How do I use setsockopt SO REUSEADDR

Question

I am running my own http server on a raspberry pi  The problem is when I stop the program and restart it  the port is no longer available  Sometimes I get the same issue when receiving lots of requests  I want to use SO REUSEADDR so that I can keep using the port even when the error occurs but have had no luck getting it set up  Below is my code  The error I get is  ERROR on binding Address already in use     include  lt stdio h gt   include  lt stdlib h gt   include  lt string h gt   include  lt unistd h gt   include  lt sys types h gt    include  lt sys socket h gt   include  lt netinet in h gt   void error const char  msg        perror msg       exit 1      int main int argc  char  argv          printf  Starting Listener n         int sockfd  newsockfd  portno       socklen t clilen       char buffer 256        struct sockaddr in serv addr  cli addr       int n       if  argc  lt  2             fprintf stderr  ERROR  no port provided n             exit 1               sockfd   socket AF INET  SOCK STREAM  0        if  sockfd  lt  0           error  ERROR opening socket         bzero  char     amp 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     amp serv addr                sizeof serv addr    lt  0                 error  ERROR on binding          printf  about to listen n         listen sockfd 5        printf  finished listening n         clilen   sizeof cli addr        printf  About to accept n          int i       for i 0  i lt 100  i              newsockfd   accept sockfd                     struct sockaddr     amp cli addr                     amp clilen             if  newsockfd  lt  0                error  ERROR on accept             bzero buffer 256            n   read newsockfd buffer 255            if  n  lt  0  error  ERROR reading from socket             printf  Here is the message   s n  buffer            n   write newsockfd  I got your message  18            if  n  lt  0  error  ERROR writing to socket             close newsockfd               close sockfd        return 0

User · Answer

Depending on the libc release it could be needed to set both SO_REUSEADDR and SO_REUSEPORT socket options as explained in socket(7) documentation :

   SO_REUSEPORT (since Linux 3.9)
          Permits multiple AF_INET or AF_INET6 sockets to be bound to an
          identical socket address.  This option must be set on each
          socket (including the first socket) prior to calling bind(2)
          on the socket.  To prevent port hijacking, all of the
          processes binding to the same address must have the same
          effective UID.  This option can be employed with both TCP and
          UDP sockets.

As this socket option appears with kernel 3.9 and raspberry use 3.12.x, it will be needed to set SO_REUSEPORT.

You can set theses two options before calling bind like this :

    int reuse = 1;
    if (setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, (const char*)&reuse, sizeof(reuse)) < 0)
        perror("setsockopt(SO_REUSEADDR) failed");

#ifdef SO_REUSEPORT
    if (setsockopt(sockfd, SOL_SOCKET, SO_REUSEPORT, (const char*)&reuse, sizeof(reuse)) < 0) 
        perror("setsockopt(SO_REUSEPORT) failed");
#endif

User · Answer

I think you should use SO LINGER options  with timeout 0    In this case  you connection will close immediately after closing your program  and next restart will be able to bind again   example   linger lin  lin l onoff   0  lin l linger   0  setsockopt fd  SOL SOCKET  SO LINGER   const char    amp lin  sizeof int      see definition  http   man7 org linux man-pages man7 socket 7 html  SO LINGER           Sets or gets the SO LINGER option   The argument is a linger           structure                 struct linger                     int l onoff        linger active                      int l linger       how many seconds to linger for                                When enabled  a close 2  or shutdown 2  will not return until           all queued messages for the socket have been successfully sent           or the linger timeout has been reached   Otherwise  the call           returns immediately and the closing is done in the background            When the socket is closed as part of exit 2   it always           lingers in the background    More about SO LINGER  TCP option SO LINGER  zero  - when it  39 s required

User · Answer

After    sockfd   socket AF INET  SOCK STREAM  0   if  sockfd  lt  0       error  ERROR opening socket      You can add  with standard C99 compound literal support     if  setsockopt sockfd  SOL SOCKET  SO REUSEADDR   amp  int  1   sizeof int    lt  0      error  setsockopt SO REUSEADDR  failed      Or    int enable   1  if  setsockopt sockfd  SOL SOCKET  SO REUSEADDR   amp enable  sizeof int    lt  0      error  setsockopt SO REUSEADDR  failed

[c] How do I use setsockopt(SO_REUSEADDR)?

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