diff --git a/include/git/os/unix.h b/include/git/os/unix.h
index 56f77b95f..475fe13a5 100644
--- a/include/git/os/unix.h
+++ b/include/git/os/unix.h
@@ -26,11 +26,16 @@
 #ifndef INCLUDE_git_os_abstraction_h__
 #define INCLUDE_git_os_abstraction_h__
 
+/** Force 64 bit off_t size on POSIX. */
+#define _FILE_OFFSET_BITS 64
+
+#include <errno.h>
 #include <unistd.h>
 #include <sys/stat.h>
 #include <fcntl.h>
 #include <sys/types.h>
 #include <time.h>
+#include <stdlib.h>
 #include <string.h>
 
 /**
@@ -95,6 +100,15 @@ GIT_EXTERN(int) git_fread(git_file fd, void *buf, size_t cnt);
  */
 GIT_EXTERN(int) git_fwrite(git_file fd, void *buf, size_t cnt);
 
+/**
+ * Get the current size of an open file.
+ * @param fd open descriptor.
+ * @return
+ * - On success, >= 0, indicating the file size in bytes.
+ * - On error, <0.
+ */
+GIT_EXTERN(off_t) git_fsize(git_file fd);
+
 /**
  * Close an open file descriptor.
  * @param fd descriptor to close.
diff --git a/src/os/unix.c b/src/os/unix.c
index 767f20ca8..8a2088414 100644
--- a/src/os/unix.c
+++ b/src/os/unix.c
@@ -23,7 +23,6 @@
  * Boston, MA 02110-1301, USA.
  */
 
-#include <errno.h>
 #include "git/common.h"
 
 int git_fopen(git_file *out, const char *path, int flags)
@@ -74,3 +73,11 @@ int git_fwrite(git_file fd, void *buf, size_t cnt)
 	}
 	return GIT_SUCCESS;
 }
+
+off_t git_fsize(git_file fd)
+{
+	struct stat sb;
+	if (fstat(fd, &sb))
+		return -1;
+	return sb.st_size;
+}