The kernel applies its own validation (iovcnt 0..IOV_MAX, iov
+ * NULL with iovcnt > 0 faults as EFAULT).
+ */
+ssize_t
+writev(int fd, const struct iovec *iov, int iovcnt);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* VLIBC_SYS_UIO_H */