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Annotated edit history of recvfrom(2) version 2, 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
10
2 PerryLorier 11 __int recvfrom(int__ ''s''__, void *__''buf''__, size_t__ ''len''__, int__ ''flags''__, struct sockaddr *__''from''__, socklen_t *__''fromlen''__);__
1 perry 12
13
2 PerryLorier 14 __int recvmsg(int__ ''s''__, struct msghdr *__''msg''__, int__ ''flags''__);__
1 perry 15
16 !!DESCRIPTION
2 PerryLorier 17 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 18
2 PerryLorier 19 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 20
2 PerryLorier 21 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 22
2 PerryLorier 23 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 24
2 PerryLorier 25 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
26 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 27 for receipt of the full amount requested.
28
2 PerryLorier 29 The select(2) or poll(2) call may be used to determine when more data arrives.
1 perry 30
2 PerryLorier 31 The ''flags'' argument to a recv call is formed by ''OR'''ing one or more of the following values:
1 perry 32
2 PerryLorier 33 ;__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 34
2 PerryLorier 35 ;__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 36
2 PerryLorier 37 ;__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 38
2 PerryLorier 39 ;__MSG_NOSIGNAL__: This flag turns off raising of [SIGPIPE] on stream sockets when the other end disappears.
1 perry 40
41
2 PerryLorier 42 ;__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 43
2 PerryLorier 44 ;__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 45 __msg_name__.
46
2 PerryLorier 47 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
48 __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 49
2 PerryLorier 50 The error is supplied in a __sock_extended_err__ structure:
1 perry 51
2 PerryLorier 52 #define SO_EE_ORIGIN_NONE 0
53 #define SO_EE_ORIGIN_LOCAL 1
54 #define SO_EE_ORIGIN_ICMP 2
55 #define SO_EE_ORIGIN_ICMP6 3
56 struct sock_extended_err
57 {
58 u_int32_t ee_errno; /* error number */
59 u_int8_t ee_origin; /* where the error originated */
60 u_int8_t ee_type; /* type */
61 u_int8_t ee_code; /* code */
62 u_int8_t ee_pad;
63 u_int32_t ee_info; /* additional information */
64 u_int32_t ee_data; /* other data */
65 /* More data may follow */
66 };
67 struct sockaddr *SO_EE_OFFENDER(struct sock_extended_err *);
1 perry 68
2 PerryLorier 69 __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.
70 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
71 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 72
2 PerryLorier 73 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
74 __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 75
2 PerryLorier 76 The __recvmsg__ call uses a ''msghdr'' structure to minimize the number of directly supplied parameters. This structure has the following form, as defined in ''<sys/socket.h>'':
1 perry 77
78
2 PerryLorier 79 struct msghdr {
80 void * msg_name; /* optional address */
81 socklen_t msg_namelen; /* size of address */
82 struct iovec * msg_iov; /* scatter/gather array */
83 size_t msg_iovlen; /* # elements in msg_iov */
84 void * msg_control; /* ancillary data, see below */
85 socklen_t msg_controllen; /* ancillary data buffer len */
86 int msg_flags; /* flags on received message */
87 };
1 perry 88
2 PerryLorier 89 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 90
91 The messages are of the form:
2 PerryLorier 92 struct cmsghdr {
93 socklen_t cmsg_len; /* data byte count, including hdr */
94 int cmsg_level; /* originating protocol */
95 int cmsg_type; /* protocol-specific type */
96 /* followed by
97 u_char cmsg_data[[]; */
98 };
1 perry 99
2 PerryLorier 100 Ancillary data should only be accessed by the macros defined in cmsg(3).
1 perry 101
2 PerryLorier 102 As an example, Linux uses this auxiliary data mechanism to pass extended errors, IP options or file descriptors over Unix sockets.
1 perry 103
2 PerryLorier 104 The ''msg_flags'' field in the msghdr is set on return of __recvmsg__(). It can contain several flags:
1 perry 105
2 PerryLorier 106 ;__MSG_EOR__: indicates end-of-record; the data returned completed a record (generally used with sockets of type __SOCK_SEQPACKET__).
1 perry 107
2 PerryLorier 108 ;__MSG_TRUNC__: indicates that the trailing portion of a datagram was discarded because the datagram was larger than the buffer supplied.
1 perry 109
2 PerryLorier 110 ;__MSG_CTRUNC__: indicates that some control data were discarded due to lack of space in the buffer for ancillary data.
1 perry 111
2 PerryLorier 112 ;__MSG_OOB__: is returned to indicate that expedited or out-of-band data were received.
1 perry 113
2 PerryLorier 114 ;__MSG_ERRQUEUE__: indicates that no data was received but an extended error from the socket error queue.
1 perry 115
2 PerryLorier 116 ;__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 117
118 !!RETURN VALUE
2 PerryLorier 119 These calls return the number of bytes received, or -1 if an error occurred.
1 perry 120
121 !!ERRORS
2 PerryLorier 122 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 123 pages.
124
2 PerryLorier 125 ;[EBADF]: The argument ''s'' is an invalid descriptor.
126 ;[ECONNREFUSED]: A remote host refused to allow the network connection (typically because it is not running the requested service).
127 ;[ENOTCONN]: The socket is associated with a connection-oriented protocol and has not been connected (see connect(2) and accept(2)).
128 ;[ENOTSOCK]: The argument ''s'' does not refer to a socket.
129 ;[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.
130 ;[EINTR]: The receive was interrupted by delivery of a signal before any data were available.
131 ;[EFAULT]: The receive buffer pointer(s) point outside the process's address space.
132 ;[EINVAL]: Invalid argument passed.
1 perry 133
134 !!CONFORMING TO
2 PerryLorier 135 4.4BSD (these function calls first appeared in 4.2BSD).
1 perry 136
137 !!NOTE
2 PerryLorier 138 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 139
140 !!SEE ALSO
2 PerryLorier 141 fcntl(2), read(2), select(2), getsockopt(2), socket(2), cmsg(3)
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