cc [ flag.\|.\|. ] file.\|.\|. -lsendfile [ library.\|.\|. ]
#include <sys/sendfile.h>

ssize_t sendfile(int out_fd, int in_fd, off_t *off, size_t len);

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.

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.

The sendfile() function will fail if:

EAFNOSUPPORT

EAGAIN

EBADF

EINVAL

EIO

ENOTCONN

EOPNOTSUPP

EPIPE

EINTR

The sendfile() function has a transitional interface for 64-bit file offsets. See lf64(5).

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;
}

See attributes(5) for descriptions of the following attributes:

open(2), libsendfile(3LIB), sendfilev(3EXT), socket(3SOCKET), attributes(5), lf64(5)