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Annotated edit history of recv(2) version 3, including all changes. View license author blame.
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1 perry 1 !!NAME
2 PerryLorier 2 recv, recvfrom, recvmsg - receive a message from a socket
1 perry 3
4 !!SYNOPSIS
2 PerryLorier 5 __#include <sys/types.h>__
6 __#include <sys/socket.h>__
1 perry 7
2 PerryLorier 8 __int recv(int__ ''s''__, void *__''buf''__, size_t__ ''len''__, int__ ''flags''__);__
1 perry 9
2 PerryLorier 10 __int recvfrom(int__ ''s''__, void *__''buf''__, size_t__ ''len''__, int__ ''flags''__, struct sockaddr *__''from''__, socklen_t *__''fromlen''__);__
1 perry 11
2 PerryLorier 12 __int recvmsg(int__ ''s''__, struct msghdr *__''msg''__, int__ ''flags''__);__
3 JohnMcPherson 13
1 perry 14 !!DESCRIPTION
2 PerryLorier 15 The __recvfrom__ and __recvmsg__ 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.
1 perry 16
2 PerryLorier 17 If ''from'' is not NULL, and the socket is not connection-oriented, the source address of the message is filled in. The argument ''fromlen'' is a value-result parameter, initialized to the size of the buffer associated with ''from'', and modified on return to indicate the actual size of the address stored there.
1 perry 18
2 PerryLorier 19 The __recv__ call is normally used only on a ''connected'' socket (see connect(2)) and is identical to __recvfrom__ with a NULL ''from'' parameter.
1 perry 20
2 PerryLorier 21 All three routines return the length of the message on successful completion. If a message is too long to fit in the supplied buffer, excess bytes may be discarded depending on the type of socket the message is received from (see socket(2)).
1 perry 22
2 PerryLorier 23 If no messages are available at the socket, the receive calls wait for a message to arrive, unless the socket is nonblocking (see fcntl(2)) in which case the value -1
24 is returned and the external variable ''errno'' set to [EAGAIN]. The receive calls normally return any data available, up to the requested amount, rather than waiting
1 perry 25 for receipt of the full amount requested.
26
2 PerryLorier 27 The select(2) or poll(2) call may be used to determine when more data arrives.
1 perry 28
2 PerryLorier 29 The ''flags'' argument to a recv call is formed by ''OR'''ing one or more of the following values:
1 perry 30
2 PerryLorier 31 ;__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.
1 perry 32
2 PerryLorier 33 ;__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.
1 perry 34
2 PerryLorier 35 ;__MSG_WAITALL__: 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.
1 perry 36
3 JohnMcPherson 37 ;[MSG_NOSIGNAL]: This flag turns off raising of [SIGPIPE] on stream sockets when the other end disappears.
1 perry 38
2 PerryLorier 39 ;__MSG_TRUNC__: Return the real length of the packet, even when it was longer than the passed buffer. Only valid for packet sockets.
1 perry 40
2 PerryLorier 41 ;__MSG_ERRQUEUE__: 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
1 perry 42 __msg_name__.
43
2 PerryLorier 44 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__ is set in the
45 __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.
1 perry 46
2 PerryLorier 47 The error is supplied in a __sock_extended_err__ structure:
1 perry 48
2 PerryLorier 49 #define SO_EE_ORIGIN_NONE 0
50 #define SO_EE_ORIGIN_LOCAL 1
51 #define SO_EE_ORIGIN_ICMP 2
52 #define SO_EE_ORIGIN_ICMP6 3
53 struct sock_extended_err
54 {
55 u_int32_t ee_errno; /* error number */
56 u_int8_t ee_origin; /* where the error originated */
57 u_int8_t ee_type; /* type */
58 u_int8_t ee_code; /* code */
59 u_int8_t ee_pad;
60 u_int32_t ee_info; /* additional information */
61 u_int32_t ee_data; /* other data */
62 /* More data may follow */
63 };
64 struct sockaddr *SO_EE_OFFENDER(struct sock_extended_err *);
1 perry 65
2 PerryLorier 66 __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.
1 perry 67
2 PerryLorier 68 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__ is set in the
69 __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.
