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simple C question

Posted on 2006-06-26
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Last Modified: 2010-04-15
Hi

I am looking at some legacy c code and i have come across

 list += temp + "\05" + " 0" + "\02";


what is "\05"  ,  "\02"

are these hex constants, binary constants, octal constants ?

thanks

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Question by:Vlearns
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12 Comments
 
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Harisha M G earned 2000 total points
ID: 16988121
Hi, They are octal constants, but no matter what they are, their value remains the same :-)

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Harish
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by:Harisha M G
ID: 16988143
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by:PaulCaswell
ID: 16988154
Hi Vlearns,

>> list += temp + "\05" + " 0" + "\02";
This isnt valid C. It might be C++ but not C.

This would be:

 list += temp + '\05' + '0' + '\02';
But even that seems wrong because that would be the same as:

 list += temp + 5 + '0' + 2;

Are you sure you copied it right?

Paul
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LVL 46

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by:Kent Olsen
ID: 16988162
Hi Vlearns,

That isn't legal syntax in modern C.  "\05" is not a character or integer value, it is a string pointer.  The keyword here is pointer.

As legacy code, it MAY have worked on old 8 and/or 16 bit systems, but it certainly isn't ANSI C in today's world.

However, replacing the double quotes with single quotes is a different matter.  (Note the spaced deleted from the " 0" string.)

 list += temp + '\05' + '0' + '\02';

In this case, the \05 and \02 are treated as octal contants so 5 and 2 are added to '0', giving the ASCII character '7'.



Good Luck!
Kent
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by:PaulCaswell
ID: 16988181
Snap! :-)
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by:Harisha M G
ID: 16988199
Vlearns, yeah I forgot.. as others said, it is valid only in C++ using string class.


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by:davidnsc1
ID: 16988460
It could be the tragic, pointless addition of string pointers :D
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by:grg99
ID: 16992373
it depends on whether this is compiled by a C compiler or a C++ compiler.
 list += temp + "\05" + " 0" + "\02";



If it is a C compiler, then the code doesnt make any sense,  as others have noted, adding double quoted strings doesnt do anything useful in C.  You end up with the sum of the addresses of some short strings, which is of no use that I can fathom.

If it's a C++ compiler, then it does make sense if "list" and "temp" are string-type variables.  Although the constants are might peculiar- an escaped zero is just a zero, and doesnt need the escaping.  

Maybe you can give us some more info, like the type of compiler, the name of this file, (.c or .cpp or what), the types of list and temp.

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by:aib_42
ID: 16999506
Here, let me back up everyone who has said this isn't valid C:

Pointer addition is invalid per §6.3.6 of ANSI 9899.
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by:aib_42
ID: 16999584
"\05", " 0" and "\02" are all string literals - the first and the last are arrays containing one character each: '\05' and '\02', respectively. (Which are octal character constants representing the ASCII characters ENQ and STX, respectively, as per mgh's reference.) The one in the middle is a two-character array containing the characters ' ' (the space) and '0' (the digit).

Maybe they are all character constants, and the one in the middle is a mistranslated \00?
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