我正在开发一个代码来实现一个停止并等待将发送500个数据包的UDP服务器。但是，我只收到255个ACK而不是500个，而且无法找出原因。
客户代码：
#include "headsock.h"
#define TRUE 1
#define FALSE 0
float str_cli(FILE *fp, int sockfd, long *len, struct sockaddr *addr, int addrlen, socklen_t *len_recvfrom); // communication function
void tv_sub(struct timeval *out, struct timeval *in); //calculate the time interval between out and in
int main(int argc, char **argv)
{
int sockfd;
float ti, rt;
long len;
struct sockaddr_in ser_addr;
char ** pptr;
struct hostent *sh;
struct in_addr **addrs;
FILE *fp;
socklen_t len_recvfrom;
if (argc != 2)
{
printf("parameters not match");
exit(0);
}
sh = gethostbyname(argv[1]); // get host's information
if (sh == NULL) {
printf("error when gethostby name");
exit(0);
}
addrs = (struct in_addr **)sh->h_addr_list;
printf("canonical name: %s\n", sh->h_name); // print the remote host's information
for (pptr=sh->h_aliases; *pptr != NULL; pptr++)
printf("the aliases name is: %s\n", *pptr);
switch(sh->h_addrtype)
{
case AF_INET: // the address family that is used for the socket you're creating (in this case an Internet Protocol address)
printf("AF_INET\n");
break;
default:
printf("unknown addrtype\n");
break;
}
sockfd = socket(AF_INET, SOCK_DGRAM, 0); // create the socket
if (sockfd <0)
{
printf("error in socket");
exit(1);
}
ser_addr.sin_family = AF_INET; // address format is host and port number
ser_addr.sin_port = htons(MYUDP_PORT);
//copies count characters from the object pointed to by src to the object pointed to by dest
memcpy(&(ser_addr.sin_addr.s_addr), *addrs, sizeof(struct in_addr));
bzero(&(ser_addr.sin_zero), 8);
if((fp = fopen ("myfile.txt","r+t")) == NULL)
{
printf("File doesn't exit\n");
exit(0);
}
// perform the transmission and receiving
ti = str_cli(fp, sockfd, &len, (struct sockaddr *)&ser_addr, sizeof(struct sockaddr_in), &len_recvfrom);
rt = (len / (float)ti); // caculate the average transmission rate
printf("Time(ms) : %.3f, Data sent(byte): %d\nData rate: %f (Kbytes/s)\n", ti, (int)len, rt);
close(sockfd);
fclose(fp);
exit(0);
}
// transmission function
float str_cli(FILE *fp, int sockfd, long *len, struct sockaddr *addr, int addrlen, socklen_t *len_recvfrom)
{
char *buf;
long lsize, ci;
char sends[DATALEN];
struct ack_so ack;
int n, slen;
float time_inv = 0.0;
struct timeval sendt, recvt;
ci = 0;
int prev_msg_acked = TRUE;
int ack_number = 1;
fseek(fp, 0, SEEK_END);
lsize = ftell (fp);
rewind(fp);
printf("The file length is %d bytes\n", (int)lsize);
printf("the packet length is %d bytes\n", DATALEN);
// allocate memory to contain the whole file.
buf = (char *) malloc(lsize);
if (buf == NULL)
exit (2);
// copy the file into the buffer.
