| sa(3) | Socket Abstraction | sa(3) |
OSSP sa - Socket Abstraction
OSSP sa 1.2.5 (02-Oct-2005)
OSSP sa is an abstraction library for the Unix Socket networking application programming interface (API), featuring stream and datagram oriented communication over Unix Domain and Internet Domain (TCP and UDP) sockets.
It provides the following key features:
OSSP sa uses three data types in its API:
SA_OK Everything Ok
SA_ERR_ARG Invalid Argument
SA_ERR_USE Invalid Use Or Context
SA_ERR_MEM Not Enough Memory
SA_ERR_MTC Matching Failed
SA_ERR_EOF End Of Communication
SA_ERR_TMT Communication Timeout
SA_ERR_SYS Operating System Error (see errno)
SA_ERR_IMP Implementation Not Available
SA_ERR_INT Internal Error
OSSP sa provides a bunch of API functions, all modelled after the same prototype:
sa_rc_t sa_name(sa_[addr_]_t *, ...)
This means, every function returns sa_rc_t
to indicate its success (SA_OK) or failure
(SA_ERR_XXX) by returning a return code (the
corresponding describing text can be determined by passing this return code
to sa_error(3)). Each function name starts with the common prefix
sa_ and receives a sa_t (or
sa_addr_t) object handle on which it operates as its
first argument.
Address Object Operations
This API part provides operations for the creation and destruction of address abstraction sa_addr_t.
Example: sa_addr_t *saa; sa_addr_create(&saa);
Example: sa_addr_destroy(saa);
Address Operations
This API part provides operations for working with the address abstraction sa_addr_t.
The supported syntax for uri is: "unix:path" for Unix Domain addresses and "inet://addr:port[#protocol]" for Internet Domain addresses.
In the URI, path can be an absolute or relative filesystem path to an existing or not-existing file. addr can be an IPv4 address in dotted decimal notation ("127.0.0.1"), an IPv6 address in colon-separated (optionally abbreviated) hexadecimal notation ("::1") or a to-be-resolved hostname ("localhost.example.com"). port has to be either a decimal port in the range 1...65535 or a port name ("smtp"). If port is specified as a name, it is resolved as a TCP port by default. To force resolving a port name via a particular protocol, protocol can be specified as either "tcp" or "udp".
The result is stored in saa on success.
Example: sa_addr_u2a(saa, "inet://192.168.0.1:smtp");
The accepted addresses for sabuf are: struct sockaddr_un (AF_LOCAL), struct sockaddr_in (AF_INET) and struct sockaddr_in6 (AF_INET6). The salen is the corresponding sizeof(...) of the particular underyling structure.
The result is stored in saa on success.
Example: sockaddr_in in; sa_addr_s2a(saa, (struct sockaddr *)&in, (socklen_t)sizeof(in));
The result is a string of the form "unix:path" for Unix Domain addresses and "inet://addr:port" for Internet Domain addresses. Notice that addr and port are returned in numerical (unresolved) way. Additionally, because usually one cannot map bidirectionally between TCP or UDP port names and the numerical value, there is no distinction between TCP and UDP here.
The result is stored in uri on success. The caller has to free(3) the uri buffer later.
Example: char *uri; sa_addr_a2u(saa, &uri);
The result is one of the following particular underlying address structures: struct sockaddr_un (AF_LOCAL), struct sockaddr_in (AF_INET) and struct sockaddr_in6 (AF_INET6).
The result is stored in sabuf and salen on success. The caller has to free(3) the sabuf buffer later.
Example: struct sockaddr sabuf, socklen_t salen; sa_addr_a2s(saa, &sa, &salen);
This compares the addresses saa1 and saa2 by only taking the prefix part of length prefixlen into account. prefixlen is number of filesystem path characters for Unix Domain addresses and number of bits for Internet Domain addresses. In case of Internet Domain addresses, the addresses are matched in network byte order and the port (counting as an additional bit/item of length 1) is virtually appended to the address for matching. Specifying prefixlen as -1 means matching the whole address (but without the virtually appended port) without having to know how long the underlying address representation (length of path for Unix Domain addresses, 32+1 [IPv4] or 128+1 [IPv6] for Internet Domain addresses) is. Specifying prefixlen as -2 is equal to -1 but additionally the port is matched, too.
This especially can be used to implement Access Control Lists (ACL) without having to fiddle around with the underlying representation. For this, make saa1 the to be checked address and saa2 plus prefixlen the ACL pattern as shown in the following example.
