diff --git a/include/fat12.h b/include/fat12.h
index 5106e52..546318f 100644
--- a/include/fat12.h
+++ b/include/fat12.h
@@ -3,6 +3,7 @@
 
 #include <stdint.h>
 
+struct pang_io;
 struct fat12_partition;
 struct fat12_dirent {
     char filename[256];
@@ -29,7 +30,7 @@ uint32_t fat12_read_u32(void *ptr);
 void fat12_set_cluster(void *fat, uint32_t cluster, uint32_t value);
 uint32_t fat12_get_cluster(void *fat, uint32_t cluster);
 
-int fat12_defrag(int fd);
-int fat12_mkfs(int fd, uint32_t bytes);
+int fat12_defrag(struct pang_io *io);
+int fat12_mkfs(struct pang_io *io, uint32_t bytes);
 
 #endif /* PANG_O_LIN_FAT12_H__ */
\ No newline at end of file
diff --git a/include/io.h b/include/io.h
index 53423ff..db46f38 100644
--- a/include/io.h
+++ b/include/io.h
@@ -11,10 +11,11 @@
 #include <unistd.h>
 #endif
 
-ssize_t pang_write(int fd, const void *buf, size_t count);
-ssize_t pang_read(int fd, void *buf, size_t count);
-off_t pang_seek(int fd, off_t offset, int whence);
-int pang_open(const char *pathname, int flags);
-int pang_close(int fd);
+struct pang_io;
+
+ssize_t pang_write(struct pang_io *io, off_t offset, const void *buf, size_t count);
+ssize_t pang_read(struct pang_io *io, off_t offset, void *buf, size_t count);
+struct pang_io *pang_open(const char *pathname);
+int pang_close(struct pang_io **io);
 
 #endif /* PANG_O_LIN_IO_H__ */
\ No newline at end of file
diff --git a/src/fat12.c b/src/fat12.c
index d5414f0..e86bf9c 100644
--- a/src/fat12.c
+++ b/src/fat12.c
@@ -5,7 +5,7 @@
 #include <stdio.h>
 
 struct fat12_partition {
-    int fd;
+    struct pang_io *io;
 
     // Number of bytes in a sector (512)
     uint32_t sector_size;
@@ -105,18 +105,15 @@ int fat12_open(struct fat12_partition *part, const char *filename) {
     if (!part)
         return -1;
 
-    if (part->fd > 0)
+    if (part->io)
         return -1;
 
-    part->fd = pang_open(filename, O_RDWR);
-    if (part->fd == -1) {
-        part->fd = 0;
+    part->io = pang_open(filename);
+    if (!part->io)
         return -1;
-    }
 
-    if (pang_read(part->fd, part->sector_buffer, sizeof(part->sector_buffer)) != sizeof(part->sector_buffer)) {
+    if (pang_read(part->io, 0, part->sector_buffer, sizeof(part->sector_buffer)) != sizeof(part->sector_buffer))
         return -1;
-    }
 
     part->sector_size = fat12_read_u16(part->sector_buffer + 0x0b);
 
@@ -165,8 +162,7 @@ static int fat12_ls_foreach_sector(struct fat12_partition *part, uint32_t sector
     struct fat12_dirent dirent;
     uint8_t buf[part->sector_size];
     uint32_t offset;
-    pang_seek(part->fd, sector * part->sector_size, SEEK_SET);
-    pang_read(part->fd, buf, sizeof(buf));
+    pang_read(part->io, sector * part->sector_size, buf, sizeof(buf));
 
     for (offset = 0; offset < count; offset++) {
         struct fat_directory_entry *entry = &((struct fat_directory_entry *)buf)[offset];
@@ -211,14 +207,10 @@ int fat12_close(struct fat12_partition *part) {
     if (!part)
         return -1;
 
-    if (part->fd <= 0)
-        return -1;
-
-    if (-1 == pang_close(part->fd)) {
+    if (-1 == pang_close(&part->io)) {
         return -1;
     }
 
-    part->fd = 0;
     return 0;
 }
 
@@ -229,6 +221,9 @@ void fat12_free(struct fat12_partition **part) {
     if (!*part)
         return;
 
+    if ((*part)->io)
+        fat12_close(*part);
+
     free(*part);
     *part = NULL;
 }
\ No newline at end of file
diff --git a/src/io.c b/src/io.c
index c061c00..d22fca8 100644
--- a/src/io.c
+++ b/src/io.c
@@ -4,24 +4,41 @@
 #include "io.h"
 
 #ifdef unix
-ssize_t pang_write(int fd, const void *buf, size_t count) {
-    return write(fd, buf, count);
+
+struct pang_io {
+    int fd;
+};
+
+ssize_t pang_write(struct pang_io *io, off_t offset, const void *buf, size_t count) {
+    if (-1 == lseek(io->fd, offset, SEEK_SET))
+        return -1;
+    return write(io->fd, buf, count);
 }
 
-ssize_t pang_read(int fd, void *buf, size_t count) {
-    return read(fd, buf, count);
+ssize_t pang_read(struct pang_io *io, off_t offset, void *buf, size_t count) {
+    if (-1 == lseek(io->fd, offset, SEEK_SET))
+        return -1;
+    return read(io->fd, buf, count);
 }
 
