Question

I wanted to write a function that takes two arguments: a void pointer type to an arbitrary memory-block and a block bytesize. Knowing the struct type of data writen in the block, the function should print the values contained.

However, at first, the code I suggested didn’t work:

#define RECORD struct record
struct record {
      char nam[32];
      double val;
};

void xprint (void *p, long j)
{
    j /= sizeof(RECORD);
    RECORD r;

    while(j--){
        r = *((RECORD *)p++);
        printf("\n..%s.., ..%lf..\n",r.nam, r.val);
    }
    return;
}

So, I came up with some alternations, mainly in the incrementing part of the code:

void print (void *p, long j)
{
    j /= sizeof(RECORD);
    RECORD r = *((RECORD *)p);

    while(j--){
        printf("\n%s,\t%8.2lf\n",r.nam, r.val);
        r = *(++(RECORD *)p);
    }
    return;
}

Now it did the job, but still the code looks less compact.

After some inspection, I found the problem lies in r = *((RECORD *)p++); line. It seems that when it comes to a postfix incrementation, p is no longer typecasted, and hence p is incremented by one byte only.

Could the xprint function be rewritten so that I would still use the postfix operator, but applied to a typecasted pointer?

Answer 1

Convert the void * to a RECORD * straight away and then use that pointer for the rest of the function.

void print (const void *p, size_t size)
{
    const RECORD *r = p;
    size_t count = size / sizeof(*r);

    while (count--) {
        printf("\n%s,\t%8.2lf\n", r->nam, r->val);
        ++r;
    }
}

I also made some stylistic changes here, such as better variable names and adding const.

---

On a side note, as Clement Rey says it’d be better to use a typedef than a define.

typedef struct record record_t;

You can even combine the typedef with the struct definition:

typedef struct {
    char nam[32];
    double val;
} record_t;