diff --git a/nbio/nbio_stdio.c b/nbio/nbio_stdio.c
index 57472a72f1..f0aaba8312 100644
--- a/nbio/nbio_stdio.c
+++ b/nbio/nbio_stdio.c
@@ -7,16 +7,23 @@ struct nbio_t {
 	void* data;
 	size_t progress;
 	size_t len;
-	char op;
+	/*
+	 * possible values:
+	 * NBIO_READ, NBIO_WRITE - obvious
+	 * -1 - currently doing nothing
+	 * -2 - the pointer was reallocated since the last operation
+	 */
+	signed char op;
 };
 
 static const char * modes[]={ "rb", "wb", "r+b" };
 struct nbio_t* nbio_open(const char * filename, enum nbio_mode_t mode)
 {
+	struct nbio_t* handle;
 	FILE* f=fopen(filename, modes[mode]);
 	if (!f) return NULL;
 	
-	struct nbio_t* handle=(struct nbio_t*)malloc(sizeof(struct nbio_t));
+	handle=(struct nbio_t*)malloc(sizeof(struct nbio_t));
 	handle->f = f;
 	if (mode != NBIO_WRITE)
 	{
@@ -26,17 +33,31 @@ struct nbio_t* nbio_open(const char * filename, enum nbio_mode_t mode)
 	else handle->len = 0;
 	handle->data = malloc(handle->len);
 	handle->progress = handle->len;
-	handle->op = -1;
+	handle->op = -2;
+	
+	return handle;
 }
 
 void nbio_begin_read(struct nbio_t* handle)
 {
+	if (handle->op >= 0)
+	{
+		puts("ERROR - attempted file read operation while busy");
+		abort();
+	}
+	fseek(handle->f, 0, SEEK_SET);
 	handle->op = NBIO_READ;
 	handle->progress = 0;
 }
 
 void nbio_begin_write(struct nbio_t* handle)
 {
+	if (handle->op >= 0)
+	{
+		puts("ERROR - attempted file write operation while busy");
+		abort();
+	}
+	fseek(handle->f, 0, SEEK_SET);
 	handle->op = NBIO_WRITE;
 	handle->progress = 0;
 }
@@ -50,27 +71,34 @@ bool nbio_iterate(struct nbio_t* handle, size_t* progress, size_t* len)
 	}
 	if (handle->op == NBIO_READ)
 	{
-		fread((char*)handle->ptr + handle->progress, 1,amount, handle->f);
+		fread((char*)handle->data + handle->progress, 1,amount, handle->f);
 	}
 	if (handle->op == NBIO_WRITE)
 	{
-		fwrite((char*)handle->ptr + handle->progress, 1,amount, handle->f);
+		fwrite((char*)handle->data + handle->progress, 1,amount, handle->f);
 	}
 	handle->progress += amount;
 	if (progress) *progress = handle->progress;
 	if (len) *len = handle->len;
 	if (handle->progress == handle->len) handle->op = -1;
-	return (handle->op == -1);
+	return (handle->op < 0);
 }
 
 void nbio_resize(struct nbio_t* handle, size_t len)
 {
+	if (handle->op >= 0)
+	{
+		puts("ERROR - attempted file resize operation while busy");
+		abort();
+	}
 	if (len < handle->len)
 	{
 		puts("ERROR - attempted file shrink operation, not implemented");
 		abort();
 	}
 	handle->len = len;
+	handle->data = realloc(handle->data, handle->len);
+	handle->op = -1;
 }
 
 void* nbio_get_ptr(struct nbio_t* handle, size_t* len)
diff --git a/nbio/nbio_test.c b/nbio/nbio_test.c
new file mode 100644
index 0000000000..30d2e5726f
--- /dev/null
+++ b/nbio/nbio_test.c
@@ -0,0 +1,49 @@
+#include "nbio.h"
+#include <string.h>
+#include <stdio.h>
+
+int main()
+{
+	struct nbio_t* write = nbio_open("test.bin", NBIO_WRITE);
+	nbio_resize(write, 1024*1024);
+	
+	size_t size;
+	void* ptr = nbio_get_ptr(write, &size);
+	if (size != 1024*1024) puts("ERROR: wrong size (1)");
+	
+	memset(ptr, 0x42, 1024*1024);
+	nbio_begin_write(write);
+	
+	size_t prog;
+	bool looped=false;
+	while (!nbio_iterate(write, &prog, &size))
+	{
+		printf("%u/%u\n", (unsigned)prog, (unsigned)size);
+		looped=true;
+	}
+	if (!looped) puts("Write finished immediately?");
+	nbio_free(write);
+	
+	
+	struct nbio_t* read = nbio_open("test.bin", NBIO_READ);
+	
+	ptr = nbio_get_ptr(read, &size);
+	if (size != 1024*1024) puts("ERROR: wrong size (2)");
+	if (ptr) puts("Read pointer is available before iterating?");
+	
+	nbio_begin_read(read);
+	
+	looped=false;
+	while (!nbio_iterate(read, &prog, &size))
+	{
+		printf("%u/%u\n", (unsigned)prog, (unsigned)size);
+		looped=true;
+	}
+	if (!looped) puts("Read finished immediately?");
+	
+	ptr = nbio_get_ptr(read, &size);
+	if (size != 1024*1024) puts("ERROR: wrong size (3)");
+	if (*(char*)ptr != 0x42 || memcmp(ptr, (char*)ptr+1, 1024*1024-1)) puts("ERROR: wrong data");
+	
+	nbio_free(read);
+}