Example:
sa_addr_t *srv_sa;
sa_addr_t *clt_saa;
sa_t *clt_sa;
sa_addr_t *acl_saa;
char *acl_addr = "192.168.0.0";
int acl_len = 24;
...
sa_addr_u2a(&acl_saa, "inet://%s:0", acl_addr);
...
while (sa_accept(srv_sa, &clt_saa, &clt_sa) == SA_OK) {
if (sa_addr_match(clt_saa, acl_saa, acl_len) != SA_OK) {
/* connection refused */
...
sa_addr_destroy(clt_saa);
sa_destroy(clt_sa);
continue;
}
...
}
...
Socket Object Operations
This API part provides operations for the creation and destruction of socket abstraction sa_t.
Example: sa_t *sa; sa_create(&sa);
Example: sa_destroy(sa);
Socket Parameter Operations
This API part provides operations for parameterizing the socket abstraction sa_t.
A socket can only be assigned a single protocol type at any time. Nevertheless one can switch the type of a socket abstraction object at any time in order to reuse it for a different communication. Just keep in mind that switching the type will stop a still ongoing communication by closing the underlying socket.
Possible values for type are SA_TYPE_STREAM (stream communication) and SA_TYPE_DATAGRAM (datagram communication). The default communication protocol type is SA_TYPE_STREAM.
Example: sa_type(sa, SA_TYPE_STREAM);
Possible values for id are: SA_TIMEOUT_ACCEPT (affecting sa_accept(3)), SA_TIMEOUT_CONNECT (affecting sa_connect(3)), SA_TIMEOUT_READ (affecting sa_read(3), sa_readln(3) and sa_recv(3)) and SA_TIMEOUT_WRITE (affecting sa_write(3), sa_writef(3), sa_send(3), and sa_sendf(3)). Additionally you can set all four timeouts at once by using SA_TIMEOUT_ALL. The default is that no communication timeouts are used which is equal to sec=0/usec=0.
Example: sa_timeout(sa, SA_TIMEOUT_ALL, 30, 0);
Possible values for id are: SA_BUFFER_READ (affecting sa_read(3) and sa_readln(3)) and SA_BUFFER_WRITE (affecting sa_write(3) and sa_writef(3)). The default is that no communication buffers are used which is equal to size=0.
Example: sa_buffer(sa, SA_BUFFER_READ, 16384);
The adjusted option is controlled by id. The number and type of the expected following argument(s) are dependent on the particular option. Currently the following options are implemented (option arguments in parenthesis):
SA_OPTION_NAGLE (int yesno) for enabling (yesno=1) or disabling (yesno == 0) Nagle's Algorithm (see RFC898 and TCP_NODELAY of setsockopt(2)).
SA_OPTION_LINGER (int amount) for enabling (amount == seconds != 0) or disabling (amount == 0) lingering on close (see SO_LINGER of setsockopt(2)). Notice: using seconds > 0 results in a regular (maximum of seconds lasting) lingering on close while using seconds < 0 results in the special case of a TCP RST based connection termination on close.
SA_OPTION_REUSEADDR (int yesno) for enabling (yesno == 1) or disabling (yesno == 0) the reusability of the address on binding via sa_bind(3) (see SO_REUSEADDR of setsockopt(2)).
SA_OPTION_REUSEPORT (int yesno) for enabling (yesno == 1) or disabling (yesno == 0) the reusability of the port on binding via sa_bind(3) (see SO_REUSEPORT of setsockopt(2)).
SA_OPTION_NONBLOCK (int yesno) for enabling (yesno == 1) or disabling (yesno == 0) non-blocking I/O mode (see O_NONBLOCK of fcntl(2)).
Example: sa_option(sa, SA_OPTION_NONBLOCK, 1);
This allows you to override mostly all I/O related system calls OSSP sa internally performs while communicating. This can be used to adapt OSSP sa to different run-time environments and requirements without having to change the source code. Usually this is used to divert the system calls to the variants of a user-land multithreading facility like GNU Pth.
The function supplied as fptr is required to fulfill the API of the replaced system call, i.e., it has to have the same prototype (if fctx is NULL). If fctx is not NULL, this prototype has to be extended to accept an additional first argument of type void * which receives the value of fctx. It is up to the callback function whether to pass the call through to the replaced actual system call or not.
Possible values for id are (expected prototypes behind fptr are given in parenthesis):
SA_SYSCALL_CONNECT: "int (*)([void *,] int, const struct sockaddr *, socklen_t)", see connect(2).
SA_SYSCALL_ACCEPT: "int (*)([void *,] int, struct sockaddr *, socklen_t *)", see accept(2).
SA_SYSCALL_SELECT: "int (*)([void *,] int, fd_set *, fd_set *, fd_set *, struct timeval *)", see select(2).
SA_SYSCALL_READ: "ssize_t (*)([void *,] int, void *, size_t)", see read(2).
SA_SYSCALL_WRITE: "ssize_t (*)([void *,] int, const void *, size_t)", see write(2).
