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).
Before Linux 2.6.33,
out_fd must refer to a socket. Since Linux 2.6.33 it
can be any file. If it is a regular file, 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.
sendfile() first appeared in Linux 2.2. The include file
<sys/sendfile.h> is present since glibc 2.1.
Not specified in POSIX.1-2001, nor in other standards.
Other UNIX systems implement
sendfile() with different semantics and
prototypes. It should not be used in portable programs.
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.
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.
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)