diff --git a/README.md b/README.md
index d1ffc40..64a10cb 100644
--- a/README.md
+++ b/README.md
@@ -21,8 +21,9 @@ funpack output.fpk [unpacked/]   # dir defaults to "output" if omitted
 ```c
 #include <fpk/fpk.h>
 
-fpk_t *pak = fpk_open("assets.fpk");
-fpk_find(pak, "some/file.bin", &data, &size);
+struct fpk *pak = fpk_open("assets.fpk");
+fpk_entry_t *entry = fpk_find(pak, "some/file.bin");
+void *data = fpk_read(pak, entry, &size);
 fpk_close(pak);
 ```
 
diff --git a/docs/format.md b/docs/format.md
new file mode 100644
index 0000000..fa8cd67
--- /dev/null
+++ b/docs/format.md
@@ -0,0 +1,52 @@
+# FPK1 format
+
+```
+[ header ][ data blobs ][ string table ][ directory entries ]
+```
+
+## Header (28 bytes)
+
+| Field           | Type      | Size | Offset |
+|-----------------|-----------|------|--------|
+| `magic`         | char[4]   | 4    | 0      |
+| `entry_count`   | uint64_t  | 8    | 4      |
+| `dir_offset`    | uint64_t  | 8    | 12     |
+| `strtab_offset` | uint64_t  | 8    | 20     |
+
+`magic` = `"FPK1"`.
+
+`FPK_HEADER_SIZE` (28): field-by-field, not `sizeof()`.
+
+## Data blobs
+
+Raw bytes, back-to-back, no padding, writer add order.
+
+## String table
+
+Paths, `/`-separated, NUL-terminated, back-to-back, entry order.
+
+`strtab_offset` points here.
+
+## Directory (25 bytes per entry)
+
+| Field         | Type     | Size | Offset |
+|---------------|----------|------|--------|
+| `path_offset` | uint64_t | 8    | 0      |
+| `data_offset` | uint64_t | 8    | 8      |
+| `data_size`   | uint64_t | 8    | 16     |
+| `method`      | uint8_t  | 1    | 24     |
+
+`path_offset` relative to `strtab_offset`. `data_offset` absolute.
+
+`FPK_DIR_ENTRY_SIZE` (25): field-by-field, not `sizeof()`.
+
+`dir_offset` points here, last region. File size = `dir_offset +
+entry_count * FPK_DIR_ENTRY_SIZE`.
+
+## Compression
+
+`method`: reserved, v1 = `FPK_METHOD_STORE` (0) only.
+
+## Endianness
+
+Little-endian on disk, always. Big-endian host swaps on read/write.
diff --git a/include/fpk.h b/include/fpk.h
new file mode 100644
index 0000000..2332454
--- /dev/null
+++ b/include/fpk.h
@@ -0,0 +1,63 @@
+#ifndef FPK_H
+#define FPK_H
+
+#include <stddef.h>
+#include <stdint.h>
+
+#define FPK_MAGIC "FPK1"
+#define FPK_MAGIC_SIZE 4
+#define FPK_VERSION 1
+
+#define FPK_HEADER_SIZE 28
+/*
+ * magic          4
+ * entry_count    8
+ * dir_offset     8
+ * strtab_offset  8
+ *               --
+ *               28
+ */
+#define FPK_DIR_ENTRY_SIZE 25
+/*
+ * path_offset  8
+ * data_offset  8
+ * data_size    8
+ * method       1
+ *             --
+ *             25
+ */
+
+#define FPK_METHOD_STORE 0
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+typedef struct {
+	const char *path;
+	uint64_t data_offset;
+	uint64_t data_size;
+	uint8_t method;
+} fpk_entry_t;
+
+struct fpk;
+struct fpk_writer;
+
+struct fpk *fpk_open(const char *path);
+void fpk_close(struct fpk *fpk);
+fpk_entry_t *fpk_find(struct fpk *fpk, const char *path);
+void *fpk_read(struct fpk *fpk, const fpk_entry_t *entry, size_t *out_size);
+uint64_t fpk_entry_count(const struct fpk *fpk);
+const fpk_entry_t *fpk_entry_at(const struct fpk *fpk, uint64_t index);
+
+struct fpk_writer *fpk_writer_open(const char *path);
+int fpk_writer_add(struct fpk_writer *w, const char *path, const void *data,
+		    size_t size);
+int fpk_writer_finalize(struct fpk_writer *w);
+void fpk_writer_close(struct fpk_writer *w);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* FPK_H */
diff --git a/include/fpk_version.h b/include/fpk_version.h
new file mode 100644
index 0000000..4deaac3
--- /dev/null
+++ b/include/fpk_version.h
@@ -0,0 +1,16 @@
+#ifndef FPK_VERSION_H
+#define FPK_VERSION_H
+
+#define FPK_VERSION_MAJOR 0
+#define FPK_VERSION_MINOR 1
+#define FPK_VERSION_PATCH 0
+
+#define FPK_STRINGIFY_HELPER(x) #x
+#define FPK_STRINGIFY(x) FPK_STRINGIFY_HELPER(x)
+
+#define FPK_VERSION_STRING                                                    \
+	FPK_STRINGIFY(FPK_VERSION_MAJOR)                                       \
+	"." FPK_STRINGIFY(FPK_VERSION_MINOR) "." FPK_STRINGIFY(                \
+	    FPK_VERSION_PATCH)
+
+#endif /* FPK_VERSION_H */