implement open_memstream

this is the first attempt, and may have bugs. only minimal testing has
been performed.

include/stdio.h

@@ -121,6 +121,7 @@ FILE *tmpfile(void);
 
 #if defined(_POSIX_SOURCE) || defined(_POSIX_C_SOURCE) \
  || defined(_XOPEN_SOURCE) || defined(_GNU_SOURCE)
+FILE *open_memstream(char **, size_t *);
 FILE *fdopen(int, const char *);
 FILE *popen(const char *, const char *);
 int pclose(FILE *);

src/stdio/open_memstream.c

@@ -0,0 +1,94 @@
+#include "stdio_impl.h"
+
+struct cookie {
+	char **bufp;
+	size_t *sizep;
+	size_t pos;
+	char *buf;
+	size_t len;
+	size_t space;
+};
+
+static off_t ms_seek(FILE *f, off_t off, int whence)
+{
+	ssize_t base;
+	struct cookie *c = f->cookie;
+	switch (whence) {
+	case SEEK_SET:
+		base = 0;
+		break;
+	case SEEK_CUR:
+		base = c->pos;
+		break;
+	case SEEK_END:
+		base = c->len;
+		break;
+	default:
+	fail:
+		errno = EINVAL;
+		return -1;
+	}
+	if (-off > base || off > SSIZE_MAX-base) goto fail;
+	return c->pos = base+off;
+}
+
+static size_t ms_write(FILE *f, const unsigned char *buf, size_t len)
+{
+	struct cookie *c = f->cookie;
+	size_t len2 = f->wpos - f->wbase;
+	char *newbuf;
+	if (len2) {
+		f->wpos = f->wbase;
+		if (ms_write(f, f->wbase, len2) < len2) return 0;
+	}
+	if (len > c->space - c->pos) {
+		len2 = 2*c->space+1 | c->space+len+1;
+		newbuf = realloc(c->buf, len2);
+		if (!newbuf) return 0;
+		*c->bufp = c->buf = newbuf;
+		memset(c->buf + c->space, 0, len2 - c->space);
+		c->space = len2;
+	}
+	memcpy(c->buf+c->pos, buf, len);
+	c->pos += len;
+	if (c->pos >= c->len) c->len = c->pos;
+	*c->sizep = c->pos;
+	return len;
+}
+
+static int ms_close(FILE *f)
+{
+	return 0;
+}
+
+FILE *open_memstream(char **bufp, size_t *sizep)
+{
+	FILE *f;
+	struct cookie *c;
+	if (!(f=malloc(sizeof *f + sizeof *c + BUFSIZ))) return 0;
+	memset(f, 0, sizeof *f + sizeof *c);
+	f->cookie = c = (void *)(f+1);
+
+	c->bufp = bufp;
+	c->sizep = sizep;
+	c->pos = c->len = c->space = 0;
+	c->buf = 0;
+
+	f->flags = F_NORD;
+	f->fd = -1;
+	f->buf = (void *)(c+1);
+	f->buf_size = BUFSIZ;
+	f->lbf = EOF;
+	f->write = ms_write;
+	f->seek = ms_seek;
+	f->close = ms_close;
+
+	if (!libc.threaded) {
+		f->lock = -1;
+		f->next = libc.ofl_head;
+		if (libc.ofl_head) libc.ofl_head->prev = f;
+		libc.ofl_head = f;
+	}
+
+	return f;
+}