SA_SYSCALL_RECVFROM: "ssize_t (*)([void *,] int, void *, size_t, int, struct sockaddr *, socklen_t *)", see recvfrom(2).
SA_SYSCALL_SENDTO: "ssize_t (*)([void *,] int, const void *, size_t, int, const struct sockaddr *, socklen_t)", see sendto(2).
Example:
ssize_t
trace_read(void *ctx, int fd, void *buf, size_t len)
{
FILE *fp = (FILE *)ctx;
ssize_t rv;
int errno_saved;
rv = read(fd, buf, len);
errno_saved = errno;
fprintf(fp, "read(%d, %lx, %d) = %d\n",
fd, (long)buf, len, rv);
errno = errno_saved;
return rv;
}
...
FILE *trace_fp = ...;
sa_syscall(sa, SA_SC_READ, trace_read, trace_fp);
...
Socket Connection Operations
This API part provides connection operations for stream-oriented data communication through the socket abstraction sa_t.
This assigns the local protocol address laddr. When a socket is created, it exists in an address family space but has no protocol address assigned. This call requests that laddr be used as the local address. For servers this is the address they later listen on (see sa_listen(3)) for incoming connections, for clients this is the address used for outgoing connections (see sa_connect(3)). Internally this directly maps to bind(2).
Example: sa_bind(sa, laddr);
This performs a connect to the remote address raddr. If the socket is of type SA_TYPE_DATAGRAM, this call specifies the peer with which the socket is to be associated; this address is that to which datagrams are to be sent, and the only address from which datagrams are to be received. If the socket is of type SA_TYPE_STREAM, this call attempts to make a connection to the remote socket. Internally this directly maps to connect(2).
Example: sa_connect(sa, raddr);
A willingness to accept incoming connections and a queue limit for incoming connections are specified by this call. The backlog argument defines the maximum length the queue of pending connections may grow to. Internally this directly maps to listen(2).
Example: sa_listen(sa, 128);
This accepts an incoming connection by extracting the first connection request on the queue of pending connections. It creates a new socket abstraction object (returned in csa) and a new socket address abstraction object (returned in caddr) describing the connection. The caller has to destroy these objects later. If no pending connections are present on the queue, it blocks the caller until a connection is present.
Example:
sa_addr_t *clt_saa;
sa_t *clt_sa;
...
while (sa_accept(srv_sa, &clt_saa, &clt_sa) == SA_OK) {
...
}
This determines the address of the communication peer and creates a new socket address abstraction object (returned in raddr) describing the peer address. The application has to destroy raddr later with sa_addr_destroy(3). Internally this maps to getpeername(2).
Example: sa_addr_t *raddr; sa_getremote(sa, &raddr);
This determines the address of the local communication side and creates a new socket address abstraction object (returned in laddr) describing the local address. The application has to destroy laddr later with sa_addr_destroy(3). Internally this maps to getsockname(2).
Example: sa_addr_t *laddr; sa_getlocal(sa, &laddr);
This performs a shut down of the connection described in sa. The flags string can be either "r" (indicating the read channel of the communication is shut down only), "w" (indicating the write channel of the communication is shut down only), or "rw" (indicating both the read and write channels of the communication are shut down). Internally this directly maps to shutdown(2).
Example: sa_shutdown(sa,
"w");
Socket Input/Output Operations (Stream Communication)
This API part provides I/O operations for stream-oriented data communication through the socket abstraction sa_t.
This peeks into the underlying socket filedescriptor OSSP sa allocated internally for the communication. This can be used for adjusting the socket communication (via fcntl(2), setsockopt(2), etc) directly.
Think twice before using this, then think once more. After all that, think again. With enough thought, the need for directly manipulating the underlying socket can often be eliminated. At least remember that all your direct socket operations fully by-pass OSSP sa and this way can leads to nasty side-effects.
Example: int fd; sa_getfd(sa, &fd);
This reads from the socket (optionally through the internal read buffer) up to a maximum of buflen bytes into buffer buf. The actual number of read bytes is stored in bufdone. This internally maps to read(2).
Example: char buf[1024]; size_t n; sa_read(sa, buf, sizeof(buf), &n);
This reads from the socket (optionally through the internal read buffer) up to a maximum of buflen bytes into buffer buf, but only as long as no line terminating newline character (0x0a) was found. The line terminating newline character is stored in the buffer plus a (not counted) terminating NUL character ('\0'), too. The actual number of read bytes is stored in bufdone. This internally maps to sa_read(3).
Keep in mind that for efficiency reasons, line-oriented I/O usually always should be performed with read buffer (see sa_option(3) and SA_BUFFER_READ). Without such a read buffer, the performance is cruel, because single character read(2) operations would be performed on the underlying socket.
