select socket call send and recv synchronization

Question

I am using the select call and accepting the connection from \"X\" no of clients. I made duplex connection i.e. server to client and client to server. When connection is establi

Answer 1

Sketch of the kind of buffer code you need. (To allow partial reads/writes, the buffers need to be persistent between calls) BTW: you really need to handle the -1 return from read() and write() because they would seriously disturb your buffer-bookkeeping. EINTR + EAGAIN/EWOULDBLOCK is very common.

struct buff {
        unsigned size;
        unsigned bot;
        unsigned top;
        char *buff;
        };
struct buff bf = {0,0,0,NULL};

initialisation:

bf.buff = malloc(SOME_SIZE);
        /* ... error checking omitted */
bp.size = SOME_SIZE;
bp.bot = bp.top =0;

reading:

unsigned todo;
int rc;

        /* (maybe) shift the buffer down to make place */
todo =  bf.top - bf.bot;
if (todo) {
        memmove (bf.buff, bf.buff + bf.bot, todo);
        bf.top = todo; bf.bot = 0;
        }

todo = bf.size - bf.top;

if (!todo) { /* maybe throttle? ... */ return; }
rc = read (fd, bf.buff+bp.top, todo);
        /* ... error checking omitted */
if (rc == -1) switch (errno) {...}
else if (rc == 0) {...}
else    {
        total_read += rc;
        bp.top +=  rc;
        }


writing:

unsigned todo; 
int rc;

todo =  bf.top - bf.bot;
if (!todo) { /* maybe juggle fd_set ... */ return; }
rc = write (fd, bf.buff+bp.bot, todo);
        /* ... error checking omitted */
if (rc == -1) switch (errno) {...}
else if (rc ==0) { ...}
else    {
        bp.bot += rc;
        total_written += rc;
        if (bp.bot == bp.top) bp.bot = bp.top =0;
        }
/* ... this is the place to juggle the fd_set for writing */