epoll_wait blocking on signalled socket - c

Question

Excuse my long post. I would post code so that it is easier to understand the problem I am facing. It seems that if a signaled socket is added to epoll instance, epoll_wait on the epoll instance will not block. The following example has let me believe this:
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <sys/types.h>
#include <time.h>
#include <sys/epoll.h>
int MAX_EVENT_COUNT = 10;
int main(int argc, char *argv[])
{
int epollfd = epoll_create( MAX_EVENT_COUNT );
if ( epollfd == -1 )
{
printf("\n Error : epoll_create \n");
return 1;
}
epoll_event ev;
memset(&ev,0,sizeof(ev));
ev.events = EPOLLIN | EPOLLET;
int pipefd[2];
if (pipe(pipefd) == -1)
{
perror("pipe");
exit(EXIT_FAILURE);
}
char ch = 'a';
write(pipefd[1], &ch, 1);
if (epoll_ctl( epollfd , EPOLL_CTL_ADD, pipefd[0], &ev ) == -1 )
{
printf("\n Error : epoll add result \n");
return 1;
}
epoll_event rawResult [MAX_EVENT_COUNT];
int32_t res = epoll_wait( epollfd, rawResult, MAX_EVENT_COUNT, -1 );
if(res!=1)
{
printf("\n Epoll problem \n");
}
else
{
printf("\n OK \n");
}
}
However the following example is pretty much the same but this time I add a signalled socket to the epoll, this time the epoll_wait call blocks
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <sys/types.h>
#include <time.h>
#include <sys/epoll.h>
int MAX_EVENT_COUNT = 10;
int main(int argc, char *argv[])
{
int listenfd = 0;
{
struct sockaddr_in serv_addr;
char sendBuff[1025];
listenfd = socket(AF_INET, SOCK_STREAM, 0);
memset(&serv_addr, '0', sizeof(serv_addr));
memset(sendBuff, '0', sizeof(sendBuff));
serv_addr.sin_family = AF_INET;
serv_addr.sin_addr.s_addr = htonl(INADDR_ANY);
serv_addr.sin_port = htons(5000);
bind(listenfd, (struct sockaddr*)&serv_addr, sizeof(serv_addr));
listen(listenfd, 10);
}
int sockfd = 0, n = 0;
char recvBuff[1024];
struct sockaddr_in serv_addr;
memset(recvBuff, '0',sizeof(recvBuff));
if((sockfd = socket(AF_INET, SOCK_STREAM, 0)) < 0)
{
printf("\n Error : Could not create socket \n");
return 1;
}
memset(&serv_addr, '0', sizeof(serv_addr));
serv_addr.sin_family = AF_INET;
serv_addr.sin_port = htons(5000);
if(inet_pton(AF_INET, "127.0.0.1", &serv_addr.sin_addr)<=0)
{
printf("\n inet_pton error occured\n");
return 1;
}
if( connect(sockfd, (struct sockaddr *)&serv_addr, sizeof(serv_addr)) < 0)
{
printf("\n Error : Connect Failed \n");
return 1;
}
int connfd = accept(listenfd, (struct sockaddr*)NULL, NULL);
int epollfd = epoll_create( MAX_EVENT_COUNT );
if ( epollfd == -1 )
{
printf("\n Error : epoll_create \n");
return 1;
}
epoll_event ev;
memset(&ev,0,sizeof(ev));
ev.events = EPOLLIN | EPOLLET;
if (epoll_ctl( epollfd , EPOLL_CTL_ADD, sockfd, &ev ) == -1 )
{
printf("\n Error : epoll add result \n");
return 1;
}
epoll_event rawResult [MAX_EVENT_COUNT];
int32_t res = epoll_wait( epollfd, rawResult, MAX_EVENT_COUNT, -1 );
}

Answer 1

epoll_wait() will block if the file descriptors it is waiting on have no events for it to report about. In your case, I don't believe you have reached epoll_wait() yet. Assuming that your program has progressed passed connect() and accept(), then you have not written any data on the connection (on the sockfd), so epoll_wait() will not detect any events on the connfd.
This is different from your first program, where you write a byte of data on the pipefd[1] before calling epoll_wait() on pipefd[0].