The flags argument¶

The flags argument is formed by ORing one or more of the following values:

MSG_CMSG_CLOEXEC (recvmsg() only; since Linux 2.6.23)

Set the close-on-exec flag for the file descriptor received via a UNIX domain file descriptor using the SCM_RIGHTS operation (described in unix(7)). This flag is useful for the same reasons as the O_CLOEXEC flag of open(2).

MSG_DONTWAIT (since Linux 2.2)

Enables nonblocking operation; if the operation would block, the call fails with the error EAGAIN or EWOULDBLOCK. This provides similar behavior to setting the O_NONBLOCK flag (via the fcntl(2) F_SETFL operation), but differs in that MSG_DONTWAIT is a per-call option, whereas O_NONBLOCK is a setting on the open file description (see open(2)), which will affect all threads in the calling process and as well as other processes that hold file descriptors referring to the same open file description.

MSG_ERRQUEUE (since Linux 2.2)

This flag specifies that queued errors should be received from the socket error queue. The error is passed in an ancillary message with a type dependent on the protocol (for IPv4 IP_RECVERR). The user should supply a buffer of sufficient size. See cmsg(3) and ip(7) for more information. The payload of the original packet that caused the error is passed as normal data via msg_iovec. The original destination address of the datagram that caused the error is supplied via msg_name.

The error is supplied in a sock_extended_err structure:

#define SO_EE_ORIGIN_NONE    0
#define SO_EE_ORIGIN_LOCAL   1
#define SO_EE_ORIGIN_ICMP    2
#define SO_EE_ORIGIN_ICMP6   3
struct sock_extended_err
{


    uint32_t ee_errno;   /* error number */


    uint8_t  ee_origin;  /* where the error originated */


    uint8_t  ee_type;    /* type */


    uint8_t  ee_code;    /* code */


    uint8_t  ee_pad;     /* padding */


    uint32_t ee_info;    /* additional information */


    uint32_t ee_data;    /* other data */


    /* More data may follow */
};
struct sockaddr *SO_EE_OFFENDER(struct sock_extended_err *);
    

ee_errno contains the errno number of the queued error. ee_origin is the origin code of where the error originated. The other fields are protocol-specific. The macro SOCK_EE_OFFENDER returns a pointer to the address of the network object where the error originated from given a pointer to the ancillary message. If this address is not known, the sa_family member of the sockaddr contains AF_UNSPEC and the other fields of the sockaddr are undefined. The payload of the packet that caused the error is passed as normal data.

For local errors, no address is passed (this can be checked with the cmsg_len member of the cmsghdr). For error receives, the MSG_ERRQUEUE flag is set in the msghdr. After an error has been passed, the pending socket error is regenerated based on the next queued error and will be passed on the next socket operation.

MSG_OOB

This flag requests receipt of out-of-band data that would not be received in the normal data stream. Some protocols place expedited data at the head of the normal data queue, and thus this flag cannot be used with such protocols.

MSG_PEEK

This flag causes the receive operation to return data from the beginning of the receive queue without removing that data from the queue. Thus, a subsequent receive call will return the same data.

MSG_TRUNC (since Linux 2.2)

For raw (AF_PACKET), Internet datagram (since Linux 2.4.27/2.6.8), netlink (since Linux 2.6.22), and UNIX datagram (since Linux 3.4) sockets: return the real length of the packet or datagram, even when it was longer than the passed buffer.

For use with Internet stream sockets, see tcp(7).

MSG_WAITALL (since Linux 2.2)

This flag requests that the operation block until the full request is satisfied. However, the call may still return less data than requested if a signal is caught, an error or disconnect occurs, or the next data to be received is of a different type than that returned. This flag has no effect for datagram sockets.

recvfrom()¶

recvfrom() places the received message into the buffer buf. The caller must specify the size of the buffer in len.

If src_addr is not NULL, and the underlying protocol provides the source address of the message, that source address is placed in the buffer pointed to by src_addr. In this case, addrlen is a value-result argument. Before the call, it should be initialized to the size of the buffer associated with src_addr. Upon return, addrlen is updated to contain the actual size of the source address. The returned address is truncated if the buffer provided is too small; in this case, addrlen will return a value greater than was supplied to the call.

If the caller is not interested in the source address, src_addr and addrlen should be specified as NULL.

recv()¶

The recv() call is normally used only on a connected socket (see connect(2)). It is equivalent to the call:

recvfrom(fd, buf, len, flags, NULL, 0);

recvmsg()¶

The recvmsg() call uses a msghdr structure to minimize the number of directly supplied arguments. This structure is defined as follows in <sys/socket.h>:

struct iovec {                    /* Scatter/gather array items */


    void  *iov_base;              /* Starting address */


    size_t iov_len;               /* Number of bytes to transfer */
};
struct msghdr {


    void         *msg_name;       /* optional address */


    socklen_t     msg_namelen;    /* size of address */


    struct iovec *msg_iov;        /* scatter/gather array */


    size_t        msg_iovlen;     /* # elements in msg_iov */


    void         *msg_control;    /* ancillary data, see below */


    size_t        msg_controllen; /* ancillary data buffer len */


    int           msg_flags;      /* flags on received message */
};

The msg_name field points to a caller-allocated buffer that is used to return the source address if the socket is unconnected. The caller should set msg_namelen to the size of this buffer before this call; upon return from a successful call, msg_namelen will contain the length of the returned address. If the application does not need to know the source address, msg_name can be specified as NULL.

The fields msg_iov and msg_iovlen describe scatter-gather locations, as discussed in readv(2).

The field msg_control, which has length msg_controllen, points to a buffer for other protocol control-related messages or miscellaneous ancillary data. When recvmsg() is called, msg_controllen should contain the length of the available buffer in msg_control; upon return from a successful call it will contain the length of the control message sequence.

The messages are of the form:

struct cmsghdr {


    size_t cmsg_len;    /* Data byte count, including header


                           (type is socklen_t in POSIX) */


    int    cmsg_level;  /* Originating protocol */


    int    cmsg_type;   /* Protocol-specific type */
/* followed by


    unsigned char cmsg_data[]; */
};

Ancillary data should be accessed only by the macros defined in cmsg(3).

As an example, Linux uses this ancillary data mechanism to pass extended errors, IP options, or file descriptors over UNIX domain sockets.

The msg_flags field in the msghdr is set on return of recvmsg(). It can contain several flags:

MSG_EOR

indicates end-of-record; the data returned completed a record (generally used with sockets of type SOCK_SEQPACKET).

MSG_TRUNC

indicates that the trailing portion of a datagram was discarded because the datagram was larger than the buffer supplied.

MSG_CTRUNC

indicates that some control data were discarded due to lack of space in the buffer for ancillary data.

MSG_OOB

is returned to indicate that expedited or out-of-band data were received.

MSG_ERRQUEUE

indicates that no data was received but an extended error from the socket error queue.