diff --git a/lib/std/c/darwin.zig b/lib/std/c/darwin.zig
index 881e52c72..2134e0e7a 100644
--- a/lib/std/c/darwin.zig
+++ b/lib/std/c/darwin.zig
@@ -63,8 +63,8 @@ pub const sf_hdtr = extern struct {
 };
 
 pub extern "c" fn sendfile(
-    out_fd: fd_t,
     in_fd: fd_t,
+    out_fd: fd_t,
     offset: off_t,
     len: *off_t,
     sf_hdtr: ?*sf_hdtr,
diff --git a/lib/std/c/freebsd.zig b/lib/std/c/freebsd.zig
index 2c4820bbe..3a0634dbb 100644
--- a/lib/std/c/freebsd.zig
+++ b/lib/std/c/freebsd.zig
@@ -15,9 +15,9 @@ pub const sf_hdtr = extern struct {
     trl_cnt: c_int,
 };
 pub extern "c" fn sendfile(
-    out_fd: fd_t,
     in_fd: fd_t,
-    offset: ?*off_t,
+    out_fd: fd_t,
+    offset: off_t,
     nbytes: usize,
     sf_hdtr: ?*sf_hdtr,
     sbytes: ?*off_t,
diff --git a/lib/std/os.zig b/lib/std/os.zig
index 90ad3e03a..a6c8b32ba 100644
--- a/lib/std/os.zig
+++ b/lib/std/os.zig
@@ -3734,7 +3734,8 @@ pub fn sendfile(
 
             while (true) {
                 var sbytes: off_t = undefined;
-                const err = errno(system.sendfile(out_fd, in_fd, in_offset, adjusted_count, hdtr, &sbytes, flags));
+                const offset = @bitCast(off_t, in_offset);
+                const err = errno(system.sendfile(in_fd, out_fd, offset, adjusted_count, hdtr, &sbytes, flags));
                 const amt = @bitCast(usize, sbytes);
                 switch (err) {
                     0 => return amt,
@@ -3813,19 +3814,17 @@ pub fn sendfile(
             while (true) {
                 var sbytes: off_t = adjusted_count;
                 const signed_offset = @bitCast(i64, in_offset);
-                const err = errno(system.sendfile(out_fd, in_fd, signed_offset, &sbytes, hdtr, flags));
+                const err = errno(system.sendfile(in_fd, out_fd, signed_offset, &sbytes, hdtr, flags));
                 const amt = @bitCast(usize, sbytes);
                 switch (err) {
                     0 => return amt,
 
+                    EBADF => unreachable, // Always a race condition.
                     EFAULT => unreachable, // Segmentation fault.
                     EINVAL => unreachable,
                     ENOTCONN => unreachable, // `out_fd` is an unconnected socket.
 
-                    // On macOS version 10.14.6, I observed Darwin return EBADF when
-                    // using sendfile on a valid open file descriptor of a file
-                    // system file.
-                    ENOTSUP, ENOTSOCK, ENOSYS, EBADF => break :sf,
+                    ENOTSUP, ENOTSOCK, ENOSYS => break :sf,
 
                     EINTR => if (amt != 0) return amt else continue,