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).
In Linux kernels before 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 appropriately.
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)/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.