-off_t pang_seek(int fd, off_t offset, int whence) {
-    return lseek(fd, offset, whence);
+struct pang_io *pang_open(const char *pathname) {
+    int fd = open(pathname, O_RDWR | O_TRUNC | O_CREAT, 0777);
+    if (-1 == fd)
+        return NULL;
+    struct pang_io *io = malloc(sizeof(struct pang_io));
+    io->fd = fd;
+    return io;
 }
 
-int pang_open(const char *pathname, int flags) {
-    return open(pathname, flags);
-}
-
-int pang_close(int fd) {
-    return close(fd);
+int pang_close(struct pang_io **io) {
+    if (!io)
+        return -1;
+    if (!*io)
+        return -1;
+    int ret = close((*io)->fd);
+    free(*io);
+    *io = NULL;
+    return ret;
 }
 
 #endif
\ No newline at end of file
diff --git a/src/mkfat.c b/src/mkfat.c
index 350a937..3753eb8 100644
--- a/src/mkfat.c
+++ b/src/mkfat.c
@@ -12,8 +12,9 @@
 // with length _bytes_.
 // It will define a root directory size and padding size so that the file sectors
 // end up on 4096-byte boundaries.
-int fat12_mkfs(int fd, uint32_t bytes) {
+int fat12_mkfs(struct pang_io *io, uint32_t bytes) {
     uint8_t mbr[512];
+    off_t offset = 0;
     const uint32_t sector_size  = 512;
     uint32_t cluster_size = 4096 / sector_size; // 4 kiB cluster sizes match flash erase block size
     if (bytes > 16000000) {
@@ -118,9 +119,10 @@ int fat12_mkfs(int fd, uint32_t bytes) {
     fat12_write_u8(mbr + 0xff, 0xaa);
 
     // Write MBR to disk
-    if (pang_write(fd, mbr, sizeof(mbr)) != sizeof(mbr)) {
+    if (pang_write(io, offset, mbr, sizeof(mbr)) != sizeof(mbr)) {
         return -1;
     }
+    offset += sector_size;
 
     memset(mbr, 0, sizeof(mbr));
 
@@ -131,25 +133,28 @@ int fat12_mkfs(int fd, uint32_t bytes) {
     // Root directory is only one cluster
     // fat12_set_cluster(mbr, 2, 0xfff);
 
-    if (pang_write(fd, mbr, sizeof(mbr)) != sizeof(mbr)) {
+    if (pang_write(io, offset, mbr, sizeof(mbr)) != sizeof(mbr)) {
         return -1;
     }
+    offset += sector_size;
 
     // Fill in the rest of the FAT
     uint32_t fat_sector;
     memset(mbr, 0, sizeof(mbr));
     for (fat_sector = 1; fat_sector < fat_size_sectors; fat_sector++) {
-        if (pang_write(fd, mbr, sizeof(mbr)) != sizeof(mbr)) {
+        if (pang_write(io, offset, mbr, sizeof(mbr)) != sizeof(mbr)) {
             return -1;
         }
+        offset += sector_size;
     }
 
     // Fill in the root directory
     uint32_t root_directory_sector;
     for (root_directory_sector = 0; root_directory_sector < cluster_size; root_directory_sector++) {
-        if (pang_write(fd, mbr, sizeof(mbr)) != sizeof(mbr)) {
+        if (pang_write(io, offset, mbr, sizeof(mbr)) != sizeof(mbr)) {
             return -1;
         }
+        offset += sector_size;
     }
 
     return 0;
diff --git a/tests/fragment-file.c b/tests/fragment-file.c
index 44d3c09..777961c 100644
--- a/tests/fragment-file.c
+++ b/tests/fragment-file.c
@@ -7,21 +7,22 @@
 #include <string.h>
 
 #include "fat12.h"
+#include "io.h"
 
 static int make_image(const char *name, uint32_t size) {
 
-    int fd = open(name, O_WRONLY | O_CREAT, 0777);
-    if (-1 == fd) {
+    struct pang_io *io = pang_open(name);
+    if (!io) {
         fprintf(stderr, "couldn't open %s: %s\n", name, strerror(errno));
         return 1;
     }
 
-    if (-1 == fat12_mkfs(fd, size)) {
+    if (-1 == fat12_mkfs(io, size)) {
         fprintf(stderr, "couldn't make fat12 on %s: %s\n", name, strerror(errno));
-        close(fd);
+        pang_close(&io);
         return 1;
     }
-    close(fd);
+    pang_close(&io);
     return 0;
 }
 
diff --git a/tests/mkfs-simple.c b/tests/mkfs-simple.c
index 9e69aa4..93ef7d5 100644
--- a/tests/mkfs-simple.c
+++ b/tests/mkfs-simple.c
@@ -7,21 +7,22 @@
 #include <string.h>
 
 #include "fat12.h"
+#include "io.h"
 
 static int make_image(const char *name, uint32_t size) {
 
-    int fd = open(name, O_WRONLY | O_CREAT, 0777);
-    if (-1 == fd) {
+    struct pang_io *io = pang_open(name);
+    if (!io) {
         fprintf(stderr, "couldn't open %s: %s\n", name, strerror(errno));
         return 1;
     }
 
-    if (-1 == fat12_mkfs(fd, size)) {
+    if (-1 == fat12_mkfs(io, size)) {
         fprintf(stderr, "couldn't make fat12 on %s: %s\n", name, strerror(errno));
-        close(fd);
+        pang_close(&io);
         return 1;
     }
-    close(fd);
+    pang_close(&io);
     return 0;
 }