Example: char buf[1024]; size_t n; sa_readln(sa, buf, sizeof(buf), &n);
This writes to the socket (optionally through the internal write buffer) buflen bytes from buffer buf. In case of a partial write, the actual number of written bytes is stored in bufdone. This internally maps to write(2).
Example: sa_write(sa, cp, strlen(cp), NULL);
This formats a string according to the printf(3)-style format specification fmt and sends the result to the socket (optionally through the internal write buffer). In case of a partial socket write, the not written data of the formatted string is internally discarded. Hence using a write buffer is strongly recommended here (see sa_option(3) and SA_BUFFER_WRITE). This internally maps to sa_write(3).
The underlying string formatting engine is just a minimal one and for security and independence reasons intentionally not directly based on s[n]printf(3). It understands only the following format specifications: "%%", "%c" (char), "%s" (char *) and "%d" (int) without any precision and padding possibilities. It is intended for minimal formatting only. If you need more sophisticated formatting, you have to format first into an own buffer via s[n]printf(3) and then write this to the socket via sa_write(3) instead.
Example: sa_writef(sa, "%s=%d\n", cp, i);
This writes all still pending outgoing data for the internal write buffer (see sa_option(3) and SA_BUFFER_WRITE) to the socket. This internally maps to write(2).
Example: sa_flush(sa);
Socket Input/Output Operations (Datagram Communication)
This API part provides I/O operations for datagram-oriented data communication through the socket abstraction sa_t.
This receives from the remote address specified in raddr via the socket up to a maximum of buflen bytes into buffer buf. The actual number of received bytes is stored in bufdone. This internally maps to recvfrom(2).
Example: char buf[1024]; size_t n; sa_recv(sa, buf, sizeof(buf), &n, saa);
This sends to the remote address specified in raddr via the socket buflen bytes from buffer buf. The actual number of sent bytes is stored in bufdone. This internally maps to sendto(2).
Example: sa_send(sa, buf, strlen(buf), NULL, saa);
This formats a string according to the printf(3)-style format specification fmt and sends the result to the socket as a single piece of data chunk. In case of a partial socket write, the not written data of the formatted string is internally discarded.
The underlying string formatting engine is just a minimal one and for security and independence reasons intentionally not directly based on s[n]printf(3). It understands only the following format specifications: "%%", "%c" (char), "%s" (char *) and "%d" (int) without any precision and padding possibilities. It is intended for minimal formatting only. If you need more sophisticated formatting, you have to format first into an own buffer via s[n]printf(3) and then send this to the remote address via sa_send(3) instead.
Example: sa_sendf(sa, saa,
"%s=%d\n", cp, i);
Socket Error Handling
This API part provides error handling operations only.
Standards
R. Gilligan, S. Thomson, J. Bound, W. Stevens: "Basic Socket Interface Extensions for IPv6", RFC 2553, March 1999.
W. Stevens: "Advanced Sockets API for IPv6", RFC 2292, February 1998.
R. Fielding, L. Masinter, T. Berners-Lee: "Uniform Resource Identifiers: Generic Syntax", RFC 2396, August 1998.
R. Hinden, S. Deering: "IP Version 6 Addressing Architecture", RFC 2373, July 1998.
R. Hinden, B. Carpenter, L. Masinter: "Format for Literal
IPv6 Addresses in URL's", RFC 2732, December 1999.
Papers
Stuart Sechrest: "An Introductory 4.4BSD Interprocess Communication Tutorial", FreeBSD 4.4 (/usr/share/doc/psd/20.ipctut/).
Samuel J. Leffler, Robert S. Fabry, William N. Joy, Phil Lapsley: "An Advanced 4.4BSD Interprocess Communication Tutorial", FreeBSD 4.4 (/usr/share/doc/psd/21.ipc/).
Craig Metz: "Protocol Independence Using the Sockets
API",
http://www.usenix.org/publications/library/proceedings/usenix2000/freenix/metzprotocol.html,
USENIX Annual Technical Conference, June 2000.
Manual Pages
socket(2), accept(2), bind(2), connect(2), getpeername(2), getsockname(2), getsockopt(2), ioctl(2), listen(2), read(2), recv(2), select(2), send(2), shutdown(2), socketpair(2), write(2), getprotoent(3), protocols(4).
OSSP sa was invented in August 2001 by Ralf S. Engelschall <rse@engelschall.com> under contract with Cable & Wireless <http://www.cw.com/> for use inside the OSSP project. Its creation was prompted by the requirement to implement an SMTP logging channel for the OSSP l2 library. Its initial code was derived from a predecessor sub-library originally written for socket address abstraction inside the OSSP lmtp2nntp tool.
Ralf S. Engelschall rse@engelschall.com www.engelschall.com
| OSSP sa 1.2.5 | 02-Oct-2005 |