1 files changed, 33 insertions, 5 deletions

diff --git a/src/lib/std.zc b/src/lib/std.zc
index 772b887..745a921 100644
--- a/src/lib/std.zc
+++ b/src/lib/std.zc
@@ -344,6 +344,24 @@ struct Num {
 // are its arguments. Host.raw fills it on each call.
 int rawblk[7];
 
+// an open file, from Host.open. its drop closes the descriptor, so the file is
+// closed when its owner's block ends, when it is freed, or when the collector
+// reclaims it (docs/SPEC.md 6.15). called as f.write(s), f.read(buf, n).
+struct File {
+	int fd;
+	// writes a string.
+	//   s: the text to write
+	//   returns: how many bytes were written, negative on an error
+	int write(char* s) { return rt.syscall(rt.SYS_WRITE, this.fd, s, rt.len_w(s)); }
+	// reads bytes, one to a word.
+	//   buf: where they go
+	//   n: the most to read
+	//   returns: how many were read, 0 at the end of the file, negative on an error
+	int read(char* buf, int n) { return rt.syscall(rt.SYS_READ, this.fd, buf, n); }
+	// the file dies: close its descriptor
+	void drop() { rt.syscall(rt.SYS_CLOSE, this.fd, 0, 0); }
+}
+
 // calls into the host that runs the vm: the environment, the clock, files and
 // raw system calls. called as std.Host.name(args).
 struct Host {
@@ -373,17 +391,27 @@ struct Host {
 	//   returns: the time in seconds since 1970
 	static int now() { return rt.syscall(rt.SYS_TIME, 0, 0, 0); }
 
+	// opens a file. the result owns the descriptor: its drop closes it (see File).
+	//   path: the file's name
+	//   mode: 0 to read, 1 to write (the file is replaced, or created)
+	//   returns: the open file, or null if it could not be opened
+	static File*.u open(char* path, int mode) {
+		int fd = rt.syscall(rt.SYS_OPEN, path, mode, 0);
+		if (fd < 0) return 0;
+		File* f = rt.new(File);
+		f.fd = fd;
+		return f;
+	}
+
 	// writes a string to a file, replacing what was there. the file is created if
 	// it is missing.
 	//   path: the file's name
 	//   s: the text to write
 	//   returns: how many bytes were written, -1 if the file could not be opened
 	static int writefile(char* path, char* s) {
-		int fd = rt.syscall(rt.SYS_OPEN, path, 1, 0), n;
-		if (fd < 0) return -1;
-		n = rt.syscall(rt.SYS_WRITE, fd, s, rt.len_w(s));
-		rt.syscall(rt.SYS_CLOSE, fd, 0, 0);
-		return n;
+		File* f = Host.open(path, 1);
+		if (!f) return -1;
+		return f.write(s);	// f's drop closes the file after the write
 	}
 }