[Qt-interest] Resizing static array data in struct fp_print_data? 0_o How?

[Constantin Makshin]

In C[++] arrays are somewhat equivalent to pointers so sometimes dynamic arrays are declared as very small small static arrays. Since one cannot [easily] malloc() or 'new' those arrays, such structures should be allocated as a whole, i.e. instead of something like:

fp_print_data pd;
// ...
pd.data = (unsigned char*)malloc(12345);

you should/must write:

fp_print_data* pd = (fp_print_data*)malloc(sizeof(fp_print_data) + 12345);
// ...

This way, all members are still the same as if you allocated the structure on the stack, but there's an extra memory block that acts as 'data' array contents.

However, I suggest using specialized libfprint functions fp_print_data_get_data() and fp_print_data_from_data() to perform [de]serialization:

unsigned char* buf;
size_t n = fp_print_get_data(&fpdata, &buf);
QByteArray byteArray((const char*)buf, qMin(n, INT_MAX));
free(buf);
// ...

fp_print_data* fpdata2 = fp_print_data_from_data((unsigned char*)byteArray.data(), byteArray.size());
// ...
fp_print_data_free(fpdata2);

On Monday 07 March 2011 04:54:55 Nicholas Shatokhin wrote:
> Hello.
> 
> I found this in structure:
> 
> enum fp_print_data_type {
>            PRINT_DATA_RAW = 0, /* memset-imposed default */
>            PRINT_DATA_NBIS_MINUTIAE,
> };
> 
> struct fp_print_data {
>            uint16_t driver_id;
>            uint32_t devtype;
>            enum fp_print_data_type type;
>            size_t length;
>            unsigned char data[0];  // OK, it's array from one element
> };
> 
> But when I enroll my finger, the data has the length 2404 elements. How it
> change it's size?
> 
> I'm porting libfprint to qt4 and I need transform this struct to
> QByteArray and back. I successfully write data to byteArray, but I have
> difficult with backward procedure. See the function:
> 
> 
> struct fp_print_data CFingerprintScanner::byteArrayToFpdata(QByteArray
> byteArray)
> {
>        quint16 driver_id;
>        quint32 devtype;
>        quint32 type;
>        quint64 length;
>        char * data;
> 
>        struct fp_print_data fpdata;
> 
>        QBuffer buffer(&byteArray);
>        buffer.open(QIODevice::ReadOnly);
> 
>        QDataStream in(&buffer);
> 
>        in >> driver_id;
>        in >> devtype;
>        in >> type;
>        in >> length;
> 
>        data = (char *) malloc(length);
> 
>        in.readRawData(data, length);
> 
>        fpdata.driver_id = (uint16_t) driver_id;
>        fpdata.devtype = (uint32_t) devtype;
>        fpdata.type = (enum fp_print_data_type) type;
>        fpdata.length = (size_t) length;
> 
>        // in this I must write data to fpdata.data, but how can I do it?
>        // it's not possible to convert char * (or unsigned char*) to  
> unsigned
> char[0]
> 
>        free(data);
> 
>        return fpdata;
> }
> 
> I'm disappointed. Somebody can tell me how can I change size of this array?
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