// read lsize data elements, each 1 byte
fread(buf, 1, lsize, fp);
// the whole file is loaded in the buffer
buf[lsize] ='\0'; // append the end byte
gettimeofday(&sendt, NULL); // get the current time
while(ci <= lsize)
{
if (prev_msg_acked) // only transmits when previous message has been acknowledged
{
if ((lsize+1-ci) <= DATALEN) // final string
slen = lsize+1-ci;
else // send message of length DATALEN
slen = DATALEN;
memcpy(sends, (buf+ci), slen);
/*************** SEND MESSAGE ***************/
if((n = sendto(sockfd, &sends, slen, 0, addr, addrlen)) == -1) {
printf("send error!\n"); // send the data
exit(1);
}
// update the expected ACK number
if (ack_number == 1)
ack_number = 0;
else
ack_number = 1;
ci += slen;
}
/*************** RECEIVE ACK ***************/
if ((n = recvfrom(sockfd, &ack, 2, 0, addr, &addrlen)) == -1)
{
printf("error when receiving\n");
exit(1);
}
if (ack.num != ack_number || ack.len != slen) // ACK wrong
{
printf("%i %i expected\n", ack_number, slen);
printf("%i %i received\n", ack.num, ack.len);
printf("ACK check fails, retransmission...\n");
prev_msg_acked = FALSE;
}
else
prev_msg_acked = TRUE;
}
gettimeofday(&recvt, NULL);
*len= ci; // get current time
tv_sub(&recvt, &sendt); // get the whole trans time
time_inv += (recvt.tv_sec)*1000.0 + (recvt.tv_usec)/1000.0;
return(time_inv);
}
//calculate the time interval between out and in
void tv_sub(struct timeval *out, struct timeval *in)
{
if ((out->tv_usec -= in->tv_usec) <0)
{
--out ->tv_sec;
out ->tv_usec += 1000000;
}
out->tv_sec -= in->tv_sec;
}
服务器代码：
#include "headsock.h"
void str_ser(int sockfd, int *ack_number); // transmitting and receiving function
int main(int argc, char *argv[])
{
int sockfd;
struct sockaddr_in my_addr;
int ack_number = 1;
//create socket
if ((sockfd = socket(AF_INET, SOCK_DGRAM, 0)) == -1) {
printf("error in socket\n");
exit(1);
}
my_addr.sin_family = AF_INET; // Address family; must be AF_INET
my_addr.sin_port = htons(MYUDP_PORT); // Internet Protocol (IP) port.
my_addr.sin_addr.s_addr = INADDR_ANY; // IP address in network byte order. INADDR_ANY is 0.0.0.0 meaning "all the addr"
// places nbyte null bytes in the string s
// this function will be used to set all the socket structures with null values
bzero(&(my_addr.sin_zero), 8);
// binds the socket to all available interfaces
if (bind(sockfd, (struct sockaddr *) &my_addr, sizeof(struct sockaddr)) == -1) {
printf("error in binding\n");
perror("socket error");
exit(1);
}
// receive and ACK
str_ser(sockfd, &ack_number);
close(sockfd);
exit(0);
}
// transmitting and receiving function
void str_ser(int sockfd, int *ack_number)
{
FILE *fp;
char buf[BUFSIZE];
char recvs[DATALEN];
int end = 0, n = 0;
long lseek = 0;
struct ack_so ack;
struct sockaddr_in addr;
socklen_t len = sizeof(struct sockaddr_in);
printf("receiving data!\n");
while(!end)
{
// receive the packet
if ((n = recvfrom(sockfd, &recvs, DATALEN, 0, (struct sockaddr *)&addr, &len)) == -1)
{
printf("error when receiving\n");
exit(1);
}
// toggle the ack_number
if (*ack_number == 1)
*ack_number = 0;
else
*ack_number = 1;
// if the last bit of the received string is the EoF
if (recvs[n-1] == '\0')
{
end = 1;
n--;
}
memcpy((buf+lseek), recvs, n);
lseek += n;
// up to here, successfully received a packet
// send ACK back
ack.num = *ack_number;
ack.len = n;
printf("%i %i as ACK sent\n", ack.num, ack.len);
if ((n = sendto(sockfd, &ack, 2, 0, (struct sockaddr *)&addr, len)) == -1)
{
printf("ACK send error!\n");
exit(1);
}
}
if ((fp = fopen ("myUDPreceive.txt", "wt")) == NULL)
{
printf("File doesn't exit\n");
exit(0);
}
fwrite (buf, 1, lseek, fp); //write data into file
fclose(fp);
printf("A file has been successfully received!\nThe total data received is %d bytes\n", (int)lseek);
}