Why is this custom network-service crashing (written in C)? - c

Question

I wrote this program which listens on a given port and then, once a connection is received, outputs a single line of text and disconnects. It runs for days, processing thousands of queries, but then (inevitably) crashes and I have to go restart it. Wondering if anyone sees anything wrong with it, or (alternatively) if anyone can suggest a way to make it more robust.
int main(int argc, char *argv[])
{
srand(time(0));
int sockfd, newsockfd, portno;
socklen_t clilen;
struct sockaddr_in serv_addr, cli_addr;
int n;
if (argc < 2) {
fprintf(stderr,"ERROR, no port provided\n");
exit(1);
}
sockfd = socket(AF_INET, SOCK_STREAM, 0);
if (sockfd < 0)
error("ERROR opening socket");
bzero((char *) &serv_addr, sizeof(serv_addr));
portno = atoi(argv[1]);
serv_addr.sin_family = AF_INET;
serv_addr.sin_addr.s_addr = INADDR_ANY;
serv_addr.sin_port = htons(portno);
if (bind(sockfd, (struct sockaddr *) &serv_addr,
sizeof(serv_addr)) < 0)
error("ERROR on binding");
listen(sockfd,5);
clilen = sizeof(cli_addr);
while (1)
{
unsigned char write_val;
unsigned char y[BYTES];
int i, j;
newsockfd = accept(sockfd,
(struct sockaddr *) &cli_addr,
&clilen);
if (newsockfd < 0)
error("ERROR on accept");
fill_buffer(y); // fills buffer y with a 128-bit string; not included here
for (i=BYTES-1; i >= 0; i--)
{
const void* ZERO = (void *)"0";
const void* ONE = (void *)"1";
for (j=7; j >= 0; j--)
write(newsockfd, (y[i] >> j) & 1 ? ONE : ZERO, 1);
}
write(newsockfd, "\n", 1);
close(newsockfd);
}
close(sockfd);
return 0;
}

Answer 1

anyone sees anything wrong with it
While this code could be made more efficient (by writing all the bytes in one single pass for example), there's no obvious flaw there.
That makes the unpublished part of your code a decent candidate for the problem:
fill_buffer(y); // fills buffer y with a 128-bit string; not included here
If you read more bytes than y[]'s size then you will crash.
or (alternatively) if anyone can suggest a way to make it more robust
Try enlarging the size of this y[] buffer (doubling it can't hurt).
And make sure that fill_buffer() can't read more than BYTES characters.
Publish this missing code in case of doubt.
You could also compile your code with debug symbols and dump a backtrace (with symbols) in a file from your signal handler. This way, if your program crashes, you will know where and why.

Answer 2

The code looks good with some comments.
One somewhat important comment:
portno should be declared as unsigned short. This works OK with an Intel-like (little endian) processor but it won't be portable to a processor with different endianness.
Anyway it was not the reason for your process crashing.
Obviously the crash occurs while executing within the 'while', and by looking at the code, if it crashes for a buffer overflow, the only possibility is within fill_buffer.
If you show the definition of BYTES and fill_buffer it will be easier to help you.
Now, if it is not a buffer overflow, there is the possibility that it aborts in the write if the client closed the connection before the server writes into the socket. In that case the process will receive a signal SIGPIPE and it will abort if the code does not handle that signal.
You can also ignore SIGPIPE with:
signal(SIGPIPE, SIG_IGN);
Other possibility is if you are doing something weird with write_val and you're not showing that code.
I hope this helps.