Index: xvfs-core.c
==================================================================
--- xvfs-core.c
+++ xvfs-core.c
@@ -47,10 +47,31 @@
 		return(NULL);
 	}
 	
 	return(pathStr + rootLen + 1);
 }
+
+static const char *xvfs_perror(int xvfs_error) {
+	if (xvfs_error >= 0) {
+		return("Not an error");
+	}
+
+	switch (xvfs_error) {
+		case XVFS_RV_ERR_ENOENT:
+			return("No such file or directory");
+		case XVFS_RV_ERR_EINVAL:
+			return("Invalid argument");
+		case XVFS_RV_ERR_EISDIR:
+			return("Is a directory");
+		case XVFS_RV_ERR_ENOTDIR:
+			return("Not a directory");
+		case XVFS_RV_ERR_EFAULT:
+			return("Bad address");
+		default:
+			return("Unknown error");
+	}
+}
 
 /*
  * Internal Tcl_Filesystem functions, with the appropriate instance info
  */
 static int xvfs_tclfs_pathInFilesystem(Tcl_Obj *path, ClientData *dataPtr, struct xvfs_tclfs_instance_info *instanceInfo) {
@@ -69,10 +90,13 @@
 	int retval;
 
 	pathStr = xvfs_relativePath(path, instanceInfo);
 	
 	retval = instanceInfo->fsInfo->getStatProc(pathStr, statBuf);
+	if (retval < 0) {
+		retval = -1;
+	}
 	
 	return(retval);
 }
 
 static Tcl_Obj *xvfs_tclfs_listVolumes(struct xvfs_tclfs_instance_info *instanceInfo) {
@@ -79,12 +103,15 @@
 	return(NULL);
 }
 
 static Tcl_Channel xvfs_tclfs_openFileChannel(Tcl_Interp *interp, Tcl_Obj *path, int mode, int permissions, struct xvfs_tclfs_instance_info *instanceInfo) {
 	const char *pathStr;
+	Tcl_WideInt length;
+	const char *data;
 
 	pathStr = xvfs_relativePath(path, instanceInfo);
+
 	/*
 	 * XXX:TODO: Do something to create the Tcl_Channel we
 	 * need to return here
 	 */
 

Index: xvfs-core.h
==================================================================
--- xvfs-core.h
+++ xvfs-core.h
@@ -7,18 +7,33 @@
 
 typedef const char **(*xvfs_proc_getChildren_t)(const char *path, Tcl_WideInt *count);
 typedef const unsigned char *(*xvfs_proc_getData_t)(const char *path, Tcl_WideInt start, Tcl_WideInt *length);
 typedef int (*xvfs_proc_getStat_t)(const char *path, Tcl_StatBuf *statBuf);
 
+/*
+ * Interface for the filesystem to fill out before registering.
+ * The protocolVersion is provided first so that if this
+ * needs to change over time it can be appropriately handled.
+ */
 struct Xvfs_FSInfo {
 	int                      protocolVersion;
 	const char               *name;
 	xvfs_proc_getChildren_t  getChildrenProc;
 	xvfs_proc_getData_t      getDataProc;
 	xvfs_proc_getStat_t      getStatProc;
 };
 
+/*
+ * Error codes for various calls.  This is part of the ABI and must
+ * not be changed.
+ */
+#define XVFS_RV_ERR_ENOENT   (-8192)
+#define XVFS_RV_ERR_EINVAL   (-8193)
+#define XVFS_RV_ERR_EISDIR   (-8194)
+#define XVFS_RV_ERR_ENOTDIR  (-8195)
+#define XVFS_RV_ERR_EFAULT   (-8196)
+
 #define XVFS_REGISTER_INTERFACE(name) int name(Tcl_Interp *interp, struct Xvfs_FSInfo *fsInfo);
 
 #if defined(XVFS_MODE_STANDALONE)
 /*
  * In standalone mode, we just redefine calls to

Index: xvfs.c.rvt
==================================================================
--- xvfs.c.rvt
+++ xvfs.c.rvt
@@ -36,10 +36,12 @@
 	
 	if (path == NULL) {
 		return(XVFS_NAME_LOOKUP_ERROR);
 	}
 
+printf("Looking for %s\n", path);
+
 	pathLen = strlen(path);
 	pathHash = Tcl_ZlibAdler32(0, (const unsigned char *) path, pathLen);
 	switch (pathHash) {
 <?
 	for {set index 0} {$index < [llength $::xvfs::outputFiles]} {incr index} {
@@ -80,19 +82,21 @@
 	/*
 	 * Get the inode from the lookup function
 	 */
 	inode = xvfs_<?= $::xvfs::fsName ?>_nameToIndex(path);
 	if (inode == XVFS_NAME_LOOKUP_ERROR) {
+		*count = XVFS_RV_ERR_ENOENT;
 		return(NULL);
 	}
 	
 	fileInfo = &xvfs_<?= $::xvfs::fsName ?>_data[inode];
 
 	/*
 	 * Ensure this is a directory
 	 */
 	if (fileInfo->type != XVFS_FILE_TYPE_DIR) {
+		*count = XVFS_RV_ERR_ENOTDIR;
 		return(NULL);
 	}
 	
 	*count = fileInfo->size;
 	return(fileInfo->data.dirChildren);
@@ -104,44 +108,48 @@
 	long inode;
 
 	/*
 	 * Validate input parameters
 	 */
+	if (length == NULL) {
+		return(NULL);
+	}
+	
 	if (start < 0) {
-		return(NULL);
-	}
-	
-	if (length == NULL) {
+		*length = XVFS_RV_ERR_EINVAL;
 		return(NULL);
 	}
 	
 	if (*length < 0) {
+		*length = XVFS_RV_ERR_EINVAL;
 		return(NULL);
 	}
 	
 	/*
 	 * Get the inode from the lookup function
 	 */
 	inode = xvfs_<?= $::xvfs::fsName ?>_nameToIndex(path);
 	if (inode == XVFS_NAME_LOOKUP_ERROR) {
+		*length = XVFS_RV_ERR_ENOENT;
 		return(NULL);
 	}
 	
 	fileInfo = &xvfs_<?= $::xvfs::fsName ?>_data[inode];
 
 	/*
 	 * Ensure this is a file that can be read
 	 */
 	if (fileInfo->type != XVFS_FILE_TYPE_REG) {
+		*length = XVFS_RV_ERR_EISDIR;
 		return(NULL);
 	}
 
 	/*
 	 * Validate the length
 	 */
 	if (start > fileInfo->size) {
-		*length = -1;
+		*length = XVFS_RV_ERR_EFAULT;
 		return(NULL);
 	}
 
 	if (*length == 0) {
 		resultLength = fileInfo->size - start;
@@ -162,19 +170,19 @@
 
 	/*
 	 * Validate input parameters
 	 */
 	if (!statBuf) {
-		return(-1);
+		return(XVFS_RV_ERR_EINVAL);
 	}
 	
 	/*
 	 * Get the inode from the lookup function
 	 */
 	inode = xvfs_<?= $::xvfs::fsName ?>_nameToIndex(path);
 	if (inode == XVFS_NAME_LOOKUP_ERROR) {
-		return(-1);
+		return(XVFS_RV_ERR_ENOENT);
 	}
 	
 	fileInfo = &xvfs_<?= $::xvfs::fsName ?>_data[inode];
 	
 	statBuf->st_dev   = 0;