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RedHat 9 (Linux i386) - man page for sendfile (redhat section 2)

SENDFILE(2)						     Linux Programmer's Manual						       SENDFILE(2)

sendfile - transfer data between file descriptors
#include <sys/sendfile.h> ssize_t sendfile(int out_fd, int in_fd, off_t *offset, size_t count);
This call copies data between one file descriptor and another. Either or both of these file descriptors may refer to a socket (but see below). in_fd should be a file descriptor opened for reading and out_fd should be a descriptor opened for writing. offset is a pointer to a variable holding the input file pointer position from which sendfile() will start reading data. When sendfile() returns, this variable will be set to the offset of the byte following the last byte that was read. count is the number of bytes to copy between file descrip- tors. Because this copying is done within the kernel, sendfile() does not need to spend time transferring data to and from user space.
Sendfile does not modify the current file pointer of in_fd, but does for out_fd. 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, please see the TCP_CORK option in tcp(7) to minimize the number of packets and to tune performance. Presently the descriptor from which data is read cannot correspond to a socket, it must correspond to a file which supports mmap()-like operations.
If the transfer was successful, the number of bytes written to out_fd is returned. On error, -1 is returned, and errno is set appropri- ately.
EBADF The input file was not opened for reading or the output file was not opened for writing. EINVAL Descriptor is not valid or locked. ENOMEM Insufficient memory to read from in_fd. EIO Unspecified error while reading from in_fd.
sendfile is a new feature in Linux 2.2. The include file <sys/sendfile.h> is present since glibc2.1. Other Unixes often implement sendfile with different semantics and prototypes. It should not be used in portable programs.
socket(2), open(2) Linux Man Page 1998-12-01 SENDFILE(2)