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9.18. recv(), recvfrom()

Receive data on a socket

#include <sys/types.h>
#include <sys/socket.h>

ssize_t recv(int s, void *buf, size_t len, int flags);
ssize_t recvfrom(int s, void *buf, size_t len, int flags,
                 struct sockaddr *from, socklen_t *fromlen);
Once you have a socket up and connected, you can read incoming data from the remote side using the recv() (for TCP SOCK_STREAM sockets) and recvfrom() (for UDP SOCK_DGRAM sockets).

Both functions take the socket descriptor s, a pointer to the buffer buf, the size (in bytes) of the buffer len, and a set of flags that control how the functions work.

Additionally, the recvfrom() takes a struct sockaddr*, from that will tell you where the data came from, and will fill in fromlen with the size of struct sockaddr. (You must also initialize fromlen to be the size of from or struct sockaddr.)

So what wondrous flags can you pass into this function? Here are some of them, but you should check your local man pages for more information and what is actually supported on your system. You bitwise-or these together, or just set flags to 0 if you want it to be a regular vanilla recv().

MSG_OOBReceive Out of Band data. This is how to get data that has been sent to you with the MSG_OOB flag in send(). As the receiving side, you will have had signal SIGURG raised telling you there is urgent data. In your handler for that signal, you could call recv() with this MSG_OOB flag.
MSG_PEEKIf you want to call recv() "just for pretend", you can call it with this flag. This will tell you what's waiting in the buffer for when you call recv() "for real" (i.e. without the MSG_PEEK flag. It's like a sneak preview into the next recv() call.
MSG_WAITALLTell recv() to not return until all the data you specified in the len parameter. It will ignore your wishes in extreme circumstances, however, like if a signal interrupts the call or if some error occurs or if the remote side closes the connection, etc. Don't be mad with it.

When you call recv(), it will block until there is some data to read. If you want to not block, set the socket to non-blocking or check with select() or poll() to see if there is incoming data before calling recv() or recvfrom().

Return Value
Returns the number of bytes actually received (which might be less than you requested in the len parameter), or -1 on error (and errno will be set accordingly.)

If the remote side has closed the connection, recv() will return 0. This is the normal method for determining if the remote side has closed the connection. Normality is good, rebel!

// stream sockets and recv()

struct addrinfo hints, *res;
int sockfd;
char buf[512];
int byte_count;

// get host info, make socket, and connect it
memset(&hints, 0, sizeof hints);
hints.ai_family = AF_UNSPEC;  // use IPv4 or IPv6, whichever
hints.ai_socktype = SOCK_STREAM;
getaddrinfo("www.example.com", "3490", &hints, &res);
sockfd = socket(res->ai_family, res->ai_socktype, res->ai_protocol);
connect(sockfd, res->ai_addr, res->ai_addrlen);

// all right!  now that we're connected, we can receive some data!
byte_count = recv(sockfd, buf, sizeof buf, 0);
printf("recv()'d %d bytes of data in buf\n", byte_count);
// datagram sockets and recvfrom()

struct addrinfo hints, *res;
int sockfd;
int byte_count;
socklen_t fromlen;
struct sockaddr_storage addr;
char buf[512];
char ipstr[INET6_ADDRSTRLEN];

// get host info, make socket, bind it to port 4950
memset(&hints, 0, sizeof hints);
hints.ai_family = AF_UNSPEC;  // use IPv4 or IPv6, whichever
hints.ai_socktype = SOCK_DGRAM;
hints.ai_flags = AI_PASSIVE;
getaddrinfo(NULL, "4950", &hints, &res);
sockfd = socket(res->ai_family, res->ai_socktype, res->ai_protocol);
bind(sockfd, res->ai_addr, res->ai_addrlen);

// no need to accept(), just recvfrom():

fromlen = sizeof addr;
byte_count = recvfrom(sockfd, buf, sizeof buf, 0, &addr, &fromlen);

printf("recv()'d %d bytes of data in buf\n", byte_count);
printf("from IP address %s\n",
        addr.ss_family == AF_INET?
            ((struct sockadd_in *)&addr)->sin_addr:
            ((struct sockadd_in6 *)&addr)->sin6_addr,
        ipstr, sizeof ipstr);
See Also
send()sendto()select()poll(), Blocking