SENDFILE(3EXT)SENDFILE(3EXT)NAMEsendfile - send files over sockets or copy files to files
SYNOPSIS
cc [ flag... ] file... -lsendfile [ library... ]
#include <sys/sendfile.h>
ssize_t sendfile(int out_fd, int in_fd, off_t *off, size_t len);
DESCRIPTION
The sendfile() function copies data from in_fd to out_fd starting at
offset off and of length len bytes. The in_fd argument should be a file
descriptor to a regular file opened for reading. See open(2). The
out_fd argument should be a file descriptor to a regular file opened
for writing or to a connected AF_INET or AF_INET6 socket of SOCK_STREAM
type. See socket(3SOCKET). The off argument is a pointer to a variable
holding the input file pointer position from which the data will be
read. After sendfile() has completed, the variable will be set to the
offset of the byte following the last byte that was read. The send‐
file() function does not modify the current file pointer of in_fd, but
does modify the file pointer for out_fd if it is a regular file.
The sendfile() function can also be used to send buffers by pointing
in_fd to SFV_FD_SELF.
RETURN VALUES
Upon successful completion, sendfile() returns the total number of
bytes written to out_fd and also updates the offset to point to the
byte that follows the last byte read. Otherwise, it returns -1, and
errno is set to indicate the error.
ERRORS
The sendfile() function will fail if:
EAFNOSUPPORT
The implementation does not support the specified
address family for socket.
EAGAIN
Mandatory file or record locking is set on either the
file descriptor or output file descriptor if it points
at regular files. O_NDELAY or O_NONBLOCK is set, and
there is a blocking record lock. An attempt has been
made to write to a stream that cannot accept data with
the O_NDELAY or the O_NONBLOCK flag set.
EBADF
The out_fd or in_fd argument is either not a valid file
descriptor, out_fd is not opened for writing. or in_fd
is not opened for reading.
EINVAL
The offset cannot be represented by the off_t struc‐
ture, or the length is negative when cast to ssize_t.
EIO
An I/O error occurred while accessing the file system.
ENOTCONN
The socket is not connected.
EOPNOTSUPP
The socket type is not supported.
EPIPE
The out_fd argument is no longer connected to the peer
endpoint.
EINTR
A signal was caught during the write operation and no
data was transferred.
USAGE
The sendfile() function has a transitional interface for 64-bit file
offsets. See lf64(5).
EXAMPLES
Example 1 Sending a Buffer Over a Socket
The following example demonstrates how to send the buffer buf over a
socket. At the end, it prints the number of bytes transferred over the
socket from the buffer. It assumes that addr will be filled up appro‐
priately, depending upon where to send the buffer.
int tfd;
off_t baddr;
struct sockaddr_in sin;
char buf[64 * 1024];
in_addr_t addr;
size_t len;
tfd = socket(AF_INET, SOCK_STREAM, 0);
if (tfd == -1) {
perror("socket");
exit(1);
}
sin.sin_family = AF_INET;
sin.sin_addr.s_addr = addr; /* Fill in the appropriate address. */
sin.sin_port = htons(2345);
if (connect(tfd, (struct sockaddr *)&sin, sizeof(sin))<0) {
perror("connect");
exit(1);
}
baddr = (off_t)buf;
len = sizeof(buf);
while (len > 0) {
ssize_t res;
res = sendfile(tfd, SFV_FD_SELF, &baddr, len);
if (res == -1)
if (errno != EINTR) {
perror("sendfile");
exit(1);
} else continue;
len -= res;
}
Example 2 Transferring Files to Sockets
The following program demonstrates a transfer of files to sockets:
int ffd, tfd;
off_t off;
struct sockaddr_in sin;
in_addr_t addr;
int len;
struct stat stat_buf;
ssize_t len;
ffd = open("file", O_RDONLY);
if (ffd == -1) {
perror("open");
exit(1);
}
tfd = socket(AF_INET, SOCK_STREAM, 0);
if (tfd == -1) {
perror("socket");
exit(1);
}
sin.sin_family = AF_INET;
sin.sin_addr = addr; /* Fill in the appropriate address. */
sin.sin_port = htons(2345);
if (connect(tfd, (struct sockaddr *) &sin, sizeof(sin)) <0) {
perror("connect");
exit(1);
}
if (fstat(ffd, &stat_buf) == -1) {
perror("fstat");
exit(1);
}
len = stat_buf.st_size;
while (len > 0) {
ssize_t res;
res = sendfile(tfd, ffd, &off, len);
if (res == -1)
if (errno != EINTR) {
perror("sendfile");
exit(1);
} else continue;
len -= res;
}
ATTRIBUTES
See attributes(5) for descriptions of the following attributes:
┌────────────────────┬─────────────────┐
│ ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
├────────────────────┼─────────────────┤
│Interface Stability │ Evolving │
├────────────────────┼─────────────────┤
│MT-Level │ MT-Safe │
└────────────────────┴─────────────────┘
SEE ALSOopen(2), libsendfile(3LIB), sendfilev(3EXT), socket(3SOCKET),
attributes(5), lf64(5)
May 31, 2006 SENDFILE(3EXT)
