sendfile - transfer data between file descriptors
#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.
copy_file_range(2), mmap(2), open(2), socket(2), splice(2)