sendfile(2) - Linux manual page (original) (raw)
sendfile(2) System Calls Manual sendfile(2)
NAME top
sendfile - transfer data between file descriptorsLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#include <sys/sendfile.h>**
**ssize_t sendfile(int** _outfd_**, int** _infd_**, off_t *_Nullable** _offset_**,**
**size_t** _count_**);**DESCRIPTION top
**sendfile**() copies data between one file descriptor and another.
Because this copying is done within the kernel, **sendfile**() is more
efficient than the combination of [read(2)](../man2/read.2.html) and [write(2)](../man2/write.2.html), which
would require transferring data to and from user space.
_infd_ should be a file descriptor opened for reading and _outfd_
should be a descriptor opened for writing.
If _offset_ is not NULL, then it points to a variable holding the
file offset from which **sendfile**() will start reading data from
_infd_. When **sendfile**() returns, this variable will be set to the
offset of the byte following the last byte that was read. If
_offset_ is not NULL, then **sendfile**() does not modify the file
offset of _infd_; otherwise the file offset is adjusted to reflect
the number of bytes read from _infd_.
If _offset_ is NULL, then data will be read from _infd_ starting at
the file offset, and the file offset will be updated by the call.
_count_ is the number of bytes to copy between the file descriptors.
The _infd_ argument must correspond to a file which supports
[mmap(2)](../man2/mmap.2.html)-like operations (i.e., it cannot be a socket). Except
since Linux 5.12 and if _outfd_ is a pipe, in which case **sendfile**()
desugars to a [splice(2)](../man2/splice.2.html) and its restrictions apply.
Before Linux 2.6.33, _outfd_ must refer to a socket. Since Linux
2.6.33 it can be any file. If it's seekable, then **sendfile**()
changes the file offset appropriately.RETURN VALUE top
If the transfer was successful, the number of bytes written to
_outfd_ is returned. Note that a successful call to **sendfile**() may
write fewer bytes than requested; the caller should be prepared to
retry the call if there were unsent bytes. See also NOTES.
On error, -1 is returned, and _[errno](../man3/errno.3.html)_ is set to indicate the error.ERRORS top
**EAGAIN** Nonblocking I/O has been selected using **O_NONBLOCK** and the
write would block.
**EBADF** The input file was not opened for reading or the output
file was not opened for writing.
**EFAULT** Bad address.
**EINVAL** Descriptor is not valid or locked, or an [mmap(2)](../man2/mmap.2.html)-like
operation is not available for _infd_, or _count_ is negative.
**EINVAL** _outfd_ has the **O_APPEND** flag set. This is not currently
supported by **sendfile**().
**EIO** Unspecified error while reading from _infd_.
**ENOMEM** Insufficient memory to read from _infd_.
**EOVERFLOW**
_count_ is too large, the operation would result in exceeding
the maximum size of either the input file or the output
file.
**ESPIPE** _offset_ is not NULL but the input file is not seekable.VERSIONS top
Other UNIX systems implement **sendfile**() with different semantics
and prototypes. It should not be used in portable programs.STANDARDS top
None.HISTORY top
Linux 2.2, glibc 2.1.
In Linux 2.4 and earlier, _outfd_ could also refer to a regular
file; this possibility went away in the Linux 2.6.x kernel series,
but was restored in Linux 2.6.33.
The original Linux **sendfile**() system call was not designed to
handle large file offsets. Consequently, Linux 2.4 added
**sendfile64**(), with a wider type for the _offset_ argument. The
glibc **sendfile**() wrapper function transparently deals with the
kernel differences.NOTES top
**sendfile**() will transfer at most 0x7ffff000 (2,147,479,552) bytes,
returning the number of bytes actually transferred. (This is true
on both 32-bit and 64-bit systems.)
If you plan to use **sendfile**() for sending files to a TCP socket,
but need to send some header data in front of the file contents,
you will find it useful to employ the **TCP_CORK** option, described
in [tcp(7)](../man7/tcp.7.html), to minimize the number of packets and to tune
performance.
Applications may wish to fall back to [read(2)](../man2/read.2.html) and [write(2)](../man2/write.2.html) in the
case where **sendfile**() fails with **EINVAL** or **ENOSYS**.
If _outfd_ refers to a socket or pipe with zero-copy support,
callers must ensure the transferred portions of the file referred
to by _infd_ remain unmodified until the reader on the other end of
_outfd_ has consumed the transferred data.
The Linux-specific [splice(2)](../man2/splice.2.html) call supports transferring data
between arbitrary file descriptors provided one (or both) of them
is a pipe.SEE ALSO top
[copy_file_range(2)](../man2/copy%5Ffile%5Frange.2.html), [mmap(2)](../man2/mmap.2.html), [open(2)](../man2/open.2.html), [socket(2)](../man2/socket.2.html), [splice(2)](../man2/splice.2.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-07-23 sendfile(2)
Pages that refer to this page:hardlink(1), copy_file_range(2), send(2), splice(2), syscalls(2), proc_pid_io(5), socket(7), tcp(7)