while (*p++);
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How do you interpret this:
while (*p++);If p initially points to a character string then after
while(*p++);p points to the0character that terminates the string. So it is semantically the same asp = p + _tcslen(p); -
I am surprised I even looked at that code. What I showed you was a litteral interpretation of the code you presented. I could even present it in assembly if required (which I will not do). Let's break it down: 1) Copy the current value into a temporary variable. 2) Increment the current value. 3) Check if the temporary value has a value other than zero. If it does not then stop looping. That is how it works! INTP Every thing is relative...
With your solution p will point to the first NULL from the beginning, which is not really correct. it will point to the char after the first null. try to trace it and see. but be careful with the string you use, or your program might crash. for eg, use "a\0bb\0ccc\0\0dddd" I can understand, the code is very deceptive. only a few get it right the first time, and I am not one of them either :) thanks!
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If p initially points to a character string then after
while(*p++);p points to the0character that terminates the string. So it is semantically the same asp = p + _tcslen(p);Carsten Leue wrote:
p = p + _tcslen(p);
Welcome to the gang! you too got it incorrect :) it, actually, is equivalent to :
p= p + _tcslen(p) + 1thanks! -
How do you interpret this:
while (*p++);step 1. obtain value of object pointed to by "p" step 2. increment "p" step 3. if value evaluates to "true" then execute statement and continue with step 1 step 4. stop
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step 1. obtain value of object pointed to by "p" step 2. increment "p" step 3. if value evaluates to "true" then execute statement and continue with step 1 step 4. stop
equivalent code:
while (true) { bool done = *p == 0; ++p; if (done) break; } -
that someone is none other than a micrsoft sample code. This piece of code is from: http://msdn.microsoft.com/library/default.asp?url=/library/en-us/fileio/fs/obtaining\_a\_file\_name\_from\_a\_file\_handle.asp where p initially points to a: A:\\0C:\\0D:\\0\0 where \0 represents a single NULL character. I dont understand what you maen by *p = 0, but this piece of code doesnt modify any data, only pointer advancement. and how that works is still mysterious to me.
It actually does the ++ first, which means that it increments the pointer until it points to NULL. Christian Graus - Microsoft MVP - C++
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It actually does the ++ first, which means that it increments the pointer until it points to NULL. Christian Graus - Microsoft MVP - C++
Christian Graus wrote:
It actually does the ++ first, which means that it increments the pointer until it points to NULL.
Welcome to the gang! you are not alone :) PS: check out other posts thanks!
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equivalent code:
while (true) { bool done = *p == 0; ++p; if (done) break; }Roger, Sunit, Ahz- you guys are awesome! Does this example fit an obfuscated C contest? :) thanks!
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Roger, Sunit, Ahz- you guys are awesome! Does this example fit an obfuscated C contest? :) thanks!
Lambu Jindu wrote:
Does this example fit an obfuscated C contest?
no
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Carsten Leue wrote:
p = p + _tcslen(p);
Welcome to the gang! you too got it incorrect :) it, actually, is equivalent to :
p= p + _tcslen(p) + 1thanks!It's a bummer! You are correct. The loop ends if
ppoints to the end of the string butpis still incremented. Carsten -
With your solution p will point to the first NULL from the beginning, which is not really correct. it will point to the char after the first null. try to trace it and see. but be careful with the string you use, or your program might crash. for eg, use "a\0bb\0ccc\0\0dddd" I can understand, the code is very deceptive. only a few get it right the first time, and I am not one of them either :) thanks!
Lets make it more accurate then:
// UNOPTIMIZED
//////////////////////////
343: char* p = "123";
0043CA51 mov dword ptr [ebp-14h],offset string "123" (004df014)
344: while( *p++ ) {};
0043CA58 mov eax,dword ptr [ebp-14h] // eax <- p
0043CA5B movsx ecx,byte ptr [eax] // value_type = p[0];
0043CA5E mov edx,dword ptr [ebp-14h] // edx <- p
0043CA61 add edx,1 // edx <- edx + 1
0043CA64 mov dword ptr [ebp-14h],edx // p <- edx
0043CA67 test ecx,ecx // if( ecx is 0 )
0043CA69 je main+4Dh (0043ca6d) // goto 0043CA6D
0043CA6B jmp main+38h (0043ca58) // goto 0043CA51
345: return 0;
0043CA6D xor eax,eax
0043CA6F jmp __tryend$_main$1+2Ch (0043cac0)// UNOPTIMIZED
START_LOOP:
value_type = *p;
++p;
if( value_type == 0 )
goto END_LOOP;
goto START_LOOP;
END_LOOP:INTP Every thing is relative...