gawkinet: CGI Lib
2.9.1 A Simple CGI Library
--------------------------
HTTP is like being married: you have to be able to handle whatever
you're given, while being very careful what you send back.
Phil Smith III,
<http://www.netfunny.com/rhf/jokes/99/Mar/http.html>
In 
A Web Service with Interaction Interacting Service, we saw
the function 'CGI_setup()' as part of the web server "core logic"
framework. The code presented there handles almost everything necessary
for CGI requests. One thing it doesn't do is handle encoded characters
in the requests. For example, an '&' is encoded as a percent sign
followed by the hexadecimal value: '%26'. These encoded values should
be decoded. Following is a simple library to perform these tasks. This
code is used for all web server examples used throughout the rest of
this Info file. If you want to use it for your own web server, store
the source code into a file named 'inetlib.awk'. Then you can include
these functions into your code by placing the following statement into
your program (on the first line of your script):
@include inetlib.awk
But beware, this mechanism is only possible if you invoke your web
server script with 'igawk' instead of the usual 'awk' or 'gawk'. Here
is the code:
# CGI Library and core of a web server
# Global arrays
# GETARG --- arguments to CGI GET command
# MENU --- menu items (path names)
# PARAM --- parameters of form x=y
# Optional variable MyHost contains host address
# Optional variable MyPort contains port number
# Needs TopHeader, TopDoc, TopFooter
# Sets MyPrefix, HttpService, Status, Reason
BEGIN {
if (MyHost == "") {
"uname -n" | getline MyHost
close("uname -n")
}
if (MyPort == 0) MyPort = 8080
HttpService = "/inet/tcp/" MyPort "/0/0"
MyPrefix = "http://" MyHost ":" MyPort
SetUpServer()
while ("awk" != "complex") {
# header lines are terminated this way
RS = ORS = "\r\n"
Status = 200 # this means OK
Reason = "OK"
Header = TopHeader
Document = TopDoc
Footer = TopFooter
if (GETARG["Method"] == "GET") {
HandleGET()
} else if (GETARG["Method"] == "HEAD") {
# not yet implemented
} else if (GETARG["Method"] != "") {
print "bad method", GETARG["Method"]
}
Prompt = Header Document Footer
print "HTTP/1.0", Status, Reason |& HttpService
print "Connection: Close" |& HttpService
print "Pragma: no-cache" |& HttpService
len = length(Prompt) + length(ORS)
print "Content-length:", len |& HttpService
print ORS Prompt |& HttpService
# ignore all the header lines
while ((HttpService |& getline) > 0)
continue
# stop talking to this client
close(HttpService)
# wait for new client request
HttpService |& getline
# do some logging
print systime(), strftime(), $0
CGI_setup($1, $2, $3)
}
}
function CGI_setup( method, uri, version, i)
{
delete GETARG
delete MENU
delete PARAM
GETARG["Method"] = method
GETARG["URI"] = uri
GETARG["Version"] = version
i = index(uri, "?")
if (i > 0) { # is there a "?" indicating a CGI request?
split(substr(uri, 1, i-1), MENU, "[/:]")
split(substr(uri, i+1), PARAM, "&")
for (i in PARAM) {
PARAM[i] = _CGI_decode(PARAM[i])
j = index(PARAM[i], "=")
GETARG[substr(PARAM[i], 1, j-1)] = \
substr(PARAM[i], j+1)
}
} else { # there is no "?", no need for splitting PARAMs
split(uri, MENU, "[/:]")
}
for (i in MENU) # decode characters in path
if (i > 4) # but not those in host name
MENU[i] = _CGI_decode(MENU[i])
}
This isolates details in a single function, 'CGI_setup()'. Decoding
of encoded characters is pushed off to a helper function,
'_CGI_decode()'. The use of the leading underscore ('_') in the
function name is intended to indicate that it is an "internal" function,
although there is nothing to enforce this:
function _CGI_decode(str, hexdigs, i, pre, code1, code2,
val, result)
{
hexdigs = "123456789abcdef"
i = index(str, "%")
if (i == 0) # no work to do
return str
do {
pre = substr(str, 1, i-1) # part before %xx
code1 = substr(str, i+1, 1) # first hex digit
code2 = substr(str, i+2, 1) # second hex digit
str = substr(str, i+3) # rest of string
code1 = tolower(code1)
code2 = tolower(code2)
val = index(hexdigs, code1) * 16 \
+ index(hexdigs, code2)
result = result pre sprintf("%c", val)
i = index(str, "%")
} while (i != 0)
if (length(str) > 0)
result = result str
return result
}
This works by splitting the string apart around an encoded character.
The two digits are converted to lowercase characters and looked up in a
string of hex digits. Note that '0' is not in the string on purpose;
'index()' returns zero when it's not found, automatically giving the
correct value! Once the hexadecimal value is converted from characters
in a string into a numerical value, 'sprintf()' converts the value back
into a real character. The following is a simple test harness for the
above functions:
BEGIN {
CGI_setup("GET",
"http://www.gnu.org/cgi-bin/foo?p1=stuff&p2=stuff%26junk" \
"&percent=a %25 sign",
"1.0")
for (i in MENU)
printf "MENU[\"%s\"] = %s\n", i, MENU[i]
for (i in PARAM)
printf "PARAM[\"%s\"] = %s\n", i, PARAM[i]
for (i in GETARG)
printf "GETARG[\"%s\"] = %s\n", i, GETARG[i]
}
And this is the result when we run it:
$ gawk -f testserv.awk
-| MENU["4"] = www.gnu.org
-| MENU["5"] = cgi-bin
-| MENU["6"] = foo
-| MENU["1"] = http
-| MENU["2"] =
-| MENU["3"] =
-| PARAM["1"] = p1=stuff
-| PARAM["2"] = p2=stuff&junk
-| PARAM["3"] = percent=a % sign
-| GETARG["p1"] = stuff
-| GETARG["percent"] = a % sign
-| GETARG["p2"] = stuff&junk
-| GETARG["Method"] = GET
-| GETARG["Version"] = 1.0
-| GETARG["URI"] = http://www.gnu.org/cgi-bin/foo?p1=stuff&
p2=stuff%26junk&percent=a %25 sign
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