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

sendfile - send files over sockets or copy files to files

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. 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.

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.

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:

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

open(2), libsendfile(3LIB), sendfilev(3EXT), socket(3SOCKET), attributes(5), lf64(5)
July 19, 2018 OmniOS