diff --git a/src/posix.c b/src/posix.c
index 9dbebbd57..4bb8c3246 100644
--- a/src/posix.c
+++ b/src/posix.c
@@ -32,9 +32,9 @@ int p_read(git_file fd, void *buf, size_t cnt)
 	return (int)(b - (char *)buf);
 }
 
-int p_write(git_file fd, void *buf, size_t cnt)
+int p_write(git_file fd, const void *buf, size_t cnt)
 {
-	char *b = buf;
+	const char *b = buf;
 	while (cnt) {
 		ssize_t r = write(fd, b, cnt);
 		if (r < 0) {
diff --git a/src/posix.h b/src/posix.h
index eaa89383d..622bf8351 100644
--- a/src/posix.h
+++ b/src/posix.h
@@ -40,7 +40,7 @@ typedef int git_file;
 extern int p_open(const char *path, int flags);
 extern int p_creat(const char *path, int mode);
 extern int p_read(git_file fd, void *buf, size_t cnt);
-extern int p_write(git_file fd, void *buf, size_t cnt);
+extern int p_write(git_file fd, const void *buf, size_t cnt);
 extern int p_getcwd(char *buffer_out, size_t size);
 
 #define p_lseek(f,n,w) lseek(f, n, w)