unsigned char write to file

[seniorc]

Hello,how can we write unsigned char to file.
i used write() but i cant

[anda_skoa]

Since that is done using write() could you show the code that you are using and how you determine that it didn' work?

Cheers,
__

[ChrisW67]

Not clear if you mean a single unsigned char or a bunch of them:

unsigned char a = 0x00;
    unsigned char data[] = { 0x00, 0x01, 0x02, 0xff };
    QFile file("test.bin");
    if (file.open(QIODevice::WriteOnly)) {
        file.write(reinterpret_cast<char *>(&a), sizeof(a));
        file.write(reinterpret_cast<char *>(&data), sizeof(data));
        file.close();
    }

unsigned char a = 0x00;
    unsigned char data[] = { 0x00, 0x01, 0x02, 0xff };
    QFile file("test.bin");
    if (file.open(QIODevice::WriteOnly)) {
        file.write(reinterpret_cast<char *>(&a), sizeof(a));
        file.write(reinterpret_cast<char *>(&data), sizeof(data));
        file.close();
    }

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$ od -tx1 test.bin
0000000 00 00 01 02 ff

$ od -tx1 test.bin
0000000 00 00 01 02 ff

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[Radek]

A possible cause of the problem: You can write either char * or QByteArray. Because of type checking, you cannot write an unsigned char *. Do as ChrisW67 has shown and reinterpret_cast.

[prkhr4u]

You can write unsigned char *.Here is one of the my sample code
Can get idea from this:

unsigned char* data3d = new unsigned char[x_size*y_size];
int i,j,k=0;
fp = fopen (output_str, "wb");
for ( i = x_size; i >= 0; i--)
for (j = 0; j < y_size; j++)
{
data3d[k++]=Buffer_in[i][j];
}
fwrite( data3d, sizeof( char ),x_size*y_size, fp);
fclose(fp);

unsigned char* data3d = new unsigned char[x_size*y_size];
int i,j,k=0;
fp = fopen (output_str, "wb");
for ( i = x_size; i >= 0; i--)
for (j = 0; j < y_size; j++)
{
data3d[k++]=Buffer_in[i][j];
}
fwrite( data3d, sizeof( char ),x_size*y_size, fp);
fclose(fp);

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[Radek]

This is ANSI C, not Qt, prkhr4u. The pointer in fwrite() is void * so that you can write anything you want. The pointer in QIODevice::write() is char * (or a reference to a QByteArray).

[seniorc]

I done writing unsigned char to file,ChrisW67's way.But i have new problem.Here is my codes.very slow run in this line.How can i do faster than now.Any idea?

QString compressed // inside bit example: 1010101010111...

QString compressed // inside bit example: 1010101010111...

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for (;!(compressed.isEmpty());compressed=compressed.mid(8)){
       unsigned char byte = 0;
       for (int j=0; j<8; ++j) {
          if (compressed.at(j) == '1')
             byte |= 0x01;
          else
             byte &= ~(0x01);
          if (j < 7) byte<<=1;
       }
       codefile.write(reinterpret_cast<char *>(&byte),sizeof(byte));
    }

for (;!(compressed.isEmpty());compressed=compressed.mid(8)){
       unsigned char byte = 0;
       for (int j=0; j<8; ++j) {
          if (compressed.at(j) == '1')
             byte |= 0x01;
          else
             byte &= ~(0x01);
          if (j < 7) byte<<=1;
       }
       codefile.write(reinterpret_cast<char *>(&byte),sizeof(byte));
    }

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Thanks

[anda_skoa]

My guess would be that the mid() call makes it slow due to having to create a new string and copy the remaining data.

Better loop over the whole string and write a byte every 8th character.

Additionally you could also preallocate a QByteArray with compressed.length / 8 size and fill it as you go, then write that block to the file in one go.

Cheers,
_

[Radek]

Make in advance everything you can do in advance and go "low level" avoiding as much of unnecessary actions you can do. For example:

const QChar  *dd      = compressed.constData();
const int     N       = compressed.size();
int           k       = 0;
unsigned char buff    = 0;
unsigned char bits[8] = { 0x01, 0x02, 0x04, 0x08, 0x10, 0x20, 0x40, 0x80 };
 
const QChar   Zero('0');
const QChar   One('1');
 
for( int i = 0; i < N; i++ )
{
  if( dd[i] == Zero ) k++;
  else if( dd[i] = One )
  {
    buff |= bits[k];
    k++;
  }
  else
  {
    report error;
    break;
  }
 
  if( k == 8 )
  {
    ff.write(reinterpret_cast<char *>(&buff),sizeof(unsigned char));
    buff = 0;
    k    = 0;
  }
}
 
if( k > 0 )    //  incomplete byte
{
  ff.write(reinterpret_cast<char *>(&buff),sizeof(unsigned char));
  set some flag, only k bits valid;
}

const QChar  *dd      = compressed.constData();
const int     N       = compressed.size();
int           k       = 0;
unsigned char buff    = 0;
unsigned char bits[8] = { 0x01, 0x02, 0x04, 0x08, 0x10, 0x20, 0x40, 0x80 };

const QChar   Zero('0');
const QChar   One('1');

for( int i = 0; i < N; i++ )
{
  if( dd[i] == Zero ) k++;
  else if( dd[i] = One )
  {
    buff |= bits[k];
    k++;
  }
  else
  {
    report error;
    break;
  }

  if( k == 8 )
  {
    ff.write(reinterpret_cast<char *>(&buff),sizeof(unsigned char));
    buff = 0;
    k    = 0;
  }
}

if( k > 0 )    //  incomplete byte
{
  ff.write(reinterpret_cast<char *>(&buff),sizeof(unsigned char));
  set some flag, only k bits valid;
}

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[seniorc]

Make in advance...

Thanks I gained twelve seconds,How can I improve better low level programming whats your suggestions,for example;thats my mistakes?I've got a new question.QMap,QHash,QList write file?I used http://doc.qt.io/qt-4.8/QDataStream but is there a better way?for example;write QMap and read QMap for easy way?

[ChrisW67]

You could save yourself a bunch of manipulations if your built your binary data up front rather than building a string full of '1' and '0' characters and then having to convert it.