[Top][All Lists]

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [Help-smalltalk] How to access environment variables ?

From: Paolo Bonzini
Subject: Re: [Help-smalltalk] How to access environment variables ?
Date: Mon, 28 Mar 2011 10:14:50 +0200
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20101209 Fedora/3.1.7-0.35.b3pre.fc14 Lightning/1.0b3pre Mnenhy/0.8.3 Thunderbird/3.1.7

On 03/28/2011 12:19 AM, Mehul Sanghvi wrote:
st>  Smalltalk getenv: 'SHELL'
st>  Smalltalk environ

Actually, I was wrong. The correct definition will use one of these cCall pragmas:

<cCall: 'environ' returning: #(#ptr #char) args: #()>
<cCall: 'environ' returning: #{CString} args: #()>

This is because a CObject always adds implicitly a "*" to the C type. A CInt is an int *, a #(#ptr #int) is an int **. Similarly:

- a #(#ptr #char) is a char **.  This is correct;

- in C, a String is a char *. So, a CString is also a char **, and a #(#ptr #string) would have been a char ***. This is why it was wrong.

So you have two ways to express a char **. The two are almost equivalent, but are used in a slightly different way.

Let's start from #(#ptr #char). Here, "env" is a pointer to the first string, and "env value" will be a CChar object (i.e. a char *, like "*env" in C):

    st> env
    st> env value

It will pointing to the first character in the string. You can see it with "env value value", which is like **env in C:

    st> env value value

So you need to convert the null-terminated C string manually to a Smalltalk String object:

    st> String fromCData: env value

If you want to obtain a Dictionary of environment variables, you have this:

    env := Smalltalk environ.
    envDict := Dictionary new.
    [ (envString := env value) isNil ] whileFalse: [
        match := (String fromCData: envString) =~ '(.*?)=(.*)'.
        envDict at: (match at: 1) put: (match at: 2).
        env incr ]

With #{CString}, instead, GNU Smalltalk will do some of the work for you. However, to avoid falling into the mistake of thinking it's magic, you should first understand the above well.

With that change, you'll get

    st> s := Smalltalk environ.

This is the same pointer to the first environment variable; note the address is the same. However, when you do "env value" GNU Smalltalk will automatically fetch the whole null-terminated string:

    st> env value

Then, to place the variables in a dictionary you need a small change to the above code:

    env := Smalltalk environ.
    envDict := Dictionary new.
    [ (envString := env value) isNil ] whileFalse: [
        match := envString =~ '(.*?)=(.*)'.
        envDict at: (match at: 1) put: (match at: 2).
        env incr ]

Regarding the rest of your message,

st>  Smalltalk environ:

I am guessing some sort of pointer issue either in cint.c or ?

No, there's no guessing required. You simply haven't completed your statement, so gst is waiting for you to type the end.

Honest question, how much do you know Smalltalk? I think you have a bigger problem than accessing environment variables.


reply via email to

[Prev in Thread] Current Thread [Next in Thread]