您需要告诉我们您的（所需）操作系统。适当的调用（或者说最合适的调用）将是非常系统特定的。
在 Linux/*BSD/Mac 中，您将使用 sendfile(2)，它处理内核空间中的复制。
概要
#include <sys/sendfile.h>
ssize_t sendfile(int out_fd, int in_fd, off_t *offset, size_t count);
描述
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.
in_fd should be a file descriptor opened for reading and out_fd should be a
descriptor opened for writing.
进一步阅读：
sendfile 示例的服务器部分：
/*
Server portion of sendfile example.
usage: server [port]
Copyright (C) 2003 Jeff Tranter.
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
*/
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <fcntl.h>
#include <sys/sendfile.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/stat.h>
#include <netinet/in.h>
int main(int argc, char **argv)
{
int port = 1234; /* port number to use */
int sock; /* socket desciptor */
int desc; /* file descriptor for socket */
int fd; /* file descriptor for file to send */
struct sockaddr_in addr; /* socket parameters for bind */
struct sockaddr_in addr1; /* socket parameters for accept */
int addrlen; /* argument to accept */
struct stat stat_buf; /* argument to fstat */
off_t offset = 0; /* file offset */
char filename[PATH_MAX]; /* filename to send */
int rc; /* holds return code of system calls */
/* check command line arguments, handling an optional port number */
if (argc == 2) {
port = atoi(argv[1]);
if (port <= 0) {
fprintf(stderr, "invalid port: %s\n", argv[1]);
exit(1);
}
} else if (argc != 1) {
fprintf(stderr, "usage: %s [port]\n", argv[0]);
exit(1);
}
/* create Internet domain socket */
sock = socket(AF_INET, SOCK_STREAM, 0);
if (sock == -1) {
fprintf(stderr, "unable to create socket: %s\n", strerror(errno));
exit(1);
}
/* fill in socket structure */
memset(&addr, 0, sizeof(addr));
addr.sin_family = AF_INET;
addr.sin_addr.s_addr = INADDR_ANY;
addr.sin_port = htons(port);
/* bind socket to the port */
rc = bind(sock, (struct sockaddr *)&addr, sizeof(addr));
if (rc == -1) {
fprintf(stderr, "unable to bind to socket: %s\n", strerror(errno));
exit(1);
}
/* listen for clients on the socket */
rc = listen(sock, 1);
if (rc == -1) {
fprintf(stderr, "listen failed: %s\n", strerror(errno));
exit(1);
}
while (1) {
/* wait for a client to connect */
desc = accept(sock, (struct sockaddr *) &addr1, &addrlen);
if (desc == -1) {
fprintf(stderr, "accept failed: %s\n", strerror(errno));
exit(1);
}
/* get the file name from the client */
rc = recv(desc, filename, sizeof(filename), 0);
if (rc == -1) {
fprintf(stderr, "recv failed: %s\n", strerror(errno));
exit(1);
}
/* null terminate and strip any \r and \n from filename */
filename[rc] = '\0';
if (filename[strlen(filename)-1] == '\n')
filename[strlen(filename)-1] = '\0';
if (filename[strlen(filename)-1] == '\r')
filename[strlen(filename)-1] = '\0';
/* exit server if filename is "quit" */
if (strcmp(filename, "quit") == 0) {
fprintf(stderr, "quit command received, shutting down server\n");
break;
}
fprintf(stderr, "received request to send file %s\n", filename);
/* open the file to be sent */
fd = open(filename, O_RDONLY);
if (fd == -1) {
fprintf(stderr, "unable to open '%s': %s\n", filename, strerror(errno));
exit(1);
}
/* get the size of the file to be sent */
fstat(fd, &stat_buf);
/* copy file using sendfile */
offset = 0;
rc = sendfile (desc, fd, &offset, stat_buf.st_size);
if (rc == -1) {
fprintf(stderr, "error from sendfile: %s\n", strerror(errno));
exit(1);
}
if (rc != stat_buf.st_size) {
fprintf(stderr, "incomplete transfer from sendfile: %d of %d bytes\n",
rc,
(int)stat_buf.st_size);
exit(1);
}
/* close descriptor for file that was sent */
close(fd);
/* close socket descriptor */
close(desc);
}
/* close socket */
close(sock);
return 0;
}