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The recv, recvfrom, recvmsg methods helps in receiving a message from a socket.

ssize_t recv (int s, void *buf, size_t len, int flags);
 
ssize_t recvfrom (int s, void * restrict buf, size_t len, int flags, 
                  struct sockaddr * restrict from, socklen_t * restrict fromlen);
 
ssize_t recvmsg (int s, struct msghdr *msg, int flags);

The recvfrom and recvmsg system calls are used to receive messages from a socket, and may be used to receive data on a socket whether or not it is connection-oriented. The recv function is normally used only on a connected socket (see connect and is identical to recvfrom with a null pointer passed as its from argument. All three routines return the length of the message on successful completion.

The recvmsg system call uses a msghdr structure to minimize the number of directly supplied arguments. This structure has the following form, as defined in sys/socket.h:

struct msghdr {
        caddr_t msg_name;       /* optional address */
        u_int   msg_namelen;    /* size of address */
        struct  iovec *msg_iov; /* scatter/gather array */
        u_int   msg_iovlen;     /* # elements in msg_iov */
        caddr_t msg_control;    /* ancillary data, see below */
        u_int   msg_controllen; /* ancillary data buffer len */
        int     msg_flags;      /* flags on received message */
};

And following code snippets shows the usage of recv, recvfrom, recvmsg functions:

#include <sys/socket.h>
#include <netinet/in.h>
#include <unistd.h>
void Recv()
{
   struct sockaddr_in serv_addr;
   int sock_fd;
   char line[10];
   int size = 10;
   serv_addr.sin_family = AF_INET;
   serv_addr.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
   serv_addr.sin_port = htons(5000);
   sock_fd = socket(AF_INET,SOCK_STREAM,IPPROTO_TCP);
   connect(sock_fd,(struct sockaddr*)&serv_addr,sizeof(serv_addr));
   recv(sock_fd, line, size, 0);
   close(sock_fd);
}

#include <sys/socket.h>
#include <netinet/in.h>
#include <unistd.h>
void Sendto()
{
   struct sockaddr_in sender_addr;
   int sock_fd;
   char line[15] = "Hello World!";
   unsigned int size = sizeof(sender_addr);
   sock_fd = socket(AF_INET,SOCK_DGRAM,IPPROTO_UDP);
   sender_addr.sin_family = AF_INET;
   sender_addr.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
   sender_addr.sin_port = htons(5000);
   recvfrom(sock_fd,line,13,0,(struct sockaddr*)&sender_addr,&size);
   close(sock_fd);
}

#include <sys/socket.h>
#include <netinet/in.h>
#include <unistd.h>
void SendMsgRecvMsg()
{
   int sock_fd;
   unsigned int sender_len;
   struct msghdr msg;
   struct iovec iov;
   struct sockaddr_in receiver_addr,sender_addr;
   char line[10];
   sock_fd = socket(AF_INET,SOCK_DGRAM,IPPROTO_UDP);
   receiver_addr.sin_family = AF_INET;
   receiver_addr.sin_addr.s_addr = htonl(INADDR_ANY);
   receiver_addr.sin_port = htons(5000);
   bind(sock_fd,(struct sockaddr*)&receiver_addr,sizeof(receiver_addr));
   sender_len = sizeof(sender_addr);
   msg.msg_name = &sender_addr;
   msg.msg_namelen = sender_len;
   msg.msg_iov = &iov;
   msg.msg_iovlen = 1;
   msg.msg_iov->iov_base = line;
   msg.msg_iov->iov_len = 10;
   msg.msg_control = 0;
   msg.msg_controllen = 0;
   msg.msg_flags = 0;
   recvmsg(sock_fd,&msg,0);
   close(sock_fd);
}

These calls return the number of bytes received, or -1 if an error occurred.

Wiki Links

Open C Sockets Overview