---
name: "sendfile"
section: "2"
platform: "macos"
package: "xnu"
version: "12377.121.6"
release: "macOS 26.5"
platforms: ["linux", "freebsd", "macos"]
url: "https://new.carta.tech/sendfile/2"
---

# sendfile(2)

send a file to a socket

## NAME

**sendfile** – send a file to a socket

## SYNOPSIS

**#include \<sys/types.h\>**

**#include \<sys/socket.h\>**

**#include \<sys/uio.h\>**

*int*\

**sendfile**(*int fd*, *int s*, *off\_t offset*, *off\_t \*len*, *struct sf\_hdtr \*hdtr*, *int flags*);

## DESCRIPTION

The **sendfile**() system call sends a regular file specified by descriptor *fd* out a stream socket specified by descriptor *s*.

The *offset* argument specifies where to begin in the file. Should *offset* fall beyond the end of file, the system will return success and report 0 bytes sent as described below.

The *len* argument is a value-result parameter, that specifies how many bytes  of the file should be sent and/or how many bytes have been sent. Initially the value pointed to by the  *len* argument specifies how many bytes should be sent with 0 having the special meaning to send until the end of file has been reached. On return the value pointed to by the  *len* argument indicates how many bytes have been sent, except when a header or  trailer is specified as shown below. The  *len* pointer may not be NULL.

An optional header and/or trailer can be sent before and after the file data by  specifying a pointer to a *struct sf\_hdtr*, which has the following structure:

```
struct sf_hdtr {
	struct iovec *headers;	/* pointer to header iovecs */
	int hdr_cnt;		/* number of header iovecs */
	struct iovec *trailers;	/* pointer to trailer iovecs */
	int trl_cnt;		/* number of trailer iovecs */
};
```

The *headers* and *trailers* pointers, if non-NULL, point to arrays of *struct iovec* structures. See the **writev**() system call for information on the iovec structure. The number of iovecs in these arrays is specified by *hdr\_cnt* and *trl\_cnt*.

When a header or trailer is specified, the value of  *len* argument indicates the maximum number of bytes in the header and/or file to be sent. It does not control the trailer; if a trailer exists, all of it will be sent. If the value of  *len* argument is 0, all of the header and/or file will be sent before the entire trailer is sent. On return, the *len* argument specifies the total number of bytes sent.

The *flags* parameter is reserved for future expansion and must be set to 0. Any other value  will cause **sendfile**() to return EINVAL.

When using a socket marked for non-blocking I/O, **sendfile**() may send fewer bytes than requested. In this case, the number of bytes successfully sent is returned in the via the  *len* parameters and the error EAGAIN is returned.

When a signal causes  **sendfile**() to return the error  EINTR, the  *len* argument may return 0 without necessarily meaning the end of file has been reached as the signal may have been caught before any data was sent.

## IMPLEMENTATION NOTES

The Mac OS X  implementation of  **sendfile**() uses 64 bits types for size and offset parameters so there is no need for  a 64 bits version  **sendfile64**() as found on some other operating systems.

## RETURN VALUES

The **sendfile()** function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

The number of bytes sent is returned via the parameter *len*. A value of 0 means the end of the file specified by descriptor  *fd* has been reached or that the value passed in  *offset* falls beyond the end of file.

## ERRORS

- \[EAGAIN\]

  The socket is marked for non-blocking I/O and not all data was sent due to the socket buffer being full. If specified, the number of bytes successfully sent will be returned in *\*len*.

- \[EBADF\]

  The *fd* argument is not a valid file descriptor.

- \[ENOTSUP\]

  The *fd* argument does not refer to a regular file.

- \[EBADF\]

  The *s* argument is not a valid socket descriptor.

- \[ENOTSOCK\]

  The *s* argument does not refer stream oriented socket.

- \[EFAULT\]

  An invalid address was specified for an argument.

- \[EINTR\]

  A signal interrupted **sendfile**() before it could be completed. If specified, the number of bytes successfully sent will be returned in *\*len*.

- \[EINVAL\]

  The *offset* argument is negative.

- \[EINVAL\]

  The *len* argument is NULL.

- \[EINVAL\]

  The *flags* argument is not set to 0.

- \[EIO\]

  An error occurred while reading from *fd*.

- \[ENOTCONN\]

  The *s* argument points to an unconnected socket.

- \[ENOTSOCK\]

  The *s* argument is not a socket.

- \[EOPNOTSUPP\]

  The file system for descriptor *fd* does not support **sendfile**().

- \[EPIPE\]

  The socket peer has closed the connection.

## SEE ALSO

[open(2)](/open/2.md), [send(2)](/send/2.md), [socket(2)](/socket/2.md), [writev(2)](/writev/2.md)

## HISTORY

The **sendfile**() system call first appeared in Darwin 9.0 (Mac OS X version 10.5) .

## AUTHORS

This manual page is based on the FreeBSD version written by David G. Lawrence ⟨dg@dglawrence.com⟩