Linux sockets: Segmentation error - c++

Question

Linux n00b here. So about a month ago I installed emacs and the gcc/g++ compiler and have gotten started with programming. I found some code online for an echo server program, copied it and compiled it to test the networking functions. It compiled but then when I tried to run it I got the error message: Segmentation fault(core dumped). When I looked carefully at the debugger details it was an error in the "fwrite()" function. I linked the code to the library libstdc++.a upon compiling and creating the output file so it does make me wonder if there is some critical error in the actual library functions and I need to go back, find the function .c sourcecode, and then add them to the headers to make it work. The code is posted below. Anybody else had this problem?
#include <sys-socket.h> /* socket definitions */
#include <sys-types.h> /* socket types */
#include <netinet-in.h> /* inet (3) functions */
#include <unistd.h> /* misc. UNIX functions */
#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <ether.h>
#include <string.h>
/* Global constants */
#define ECHO_PORT 2002
#define MAX_LINE 1000
#define LISTENQ 5
ssize_t Readline(int sockd, char *vptr,size_t maxlen) {
ssize_t n, rc;
char* c;
msghdr* buffer;
buffer->msg_iov->iov_base = vptr;
buffer->msg_iov->iov_len = maxlen;
for ( n = 1; n < maxlen; n++ ) {
if ( (rc = recvmsg(sockd,buffer, 1)) == 1 ) {
c = buffer->msg_iov->iov_base++;
if (*c == '\n' )
break;
}
else if ( rc == 0 ) {
if ( n == 1 )
return 0;
else
break;
}
else {
if (rc < 0 )
continue;
return -1;
}
}
buffer->msg_iov->iov_base = 0;
return n;
}
/* Write a line to a socket */
ssize_t Writeline(int sockd, char *vptr) {
msghdr *buffer;
buffer->msg_iov->iov_base = vptr;
size_t nleft = buffer->msg_iov->iov_len;
ssize_t nwritten;
while ( nleft > 0 ) {
if ( (nwritten = sendmsg(sockd, buffer, nleft)) < 0 ) {
return -1;
}
nleft -= nwritten;
buffer += nwritten;
}
return nwritten;
}
int main(int argc, char *argv[]) {
int list_s; /* listening socket */
int conn_s; /* connection socket */
short int port; /* port number */
struct sockaddr_in servaddr; /* socket address structure */
char *endptr; /* for strtol() */
char buffer[MAX_LINE];
port = 5000;
/* Create the listening socket */
if ( (list_s = socket(AF_INET, SOCK_STREAM, 0)) < 0 ) {
fprintf(stderr, "ECHOSERV: Error creating listening socket.\n");
exit(EXIT_FAILURE);
}
/* Set all bytes in socket address structure to
zero, and fill in the relevant data members */
memset(&servaddr, 0, sizeof(servaddr));
servaddr.sin_family = AF_INET;
servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
servaddr.sin_port = htons(port);
/* Bind our socket addresss to the
listening socket, and call listen() */
if ( bind(list_s, (struct sockaddr *) &servaddr, sizeof(servaddr)) < 0 ) {
fprintf(stderr, "ECHOSERV: Error calling bind()\n");
exit(EXIT_FAILURE);
}
if ( listen(list_s, LISTENQ) < 0 ) {
fprintf(stderr, "ECHOSERV: Error calling listen()\n");
exit(EXIT_FAILURE);
}
/* Enter an infinite loop to respond
to client requests and echo input */
while ( 1 ) {
/* Wait for a connection, then accept() it */
if ( (conn_s = accept(list_s, NULL, NULL) ) < 0 ) {
fprintf(stderr, "ECHOSERV: Error calling accept()\n");
exit(EXIT_FAILURE);
}
/* Retrieve an input line from the connected socket
then simply write it back to the same socket. */
Readline(conn_s, buffer, MAX_LINE-1);
Writeline(conn_s, buffer);
/* Close the connected socket */
if ( shutdown(conn_s,0) < 0 ) {
fprintf(stderr, "ECHOSERV: Error calling close()\n");
exit(EXIT_FAILURE);
}
}
}

Answer 1

ssize_t Writeline(int sockd, char *vptr) {
msghdr *buffer;
buffer->msg_iov->iov_base = vptr;
your pointer buffer is not initialized. You might look at this code snippet to do it correctly:
/* This structure contains parameter information for sendmsg. */
struct msghdr mh;
/* The message header contains parameters for sendmsg. */
mh.msg_name = (caddr_t) &dest;
mh.msg_namelen = sizeof(dest);
mh.msg_iov = iov;
mh.msg_iovlen = 3;
mh.msg_accrights = NULL; /* irrelevant to AF_INET */
mh.msg_accrightslen = 0; /* irrelevant to AF_INET */
rc = sendmsg(s, &mh, 0); /* no flags used */
if (rc == -1) {
perror("sendmsg failed");
return -1;
}
return 0;
}
sendmsg

Answer 2

You are not setting the buffer pointer variables in your Writeline() and ReadLine() functions.
ssize_t Writeline(int sockd, char *vptr) {
msghdr *buffer;
//this is not appropriate as buffer does not point to appropriate memory.
buffer->msg_iov->iov_base = vptr;
size_t nleft = buffer->msg_iov->iov_len;
ssize_t nwritten;
...
return nwritten;
}
Accessing buffer->msg_iov->iov_base or buffer->msg_iov->iov_len or even buffer is not appropriate without allocating it or setting to appropriate memory is not valid.

Answer 3

Did you actually COPY this, or did you copy bits and then paste it together yourself?
It's not very hard to fix at least to the point where it doesn't crash by itself - I didn't get further because my firewall settings are too strict to just fire up a program and use a random port, and I don't feel like messing up my firewall setting just to test your code.
So, as pointed out msghdr *buffer; means that the pointer for buffer is uninitialized. The easy fix is to NOT use a pointer, and instead use the address of buffer when you need it. You then need to have an iov data structure.
So, in receive, you end up with something like this:
msghdr buffer;
iovec iov;
buffer.msg_iov = &iov;
...
if ( (rc = recvmsg(sockd, &buffer, 1)) == 1 ) {
c = vptr++;
buffer.msg_iov->iov_base = vptr;
Note the & in front of buffer. I also changed the next line, as it was doing ++ on a void pointer, which is not clearly defined in C++, so the compiler gave a warning. (There's also a warning for buffer not initialized).
A similar treatment is needed in the `WriteLine function.
iovec iov;
buffer.msg_iov = &iov;
buffer.msg_iov->iov_base = vptr;
size_t nleft = MAX_LINE;
...
if ( (nwritten = sendmsg(sockd, &buffer, nleft)) < 0 ) {
....
nleft -= nwritten;
vptr += nwritten;
buffer.msg_iov->iov_base = vptr;
Again, the increment of iov_base is incrementing a void *, which hasn't been defined since I wrote it above, so need to make sure that the pointer has a different type - reusing vptr is decent here.
As an aside, I changed the nleft to set to MAX_LINE, as you don't pass in the size of the line. I would suggest that you change it so that it does take the size as an argument, similar to the ReadLine function.
Finally, please do yourself a favour and use -Wall -Werror when compiling the code - that means that you will get warnings when you do "silly" things - it may work, but it may also NOT work. Nearly all warnings from the compiler are USEFUL.
Remember when using a pointer in C or C++, you should make sure it points at something. Just writing T* ptr; only gives you a pointer, there is no memory attached to the pointer, so before you USE that pointer, you should assign it in some way.
I'm far from convinced this covers everything - but it should get you somewhat on the way to getting something working.