HTTP is like being married: you have to be able to handle whatever you’re given, while being very careful what you send back.
In A Web Service with Interaction,
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
throughout the rest of this web page.
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