diff --git a/libsparse/include/sparse/sparse.h b/libsparse/include/sparse/sparse.h
index 356f65f..1b91ead 100644
--- a/libsparse/include/sparse/sparse.h
+++ b/libsparse/include/sparse/sparse.h
@@ -18,6 +18,7 @@
 #define _LIBSPARSE_SPARSE_H_
 
 #include <stdbool.h>
+#include <stddef.h>
 #include <stdint.h>
 
 #ifdef	__cplusplus
@@ -26,6 +27,11 @@
 
 struct sparse_file;
 
+// The callbacks in sparse_file_callback() and sparse_file_foreach_chunk() take
+// size_t as the length type (was `int` in past). This allows clients to keep
+// their codes compatibile with both versions as needed.
+#define	SPARSE_CALLBACK_USES_SIZE_T
+
 /**
  * sparse_file_new - create a new sparse file cookie
  *
@@ -201,7 +207,7 @@
  * Returns 0 on success, negative errno on error.
  */
 int sparse_file_callback(struct sparse_file *s, bool sparse, bool crc,
-		int (*write)(void *priv, const void *data, int len), void *priv);
+		int (*write)(void *priv, const void *data, size_t len), void *priv);
 
 /**
  * sparse_file_foreach_chunk - call a callback for data blocks in sparse file
@@ -218,7 +224,7 @@
  * Returns 0 on success, negative errno on error.
  */
 int sparse_file_foreach_chunk(struct sparse_file *s, bool sparse, bool crc,
-	int (*write)(void *priv, const void *data, int len, unsigned int block,
+	int (*write)(void *priv, const void *data, size_t len, unsigned int block,
 		     unsigned int nr_blocks),
 	void *priv);
 /**
diff --git a/libsparse/output_file.c b/libsparse/output_file.c
index 51e60ef..002ad27 100644
--- a/libsparse/output_file.c
+++ b/libsparse/output_file.c
@@ -109,7 +109,7 @@
 struct output_file_callback {
 	struct output_file out;
 	void *priv;
-	int (*write)(void *priv, const void *buf, int len);
+	int (*write)(void *priv, const void *buf, size_t len);
 };
 
 #define to_output_file_callback(_o) \
@@ -634,7 +634,8 @@
 	return &outn->out;
 }
 
-struct output_file *output_file_open_callback(int (*write)(void *, const void *, int),
+struct output_file *output_file_open_callback(
+		int (*write)(void *, const void *, size_t),
 		void *priv, unsigned int block_size, int64_t len,
 		int gz __unused, int sparse, int chunks, int crc)
 {
diff --git a/libsparse/output_file.h b/libsparse/output_file.h
index b67e94e..690f610 100644
--- a/libsparse/output_file.h
+++ b/libsparse/output_file.h
@@ -27,7 +27,8 @@
 
 struct output_file *output_file_open_fd(int fd, unsigned int block_size, int64_t len,
 		int gz, int sparse, int chunks, int crc);
-struct output_file *output_file_open_callback(int (*write)(void *, const void *, int),
+struct output_file *output_file_open_callback(
+		int (*write)(void *, const void *, size_t),
 		void *priv, unsigned int block_size, int64_t len, int gz, int sparse,
 		int chunks, int crc);
 int write_data_chunk(struct output_file *out, unsigned int len, void *data);
diff --git a/libsparse/sparse.c b/libsparse/sparse.c
index b175860..466435f 100644
--- a/libsparse/sparse.c
+++ b/libsparse/sparse.c
@@ -179,7 +179,7 @@
 }
 
 int sparse_file_callback(struct sparse_file *s, bool sparse, bool crc,
-		int (*write)(void *priv, const void *data, int len), void *priv)
+		int (*write)(void *priv, const void *data, size_t len), void *priv)
 {
 	int ret;
 	int chunks;
@@ -203,11 +203,11 @@
 	void		*priv;
 	unsigned int	block;
 	unsigned int	nr_blocks;
-	int (*write)(void *priv, const void *data, int len, unsigned int block,
-		     unsigned int nr_blocks);
+	int (*write)(void *priv, const void *data, size_t len,
+		     unsigned int block, unsigned int nr_blocks);
 };
 
-static int foreach_chunk_write(void *priv, const void *data, int len)
+static int foreach_chunk_write(void *priv, const void *data, size_t len)
 {
 	struct chunk_data *chk = priv;
 
@@ -215,7 +215,7 @@
 }
 
 int sparse_file_foreach_chunk(struct sparse_file *s, bool sparse, bool crc,
-	int (*write)(void *priv, const void *data, int len, unsigned int block,
+	int (*write)(void *priv, const void *data, size_t len, unsigned int block,
 		     unsigned int nr_blocks),
 	void *priv)
 {
@@ -250,7 +250,7 @@
 	return ret;
 }
 
-static int out_counter_write(void *priv, const void *data __unused, int len)
+static int out_counter_write(void *priv, const void *data __unused, size_t len)
 {
 	int64_t *count = priv;
 	*count += len;