QByteArray data format explanation

Question

I am confused about QByteArray data. Can someone please explain the below scenario for me.

Here data type for each array index is char. I understand some of those values. Like 74 'J' I understand the first one is ASCII and second one is the corresponding character. But what is the meaning of -1 '\\377'

Also what does the below gui means?? I sent the QByteArray of above to a function which takes the QByteArray as unsigned char* source. The below gui is showing the value of that source.

The main confusing part is the first line's value 0x87089e8 "\377\330\377\340"

Answer 1

char in C/C++ is a signed 1-byte integer. This GUI is simply expressing that value as a signed decimal number, and the equivalent ASCII character.

You're asking about the byte value -1, which can be interpretted in the following ways:

Binary     11111111
Octal      0377
Hex        0xFF
Decimal    -1    (Signed)
           255   (Unsigned)

ASCII      \377
           \xFF

Note that there isn't a standard printable ASCII character for 255, which is why they show it like they do.

Another Example:

Binary     01001010
Octal      0112
Hex        0x4A
Decimal    74    (Signed)
           74   (Unsigned)
ASCII      'J'

Answer 2

I'm answering about the meaning of 0x87089e8 "\377\330\377\340".

0x87089e8 is the value of the source pointer, i.e. it's an address in memory. "\377\330\377\340" is the character string stored at that address, written as octal escape sequences. It's written this way because none of these characters is ASCII (ASCII goes only from 0 to 127). In hex, the string of bytes is ff d8 ff e0 00. The 00 at the end is interpreted as and end-of-string mark (ASCII NULL).