pointersshared-librariesgetlinered-langred-system

How to create a Red/System binding to a function that takes a pointer to a pointer?


I was looking at how Red/System hooks up with C library functions from Windows DLLs, Linux/Android .so shared libraries, and OS/X .dylib, with the #import syntax:

#import [
    "libc.so.6" cdecl [

        allocate: "malloc" [
            size [integer!]
            return: [byte-ptr!]
        ]

        free: "free" [
            block [byte-ptr!]
        ]

        /* ... */
]

Works for all the basic types, but what do you do when a function wants a pointer to a pointer? For instance, what if you were trying to bind to something like getline(), which has the prototype:

ssize_t getline(char **lineptr, size_t *n, FILE *stream);

One way of calling this (the way I want to call it...) is where the incoming lineptr is not a preallocated buffer, but rather null. In this case, getline allocates the buffer for you and modifies the char* so it is no longer null.

As a first try, I did:

getline: "getline" [
    lineptr [byte-ptr!]       ;-- char **
    n [pointer! [integer!]]   ;-- size_t *
    stream [byte-ptr!]        ;-- FILE *
    return: [integer!]        ;-- ssize_t
]

But since there isn't such a thing as a pointer! [c-string!] or similar, I don't see how to call it. Can I get the address of a c-string! local variable and pass it as a byte-ptr! somehow?


Solution

  • Red/System [
        Title: "derp"
    ]
    
    #import [
        LIBC-file cdecl [
            _fdopen: "fdopen" [
                fd   [integer!]
                type [c-string!]
    
                return: [pointer! [byte!]]
            ]
    
            _getline: "getline" [
                line   [pointer! [integer!]]
                n      [pointer! [integer!]]
                stream [pointer! [byte!]]
    
                return: [integer!]
            ]
        ]
    ]
    
    getline: func [
        stream  [pointer! [byte!]]
        return: [c-string!]
    
        /local
            line [integer!]
            n    [integer!]
    ] [
        line: 0
        n:    0
    
        _getline :line :n stream
        as c-string! line
    ]
    
    stream: _fdopen 0 "r"
    line:  getline stream
    
    print line
    

    This works.