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urlinformix4gl

Validate URL in Informix 4GL program

In my Informix 4GL program, I have an input field where the user can insert a URL and the feed is later being sent over to the web via a script. How can I validate the URL at the time of input, to ensure that it's a live link? Can I make a call and see if I get back any errors?

Solution

  • I4GL checking the URL

    There is no built-in function to do that (URLs didn't exist when I4GL was invented, amongst other things).

    If you can devise a C method to do that, you can arrange to call that method through the C interface. You'll write the method in native C, and then write an I4GL-callable C interface function using the normal rules. When you build the program with I4GL c-code, you'll link the extra C functions too. If you build the program with I4GL-RDS (p-code), you'll need to build a custom runner with the extra function(s) exposed. All of this is standard technique for I4GL.

    In general terms, the C interface code you'll need will look vaguely like this:

    #include <fglsys.h>

// Standard interface for I4GL-callable C functions
extern int i4gl_validate_url(int nargs);

// Using obsolescent interface functions
int i4gl_validate_url(int nargs)
{
    if (nargs != 1)
        fgl_fatal(__FILE__, __LINE__, -1318);
    char url[4096];
    popstring(url, sizeof(url));
    int r = validate_url(url);     // Your C function
    retint(r);
    return 1;
}

    You can and should check the manuals but that code, using the 'old style' function names, should compile correctly. The code can be called in I4GL like this:

    DEFINE url CHAR(256)
DEFINE rc  INTEGER

LET url = "http://www.google.com/"
LET rc = i4gl_validate_url(url)

IF rc != 0 THEN
    ERROR "Invalid URL"
ELSE
    MESSAGE "URL is OK"
END IF

    Or along those general lines. Exactly what values you return depends on your decisions about how to return a status from validate_url(). If need so be, you can return multiple values from the interface function (e.g. error number and text of error message). Etc. This is about the simplest possible design for calling some C code to validate a URL from within an I4GL program.

    Modern C interface functions

    The function names in the interface library were all changed in the mid-00's, though the old names still exist as macros. The old names were:

    • popstring(char *buffer, int buflen)
    • retint(int retval)
    • fgl_fatal(const char *file, int line, int errnum)

    You can find the revised documentation at IBM Informix 4GL v7.50.xC3: Publication library in PDF in the 4GL Reference Manual, and you need Appendix C "Using C with IBM Informix 4GL".

    The new names start ibm_lib4gl_:

    • ibm_libi4gl_popMInt()
    • ibm_libi4gl_popString()

    As to the error reporting function, there is one — it exists — but I don't have access to documentation for it any more. It'll be in the fglsys.h header. It takes an error number as one argument; there's the file name and a line number as the other arguments. And it will, presumably, be ibm_lib4gl_… and there'll be probably be Fatal or perhaps fatal (or maybe Err or err) in the rest of the name.

    I4GL running a script that checks the URL

    Wouldn't it be easier to write a shell script to get the status code? That might work if I can return the status code or any existing results back to the program into a variable? Can I do that?

    Quite possibly. If you want the contents of the URL as a string, though, you'll might end up wanting to call C. It is certainly worth thinking about whether calling a shell script from within I4GL is doable. If so, it will be a lot simpler (RUN "script", IIRC, where the literal string would probably be replaced by a built-up string containing the command and the URL). I believe there are file I/O functions in I4GL now, too, so if you can get the script to write a file (trivial), you can read the data from the file without needing custom C. For a long time, you needed custom C to do that.

    I just need to validate the URL before storing it into the database. I was thinking about:

    #!/bin/bash

read -p "URL to check: " url
if curl --output /dev/null --silent --head --fail "$url"; then
    printf '%s\n' "$url exist"
else
    printf '%s\n' "$url does not exist"
fi

    but I just need the output instead of /dev/null to be into a variable. I believe the only option is to dump the output into a temp file and read from there.

    Instead of having I4GL run the code to validate the URL, have I4GL run a script to validate the URL. Use the exit status of the script and dump the output of curl into /dev/null.

    FUNCTION check_url(url)

    DEFINE url VARCHAR(255)
    DEFINE command_line VARCHAR(255)
    DEFINE exit_status  INTEGER

    LET command_line = "check_url ", url
    RUN command_line RETURNING exit_status

    RETURN exit_status

END FUNCTION {check_url}

    Your calling code can analyze exit_status to see whether it worked. A value of 0 indicates success; non-zero indicates a problem of some sort, which can be deemed 'URL does not work'.

    Make sure the check_url script (a) exits with status zero on success and non-zero on any sort of failure, and (b) doesn't write anything to standard output (or standard error) by default. The writing to standard error or output will screw up screen layouts, etc, and you do not want that. (You can obviously have options to the script that enable standard output, or you can invoke the script with options to suppress standard output and standard error, or redirect the outputs to /dev/null; however, when used by the I4GL program, it should be silent.)

    Your 'script' (check_url) could be as simple as:

    #!/bin/bash

exec curl --output /dev/null --silent --head --fail "${1:-http://www.example.com/"

    This passes the first argument to curl, or the non-existent example.com URL if no argument is given, and replaces itself with curl, which generates a zero/non-zero exit status as required. You might add 2>/dev/null to the end of the command line to ensure that error messages are not seen. (Note that it will be hell debugging this if anything goes wrong; make sure you've got provision for debugging.)

    The exec is a minor optimization; you could omit it with almost no difference in result. (I could devise a scheme that would probably spot the difference; it involves signalling the curl process, though — kill -9 9999 or similar, where the 9999 is the PID of the curl process — and isn't of practical significance.)

    Given that the script is just one line of code that invokes another program, it would be possible to embed all that in the I4GL program. However, having an external shell script (or Perl script, or …) has merits of flexibility; you can edit it to log attempts, for example, without changing the I4GL code at all. One more file to distribute, but better flexibility — keep a separate script, even though it could all be embedded in the I4GL.