I've just started started socket programming so there's a few things I'm wondering about:
I found a set of socket programming codes here:
http://cs.baylor.edu/~donahoo/practical/CSockets/textcode.html
And the thing I don't really get is here in TCPEchoClient.c:
while (totalBytesRcvd < echoStringLen)
{
/* Receive up to the buffer size (minus 1 to leave space for
a null terminator) bytes from the sender */
if ((bytesRcvd = recv(sock, echoBuffer, RCVBUFSIZE - 1, 0)) <= 0)
DieWithError("recv() failed or connection closed prematurely");
totalBytesRcvd += bytesRcvd; /* Keep tally of total bytes */
echoBuffer[bytesRcvd] = '\0'; /* Terminate the string! */
printf("%s", echoBuffer); /* Print the echo buffer */
}Why did he use a while loop for this? Wouldn't you recieve the same data by just using
if ((bytesRcvd = recv(sock, echoBuffer, RCVBUFSIZE - 1, 0)) <= 0)
DieWithError("recv() failed or connection closed prematurely");In the project I'm doing, I don't really know how long the string I will be recieving is so can I/should I just write something like
if ((bytesRcvd = recv(sock, echoBuffer, RCVBUFSIZE - 1, 0)) <= 0)
DieWithError("recv() failed or connection closed prematurely");without the while loop?
EDIT: I tried it out and yeah, I recieved the same message with or without the while loop so my question still stands, why use a while loop?
But more importantly... the thing I'm having trouble with in my project is...
Okay, say I have a message to send like this:
char message[50] = "Not enough to be 50";
char *msg = message;Will there be any problems if I send the message over like this:
send(int fd, msg, 50,int flags);That is, can I send my message based on the length of the char array rather than the actual number of characters in the string?
The thing is, the length of my message differs depending on the situation. If it is an error message the length would be 51, if it is an acknowledgement message it would be length 10. Is there any way to send the message without using if clauses like:
if there's an error, send message of length 51
else send message of length 10
Also, if anyone needs to see the original TCPEchoClient.c code:
#include <stdio.h> /* for printf() and fprintf() */
#include <sys/socket.h> /* for socket(), connect(), send(), and recv() */
#include <arpa/inet.h> /* for sockaddr_in and inet_addr() */
#include <stdlib.h> /* for atoi() and exit() */
#include <string.h> /* for memset() */
#include <unistd.h> /* for close() */
#define RCVBUFSIZE 32 /* Size of receive buffer */
void DieWithError(char *errorMessage); /* Error handling function */
int main(int argc, char *argv[])
{
int sock; /* Socket descriptor */
struct sockaddr_in echoServAddr; /* Echo server address */
unsigned short echoServPort; /* Echo server port */
char *servIP; /* Server IP address (dotted quad) */
char *echoString; /* String to send to echo server */
char echoBuffer[RCVBUFSIZE]; /* Buffer for echo string */
unsigned int echoStringLen; /* Length of string to echo */
int bytesRcvd, totalBytesRcvd; /* Bytes read in single recv()
and total bytes read */
if ((argc < 3) || (argc > 4)) /* Test for correct number of arguments */
{
fprintf(stderr, "Usage: %s <Server IP> <Echo Word> [<Echo Port>]\n",
argv[0]);
exit(1);
}
servIP = argv[1]; /* First arg: server IP address (dotted quad) */
echoString = argv[2]; /* Second arg: string to echo */
if (argc == 4)
echoServPort = atoi(argv[3]); /* Use given port, if any */
else
echoServPort = 7; /* 7 is the well-known port for the echo service */
/* Create a reliable, stream socket using TCP */
if ((sock = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP)) < 0)
DieWithError("socket() failed");
/* Construct the server address structure */
memset(&echoServAddr, 0, sizeof(echoServAddr)); /* Zero out structure */
echoServAddr.sin_family = AF_INET; /* Internet address family */
echoServAddr.sin_addr.s_addr = inet_addr(servIP); /* Server IP address */
echoServAddr.sin_port = htons(echoServPort); /* Server port */
/* Establish the connection to the echo server */
if (connect(sock, (struct sockaddr *) &echoServAddr, sizeof(echoServAddr)) < 0)
DieWithError("connect() failed");
echoStringLen = strlen(echoString); /* Determine input length */
/* Send the string to the server */
if (send(sock, echoString, echoStringLen, 0) != echoStringLen)
DieWithError("send() sent a different number of bytes than expected");
/* Receive the same string back from the server */
totalBytesRcvd = 0;
printf("Received: "); /* Setup to print the echoed string */
while (totalBytesRcvd < echoStringLen)
{
/* Receive up to the buffer size (minus 1 to leave space for
a null terminator) bytes from the sender */
if ((bytesRcvd = recv(sock, echoBuffer, RCVBUFSIZE - 1, 0)) <= 0)
DieWithError("recv() failed or connection closed prematurely");
totalBytesRcvd += bytesRcvd; /* Keep tally of total bytes */
echoBuffer[bytesRcvd] = '\0'; /* Terminate the string! */
printf("%s", echoBuffer); /* Print the echo buffer */
}
printf("\n"); /* Print a final linefeed */
close(sock);
exit(0);
}