Does fgets change the file descriptor set? - c

Question

So, I can use FD_ZERO, FD_SET and FD_ISSET to mess around with the file descriptor set. The select() function may now be used to observe if a certain file object becomes readable (hope my wording is right here). What I want to do is: Read in a sequence of characters from stdin and then send this sequence to a server. I leave out some error messages of the socket stuff here, however, because my question is about FD_ISSET.
#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet.in.h>
#include <arpa/inet.h>
int main(int argc, char *argv[])
{
static char buffer[256];
int sock_fd, length, port;
struct sockaddr_in server_addr;
fd_set input_fdset
sock_fd = socket(PF_INET, SOCK_STREAM, 0);
server_addr.sin_family = AF_INET;
server_addr.sin_port = htons(port);
connect(sock_fd, (struct sockaddr *) &server_addr, sizeof(struct sockaddr_in));
while (1)
{
FD_ZERO(&input_fdset);
FD_SET(STDIN_FILENO, &input_fdset);
FD_SET(sock_fd, &input_fdset);
if (select(sock_fd+1, &input_fdset, NULL, NULL, NULL) == -1)
perror("some error");
if (FD_ISSET(STDIN_FILENO, &input_fdset))
{
if(fgets(buffer, 256, stdin) == NULL)
{
printf("connect: Closing socket.");
break;
}
length = strlen(buffer);
send(sock_fd, buffer, length, 0);
}
else
{
length = recv(sock_fd, buffer, 256, 0);
if(length == 0)
{
printf( "Connection closed by remote host.");
break;
}
write(STDOUT_FILENO, buffer, length);
}
}
close(sock_fd);
return(0);
}
You can probably guess I came across this code.
I have troubles to understand the following: Within the while-loop, I add both the socket object and stdin to the file descriptor set. In the next step select checks whether there is anything to read from either of these two objects. Then the if(FD_ISSET(STDIN_FILENO, &input_fdset)) should ALWAYS be executed, right? I mean the condition is always fulfilled, because we set the STDIN_FILENO using FD_SET(STDIN_FILENO, &input_fdset). Now, I am within this if-statement. fgets reads in my terminal input line by line. When I end this input with ENTER, a newline-character is appended to my buffer and reading from stdin stops. But only until the while-loop is executed again, right? Because in that case fgets gets called again as well. Btw I think it is kind of weird to have fgets only within this if-statement. Shouldn't it be called before? How can fgets become NULL? Only if EOF is reached without reading in any character. This won't happen though, will it? I cannot reach EOF when I am reading from stdin. I think. And now the last question: When would I execute the else-block? For this to happen, the previous if-statement would need to fail. But as I said, if(FD_ISSET(STDIN_FILENO, &input_fdset) will never fail.
Ofc I realize I am missunderstanding this code.
Can somebody help me figure this out?

Answer 1

From the documentation for select (man select):
"On return, select() replaces the given descriptor sets with subsets consisting of those descriptors that are ready for the requested operation."
The purpose of FD_ISSET is to determine which sets are ready to be read. The set is changed by the select call, by design.