SENDFILE(3EXT) Extended Library Functions SENDFILE(3EXT)


NAME


sendfile - 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 sendfile() 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. In some error cases sendfile() may still write some
data before encountering an error and returning -1. When that occurs,
off is updated to point to the byte that follows the last byte copied and
should be compared with its value before calling sendfile() to determine
how much data was sent.

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 structure,
or the length is negative when cast to ssize_t.

Fewer bytes were transferred than were requested.


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. The SIGPIPE signal is generated to the calling
thread. The process dies unless special provisions were
taken to catch or ignore the signal.


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(7).

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
appropriately, 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(7) for descriptions of the following attributes:


+--------------------+-----------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-----------------+
|Interface Stability | Evolving |
+--------------------+-----------------+
|MT-Level | MT-Safe |
+--------------------+-----------------+

SEE ALSO


open(2), sendfilev(3EXT), libsendfile(3LIB), socket(3SOCKET),
attributes(7), lf64(7)


July 19, 2018 SENDFILE(3EXT)