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How to check if there are available data to read before to call recv in a blocking socket context on Windows PCs

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0

I wrote a short TCP client application to test a SW broker using blocking sockets. It works well and I am happy with it. Now I need to modify it and I have a problem reading not solecited messages (messages coming from the server not initiated by the client) because recv() blocks the program flow until there is no data to read.
Good solution for me may be to find a way to check if there are available data to read before calling recv() or that recv() may exit if there are no data to read within a certain period.
So I tried with this function:

static bool isDataAvailable(int socket)

 fd_set sready;
 struct timeval nowait;

 FD_ZERO(&sready);
 FD_SET((unsigned int) socket, &sready);
 memset((char *) &nowait, 0, sizeof(nowait));

 bool res = select(socket + 1, &sready, NULL, NULL, &nowait);
 if(FD_ISSET(socket, &sready))
 res = true;
 else
 res = false;
 return res;

//------------------------------------------------------------------------

but the result is that after the call all my application works as I selected to work using non blocking sockets. I normally use it on Linux and it works. I found some info around the web about select() and seems to me it would work also under Windows. But... something is going wrong.

Any help and suggestion will be appreciated.
Thanks,
Massimo

c windows sockets

share|improve this question

asked Mar 27 at 18:54

Massimo MancaMassimo Manca

2655 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  How do you use this function? What behavior did you expect? If you only call recv after this function returns true, then of course your program will (in a way) behave like the socket was non-blocking, you don't call the recv function which does the blocking.
  
  – Some programmer dude
  Mar 27 at 19:04
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Also, don't forget to check for errors from select.
  
  – Some programmer dude
  Mar 27 at 19:04
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Well... I would expect select returns a positive value if there are available data to read calling recv(). If selects returns a negative value may be a select error or the socket is closed. So, I started as simple as possible to see it working or not just in case of no data available.
  
  – Massimo Manca
  Mar 27 at 19:37
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Yes, sure, I can check better select errors at least now to understand how to implement my function.
  
  – Massimo Manca
  Mar 27 at 19:37
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  select() will return > 0 if the socket is readable, 0 on timeout, and -1 on error. So, since you are only checking 1 socket, you can do this instead: return select(...) > 0;
  
  – Remy Lebeau
  Mar 27 at 20:11
  
  

  
  
  
  

add a comment | 

0

I wrote a short TCP client application to test a SW broker using blocking sockets. It works well and I am happy with it. Now I need to modify it and I have a problem reading not solecited messages (messages coming from the server not initiated by the client) because recv() blocks the program flow until there is no data to read.
Good solution for me may be to find a way to check if there are available data to read before calling recv() or that recv() may exit if there are no data to read within a certain period.
So I tried with this function:

static bool isDataAvailable(int socket)

 fd_set sready;
 struct timeval nowait;

 FD_ZERO(&sready);
 FD_SET((unsigned int) socket, &sready);
 memset((char *) &nowait, 0, sizeof(nowait));

 bool res = select(socket + 1, &sready, NULL, NULL, &nowait);
 if(FD_ISSET(socket, &sready))
 res = true;
 else
 res = false;
 return res;

//------------------------------------------------------------------------

but the result is that after the call all my application works as I selected to work using non blocking sockets. I normally use it on Linux and it works. I found some info around the web about select() and seems to me it would work also under Windows. But... something is going wrong.

Any help and suggestion will be appreciated.
Thanks,
Massimo

c windows sockets

share|improve this question

asked Mar 27 at 18:54

Massimo MancaMassimo Manca

2655 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  How do you use this function? What behavior did you expect? If you only call recv after this function returns true, then of course your program will (in a way) behave like the socket was non-blocking, you don't call the recv function which does the blocking.
  
  – Some programmer dude
  Mar 27 at 19:04
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Also, don't forget to check for errors from select.
  
  – Some programmer dude
  Mar 27 at 19:04
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Well... I would expect select returns a positive value if there are available data to read calling recv(). If selects returns a negative value may be a select error or the socket is closed. So, I started as simple as possible to see it working or not just in case of no data available.
  
  – Massimo Manca
  Mar 27 at 19:37
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Yes, sure, I can check better select errors at least now to understand how to implement my function.
  
  – Massimo Manca
  Mar 27 at 19:37
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  select() will return > 0 if the socket is readable, 0 on timeout, and -1 on error. So, since you are only checking 1 socket, you can do this instead: return select(...) > 0;
  
  – Remy Lebeau
  Mar 27 at 20:11
  
  

  
  
  
  

add a comment | 

I wrote a short TCP client application to test a SW broker using blocking sockets. It works well and I am happy with it. Now I need to modify it and I have a problem reading not solecited messages (messages coming from the server not initiated by the client) because recv() blocks the program flow until there is no data to read.
Good solution for me may be to find a way to check if there are available data to read before calling recv() or that recv() may exit if there are no data to read within a certain period.
So I tried with this function:

static bool isDataAvailable(int socket)

 fd_set sready;
 struct timeval nowait;

 FD_ZERO(&sready);
 FD_SET((unsigned int) socket, &sready);
 memset((char *) &nowait, 0, sizeof(nowait));

 bool res = select(socket + 1, &sready, NULL, NULL, &nowait);
 if(FD_ISSET(socket, &sready))
 res = true;
 else
 res = false;
 return res;

//------------------------------------------------------------------------

but the result is that after the call all my application works as I selected to work using non blocking sockets. I normally use it on Linux and it works. I found some info around the web about select() and seems to me it would work also under Windows. But... something is going wrong.

Any help and suggestion will be appreciated.
Thanks,
Massimo

c windows sockets

share|improve this question

asked Mar 27 at 18:54

Massimo MancaMassimo Manca

2655 bronze badges

static bool isDataAvailable(int socket)

 fd_set sready;
 struct timeval nowait;

 FD_ZERO(&sready);
 FD_SET((unsigned int) socket, &sready);
 memset((char *) &nowait, 0, sizeof(nowait));

 bool res = select(socket + 1, &sready, NULL, NULL, &nowait);
 if(FD_ISSET(socket, &sready))
 res = true;
 else
 res = false;
 return res;

//------------------------------------------------------------------------

share|improve this question

asked Mar 27 at 18:54

Massimo MancaMassimo Manca

2655 bronze badges

share|improve this question

asked Mar 27 at 18:54

Massimo MancaMassimo Manca

2655 bronze badges

asked Mar 27 at 18:54

Massimo MancaMassimo Manca

2655 bronze badges

asked Mar 27 at 18:54

Massimo MancaMassimo Manca

2655 bronze badges