3

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?

HostileFork says dont trust SE
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2 Answers2

7
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.

meh.
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  • Thanks! This works, although the return from LIBC's getline is the length of the line retrieved (not the string), so it would be an integer... you can edit your answers (I could too). But I'll let you do it. :-) – HostileFork says dont trust SE Dec 16 '13 at 02:39
2

In a world of little Red documentation, one actual Red/System Specification Document is pretty useful, and that section contains the answer indirectly in another example:

p: declare pointer! [byte!]            ;-- char *p;

which would result in:

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