Python writes to C++ program's stdin - c++

Question

I have a client - server model using 2 C++ files. The client and server communicate through a socket. The user enters strings to the client's stdin and then the client passes that to the server. However, I want to simulate the user using a python program. This is my attemp:
from subprocess import Popen, PIPE, STDOUT
host_name = raw_input("Enter host name: ")
port = raw_input("Enter port: ")
p = Popen(["./client", host_name, port], stdout=PIPE, stdin=PIPE, stderr=STDOUT)
p.stdin.write("DEVICE:3:MALFUNCTIONING")
This doesn't work. It starts the client process and the arguments, but does not write the string DEVICE:3:MALFUNCTIONING to it. Any suggestion?
This is briefly my client code:
if (connect(sockfd,(struct sockaddr *) &serv_addr,sizeof(serv_addr)) < 0)
error("ERROR connecting");
printf("Please enter the message: ");
memset(buffer, 0, 256);
fgets(buffer,255,stdin);
n = write(sockfd,buffer,strlen(buffer));
if (n < 0)
error("ERROR writing to socket");
memset(buffer, 0, 256);
n = read(sockfd,buffer,255);
if (n < 0)
error("ERROR reading from socket");
printf("%s\n",buffer);
close(sockfd);

Answer 1

I am not sure how you create your client and server, but here's my implementation which is pretty much the same as yours for the python program simulating the user.
Github gist: https://gist.github.com/yanhan/5791613
I have a few suspicions of what may be going wrong based on your code snippets for the client:
server_addr variable not set properly
bug in the server code
Like mata said, it'll be more helpful if you could show any output from stdout/stderr and stacktraces.
EDIT: Just wanted to add that the following links might help:
http://en.wikibooks.org/wiki/C_Programming/Networking_in_UNIX
http://rabbit.eng.miami.edu/info/functions/internet.html#clientsample
http://rabbit.eng.miami.edu/info/functions/internet.html#serversample
I am pasting my code here for convenience.
sim.py:
from subprocess import Popen, PIPE
def run():
# this should work if you change to
# host_name = raw_input
# port = raw_input
host_name = '127.0.0.1'
port = '8124'
p = Popen(['./client', host_name, port], stdin=PIPE, close_fds=True)
myInput = raw_input()
p.stdin.write(myInput)
p.communicate()
if __name__ == '__main__':
run()
client.c:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#define BUFSZ 256
int main(int argc, char *argv[])
{
int sockfd, ret, exitCode = 0;
char buffer[BUFSZ];
memset(buffer, 0, BUFSZ);
fgets(buffer, sizeof(buffer), stdin);
char *hostname = argv[1];
int port = atoi(argv[2]);
// echo out the stuff just to see
printf("hostname = %s\n", hostname);
printf("port = %d\n", port);
printf("msg = %s\n", buffer);
// creates the socket
sockfd = socket(AF_INET, SOCK_STREAM, 0);
if (sockfd == -1) {
fprintf(stderr, "client: socket() failed\n");
exit(1);
}
struct sockaddr_in sockAddr;
memset(&sockAddr, 0, sizeof(struct sockaddr_in));
sockAddr.sin_family = AF_INET;
sockAddr.sin_port = htons(port);
sockAddr.sin_addr.s_addr = inet_addr(hostname);
// connect to server
ret = connect(sockfd, (const struct sockaddr *)&sockAddr, sizeof(struct sockaddr));
if (ret == -1) {
fprintf(stderr, "client: connect failed\n");
exitCode = 1;
goto done;
}
// send buffer
ssize_t charsSent = send(sockfd, buffer, strlen(buffer)+1, 0);
printf("client: chars sent: %d\n", (int)charsSent);
/*
// this seems to work as well
write(sockfd, buffer, strlen(buffer)+1);
*/
done:
close(sockfd);
return exitCode;
}
server.c:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#define PORTNUM 8124
#define BUFSZ 256
int main(int argc, char *argv[])
{
char buf[BUFSZ];
int ret, exitCode = 1;
// create socket
int sockfd = socket(AF_INET, SOCK_STREAM, 0);
int recvFd;
if (sockfd == -1) {
fprintf(stderr, "server: Error creating socket\n");
exit(1);
}
struct sockaddr_in sockAddr;
memset(&sockAddr, 0, sizeof(struct sockaddr_in));
sockAddr.sin_family = AF_INET;
sockAddr.sin_port = htons(PORTNUM);
sockAddr.sin_addr.s_addr = htonl(INADDR_ANY);
// bind socket
ret = bind(sockfd, (const struct sockaddr *)&sockAddr, sizeof(struct sockaddr));
if (ret == -1) {
fprintf(stderr, "server: bind failed\n");
exitCode = 1;
goto done;
}
// listen for connection. max of 1 connection
ret = listen(sockfd, 1);
if (ret == -1) {
fprintf(stderr, "server: listen failed\n");
exitCode = 1;
goto done;
}
struct sockaddr_in dest;
socklen_t sockLen;
ssize_t bytesReceived;
while (1) {
// accept connection from client
recvFd = accept(sockfd, (struct sockaddr *)&dest, &sockLen);
if (recvFd == -1) {
fprintf(stderr, "server: accept failed\n");
exitCode = 1;
break;
}
// receive message from client
bytesReceived = recv(recvFd, buf, BUFSZ, 0);
if (bytesReceived == -1) {
fprintf(stderr, "server: recv failed\n");
exitCode = 1;
break;
} else {
printf("server: received %s", buf);
fflush(stdout);
}
}
done:
close(sockfd);
return exitCode;
}