NAME
sendfile - transfer data between file descriptors SYNOPSIS
#include
ssizet sendfile(int outfd, int infd, offt *offset, sizet count); DESCRIPTION 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. infd should be a file descriptor opened for reading and outfd 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 infd. 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 current file offset of infd; otherwise the current file offset is adjusted to reflect the number of bytes read from infd. If offset is NULL, then data will be read from infd starting at the current 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 infd 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, outfd must refer to a socket. Since Linux 2.6.33 it can be any file. If it is a regular file, then send‐ file() changes the file offset appropriately. RETURN VALUE If the transfer was successful, the number of bytes written to outfd
is returned. On error, -1 is returned, and errno is set appropriately. ERRORS EAGAIN Nonblocking I/O has been selected using ONONBLOCK 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 infd. EIO Unspecified error while reading from infd. ENOMEM Insufficient memory to read from infd. VERSIONS sendfile() is a new feature in Linux 2.2. The include file
is present since glibc 2.1. CONFORMING TO Not specified in POSIX.1-2001, or other standards. Other UNIX systems implement sendfile() with different semantics and prototypes. It should not be used in portable programs. NOTES 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 TCPCORK option, described in tcp(7), to minimize the number of packets and to tune performance. In Linux 2.4 and earlier, outfd could also refer to a regular file, and sendfile() changed the current offset of that file. 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. Applications may wish to fall back to read(2)/write(2) in the case where sendfile() fails with EINVAL or ENOSYS.
The Linux-specific splice(2) call supports transferring data between arbitrary files (e.g., a pair of sockets). SEE ALSO mmap(2), open(2), socket(2), splice(2) COLOPHON
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Linux 2011-09-14 SENDFILE(2)