Re: function pointer casting

From: Barry Schwarz (schwarzb_at_deloz.net)
Date: 05/01/04

  • Next message: fix: "Re: Array of pointers in a struct" 
    Date: 1 May 2004 06:29:37 GMT

    On 30 Apr 2004 21:17:25 -0700, joe_bruin@hotmail.com (joe bruin)
    wrote:

    >hello all.
    >
    >i am trying to get rid of some warnings and do "the right thing".
    >although in this particular case, i am not sure what the right thing
    >is.
    >
    >the code:
    >
    >typedef struct
    >{
    > void *ctx;
    > void (*init)(void *);

    Here you say the second member of the struct is a function pointer
    whose argument is void*.

    > void (*update)(void *, const void *, unsigned long);
    > void (*final)(unsigned char *, void *);
    > int mdlen;
    > char *name;
    >} digest_t;
    >
    >MD2_CTX md2ctx;
    >MD4_CTX md4ctx;
    >MD5_CTX md5ctx;
    >SHA_CTX shactx;
    >RIPEMD160_CTX rmdctx;
    >
    >digest_t dig[] =
    >{
    > { &md2ctx, MD2_Init, MD2_Update, MD2_Final,
    > MD2_DIGEST_LENGTH, "MD2" },
    > { &md4ctx, MD4_Init, MD4_Update, MD4_Final,
    > MD4_DIGEST_LENGTH, "MD4" },
    > { &md5ctx, MD5_Init, MD5_Update, MD5_Final,
    > MD5_DIGEST_LENGTH, "MD5" },

    Here you initialize an instance of the struct and the second member is
    initialized to the address of MD5_Init.

    > { &shactx, SHA_Init, SHA_Update, SHA_Final,
    > SHA_DIGEST_LENGTH, "SHA" },
    > { &shactx, SHA1_Init, SHA1_Update, SHA1_Final,
    > SHA_DIGEST_LENGTH, "SHA1" },
    > { &rmdctx, RIPEMD160_Init, RIPEMD160_Update, RIPEMD160_Final,
    > RIPEMD160_DIGEST_LENGTH, "RIPEMD160" }
    >};
    >
    >this code will yield a "warning: initialization from incompatible
    >pointer type", on every Init, Update, and Final function in dig[].
    >these functions are all in the format:
    >
    >void MD5_Init(MD5_CTX *c);

    But the function MD5_Init actually takes a pointer to MD5_CTX which
    apparently is not the same as pointer to void.

    When calling a function that expects a void*, you can pass any kind of
    object pointer you want because the types are compatible for the
    implied assignment of the argument to the parameter. Consider that
    the following is legal because the implied conversion is allowed
            void *x;
            MD5_CTX y;
            x = &y;

    But a function taking a void* is not the same type as a function
    taking a MD5_CTX*. More importantly, they are not compatible for the
    implied assignment. Consider that the following is not legal because
    the implied conversion is not allowed (you could cast but that is a
    different story)
            void (*func)(void*);
            void MD5_Init(MD5_CTX *c);
            func = MD5_INIT;

    If you can't do it with an assignment, you can't do it with
    initialization (excluding the obvious exception of initializing a char
    array with a string which cannot be done with assignment).

    >void MD5_Update(MD5_CTX *c, const void *data, unsigned long len);
    >void MD5_Final(unsigned char *md, MD5_CTX *c);
    >
    >with different types for the context. short of writing a wrapper
    >function for each of these (or one smart wrapper function for them
    >all), is there a safe solution to fix these assignments? while i'm at
    >it, please post other corrections are you see fit to call them.
    >
    Why not declare and define the functions to match the struct members
    (take void* as arguments) and inside each function convert the void*
    parameter to the correct pointer type. Something like
            void MD5_Init(void *x){
                    MD5_CTX *c = x;
    and the rest of your function body can remain unchanged.

    <<Remove the del for email>>

  • Next message: fix: "Re: Array of pointers in a struct"

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