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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | STANDARDS | HISTORY | NOTES | SEE ALSO | COLOPHON |
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sendfile(2) System Calls Manual sendfile(2)
sendfile - transfer data between file descriptors
Standard C library (libc, -lc)
#include <sys/sendfile.h>
ssize_t sendfile(int out_fd, int in_fd, off_t *_Nullable offset,
size_t count);
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) and write(2), which
would require transferring data to and from user space.
in_fd should be a file descriptor opened for reading and out_fd
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
in_fd. 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 in_fd; otherwise the file offset is adjusted to reflect
the number of bytes read from in_fd.
If offset is NULL, then data will be read from in_fd 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 in_fd argument must correspond to a file which supports
mmap(2)-like operations (i.e., it cannot be a socket). Except
since Linux 5.12 and if out_fd is a pipe, in which case sendfile()
desugars to a splice(2) and its restrictions apply.
Before Linux 2.6.33, out_fd 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.
If the transfer was successful, the number of bytes written to
out_fd 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 is set to indicate the error.
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)-like
operation is not available for in_fd, or count is negative.
EINVAL out_fd has the O_APPEND flag set. This is not currently
supported by sendfile().
EIO Unspecified error while reading from in_fd.
ENOMEM Insufficient memory to read from in_fd.
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.
Other UNIX systems implement sendfile() with different semantics
and prototypes. It should not be used in portable programs.
None.
Linux 2.2, glibc 2.1.
In Linux 2.4 and earlier, out_fd 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.
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), to minimize the number of packets and to tune
performance.
Applications may wish to fall back to read(2) and write(2) in the
case where sendfile() fails with EINVAL or ENOSYS.
If out_fd refers to a socket or pipe with zero-copy support,
callers must ensure the transferred portions of the file referred
to by in_fd remain unmodified until the reader on the other end of
out_fd has consumed the transferred data.
The Linux-specific splice(2) call supports transferring data
between arbitrary file descriptors provided one (or both) of them
is a pipe.
copy_file_range(2), mmap(2), open(2), socket(2), splice(2)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.15 2025-05-17 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)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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