sendfile(2) System Calls Manual sendfile(2)
NAME
sendfile - transfer data between file descriptors
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <sys/sendfile.h>
ssize_t sendfile(int out_fd, int in_fd, off_t *_Nullable offset,
size_t count);
DESCRIPTION
sendfile() copies data between one file descriptor and another. Be-
cause this copying is done within the kernel, sendfile() is more effi-
cient 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.
RETURN VALUE
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.
ERRORS
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 sup-
ported 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.
VERSIONS
Other UNIX systems implement sendfile() with different semantics and
prototypes. It should not be used in portable programs.
STANDARDS
None.
HISTORY
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 func-
tion transparently deals with the kernel differences.
NOTES
sendfile() will transfer at most 0x7ffff000 (2,147,479,552) bytes, re-
turning 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 con-
sumed 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.
SEE ALSO
copy_file_range(2), mmap(2), open(2), socket(2), splice(2)
Linux man-pages 6.7 2023-12-21 sendfile(2)
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