1 perry 70
2 PerryLorier 71 The __recvmsg__ call uses a ''msghdr'' structure to minimize the number of directly supplied parameters. This structure has the following form, as defined in '''':
1 perry 72
2 PerryLorier 73 struct msghdr {
74 void * msg_name; /* optional address */
75 socklen_t msg_namelen; /* size of address */
76 struct iovec * msg_iov; /* scatter/gather array */
77 size_t msg_iovlen; /* # elements in msg_iov */
78 void * msg_control; /* ancillary data, see below */
79 socklen_t msg_controllen; /* ancillary data buffer len */
80 int msg_flags; /* flags on received message */
81 };
1 perry 82
2 PerryLorier 83 Here ''msg_name'' and ''msg_namelen'' specify the destination address if the socket is unconnected; ''msg_name'' may be given as a null pointer if no names are desired or required. 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.
1 perry 84
85 The messages are of the form:
2 PerryLorier 86 struct cmsghdr {
87 socklen_t cmsg_len; /* data byte count, including hdr */
88 int cmsg_level; /* originating protocol */
89 int cmsg_type; /* protocol-specific type */
90 /* followed by
91 u_char cmsg_data[[]; */
92 };
1 perry 93
2 PerryLorier 94 Ancillary data should only be accessed by the macros defined in cmsg(3).
1 perry 95
2 PerryLorier 96 As an example, Linux uses this auxiliary data mechanism to pass extended errors, IP options or file descriptors over Unix sockets.
1 perry 97
2 PerryLorier 98 The ''msg_flags'' field in the msghdr is set on return of __recvmsg__(). It can contain several flags:
1 perry 99
2 PerryLorier 100 ;__MSG_EOR__: indicates end-of-record; the data returned completed a record (generally used with sockets of type __SOCK_SEQPACKET__).
1 perry 101
2 PerryLorier 102 ;__MSG_TRUNC__: indicates that the trailing portion of a datagram was discarded because the datagram was larger than the buffer supplied.
1 perry 103
2 PerryLorier 104 ;__MSG_CTRUNC__: indicates that some control data were discarded due to lack of space in the buffer for ancillary data.
1 perry 105
2 PerryLorier 106 ;__MSG_OOB__: is returned to indicate that expedited or out-of-band data were received.
1 perry 107
2 PerryLorier 108 ;__MSG_ERRQUEUE__: indicates that no data was received but an extended error from the socket error queue.
1 perry 109
3 JohnMcPherson 110 ;__MSG_DONTWAIT__: Enables non-blocking operation; if the operation would block, [EAGAIN] is returned (this can also be enabled using the __O_NONBLOCK__ with the __F_SETFL__ fcntl(2)).
1 perry 111
112 !!RETURN VALUE
2 PerryLorier 113 These calls return the number of bytes received, or -1 if an error occurred.
1 perry 114
115 !!ERRORS
2 PerryLorier 116 These are some standard errors generated by the socket layer. Additional errors may be generated and returned from the underlying protocol modules; see their manual
1 perry 117 pages.
118
2 PerryLorier 119 ;[EBADF]: The argument ''s'' is an invalid descriptor.
120 ;[ECONNREFUSED]: A remote host refused to allow the network connection (typically because it is not running the requested service).
121 ;[ENOTCONN]: The socket is associated with a connection-oriented protocol and has not been connected (see connect(2) and accept(2)).
122 ;[ENOTSOCK]: The argument ''s'' does not refer to a socket.
123 ;[EAGAIN]: The socket is marked non-blocking and the receive operation would block, or a receive timeout had been set and the timeout expired before data was received.
124 ;[EINTR]: The receive was interrupted by delivery of a signal before any data were available.
125 ;[EFAULT]: The receive buffer pointer(s) point outside the process's address space.
126 ;[EINVAL]: Invalid argument passed.
1 perry 127
128 !!CONFORMING TO
2 PerryLorier 129 4.4BSD (these function calls first appeared in 4.2BSD).
1 perry 130 !!NOTE
2 PerryLorier 131 The prototypes given above follow glibc2. The Single Unix Specification agrees, except that it has return values of type `ssize_t' (while BSD 4.* and libc4 and libc5 all have `int'). The ''flags'' argument is `int' in BSD 4.*, but `unsigned int' in libc4 and libc5. The ''len'' argument is `int' in BSD 4.*, but `size_t' in libc4 and libc5. The ''fromlen'' argument is `int *' in BSD 4.*, libc4 and libc5. The present `socklen_t *' was invented by POSIX. See also accept(2).
1 perry 132
133 !!SEE ALSO
2 PerryLorier 134 fcntl(2), read(2), select(2), getsockopt(2), socket(2), cmsg(3)
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