partially-written draft of fmemopen, still in #if 0

src/stdio/fmemopen.c

@@ -3,14 +3,59 @@
 
 static ssize_t mread(FILE *f, unsigned char *buf, size_t len)
 {
-	memcpy(buf, 
+	size_t rem = f->memsize - f->mempos;
+	if (len > rem) len = rem;
+	memcpy(buf, f->membuf+f->mempos, len);
+	f->mempos += len;
+	return len;
+}
+
+static ssize_t mwrite(FILE *f, const unsigned char *buf, size_t len)
+{
+	size_t rem;
+	if (f->memmode == 'a') f->mempos = f->memsize;
+	rem = f->memlim - f->mempos;
+	if (len > rem) len = rem;
+	memcpy(f->membuf+f->mempos, buf, len);
+	f->mempos += len;
+	if (f->mempos >= f->memsize) {
+		f->memsize = f->mempos;
+	}
+	return len;
 }
 
 FILE *fmemopen(void *buf, size_t size, const char *mode)
 {
-	FILE *f = calloc(sizeof(FILE), 1);
+	FILE *f;
+	int plus = !!strchr(mode, '+');
+	
+	if (!size || !strchr("rwa", *mode)) {
+		errno = EINVAL;
+		return 0;
+	}
+
+	if (!buf && size > SIZE_MAX-sizeof(FILE)-BUFSIZ-UNGET) {
+		errno = ENOMEM;
+		return 0;
+	}
+
+	f = calloc(sizeof(FILE) + UNGET + BUFSIZ + (buf?0:size), 1);
 	if (!f) return 0;
+	f->fd = -1;
+	f->lbf = EOF;
+	f->buf = (unsigned char *)(f+1) + UNGET;
+	f->buf_size = BUFSIZ;
+	if (!buf) buf = f->buf + BUFSIZ;
 	
-	//
+	if (!plus) f->flags = (*mode == 'r') ? F_NOWR : F_NORD;
+	if (*mode == 'a') f->mempos = strchr(buf, 0)-buf;
+
+	f->read = mread;
+	f->write = mwrite;
+	f->seek = mseek;
+	f->flush = mflush;
+	f->close = mclose;
+
+	return f;
 }
 #endif