There's this Ghidra decompiled C code.
I understand that local_60 is a quad word, but I don't understand indexing it
What does local_60._3_1_ refer to here?
local_60 = 0x6c46575935676a5a;
local_28 = 0x7945474e3563544f;
printf("Enter access code: ");
__isoc99_scanf(&DAT_0010201c,&DAT_001040c0);
if ((((DAT_001040c0 == 'f') && (DAT_001040c1 == 'b')) && (DAT_001040c2 == local_60._3_1_)) &&
((DAT_001040c3 == '6' && (DAT_001040c4 == local_28._2_1_)))) {
You can usually click on the part of decompiled C code and highlight what part of original assembly produced it, this could help you understand what ghidra means by _3_1_ and _2_1_.
From my experience, the _X_Y_ syntax usually means that the code tries to "index" into an integer like an array, by taking a value of a single byte out of 8 into account. In addition to that, if you inspect byte values from the two constants 0x6c46575935676a5a and 0x7945474e3563544f you may notice that all the bytes are proper ASCII characters. These two things would suggest that local_60 and local_28 should instead be char[8] instead of integers. You should be able to right-click on the variable declarations and change their type manually, which may make code more readable by changing syntax into array indexing and array initialization.
Related
I got the task in university to realize an input of a maximum of 10 integers, which shall be stored in a one dimensional vector. Afterwards, every integer of the vector needs to be displayed on the display (via printf).
However, I don't know how to check the vector for each number. I thought something along the lines of letting the pointer of the vector run from 0 to 9 and comparing the value of each element with all elements again, but I am sure there is a much smarter way. I don't in any case know how to code this idea since I am new to C.
Here is what I have tried:
#include <stdio.h>
int main(void)
{
int vector[10];
int a;
int b;
int c;
a = 0;
b = 0;
c = 0;
printf("Please input 10 integers.\n\n");
while (a <= 10);
{
for (scanf_s("%lf", &vektor[a]) == 0)
{
printf("This is not an integer. Please try again.\n");
fflush(stdin);
}
a++;
}
for (b <= 10);
{
if (vector[b] != vector[c]);
{
printf("&d", vector[b]);
c++;
}
b++;
}
return 0;
}
Your code has several problems, some syntactic and some semantic. Your compiler will help with many of the former kind, such as
misspelling of variable name vector in one place (though perhaps this was a missed after-the-fact edit), and
incorrect syntax for a for loop
Some compilers will notice that your scanf format is mismatched with the corresponding argument. Also, you might even get a warning that clues you in to the semicolons that are erroneously placed between your loop headers and their intended bodies. I don't know any compiler that would warn you that bad input will cause your input loop to spin indefinitely, however.
But I guess the most significant issue is that the details of your approach to printing only non-duplicate elements simply will not serve. For this purpose, I recommend figuring out how to describe in words how the computer (or a person) should solve the problem before trying to write C code to implement it. These are really two different exercises, especially for someone whose familiarity with C is limited. You can reason about the prose description without being bogged down and distracted by C syntax.
For example, here are some words that might suit:
Consider each element, E, of the array in turn, from first to last.
Check all the elements preceding E in the array for one that contains the same value.
If none of the elements before E contains the same value as E then E contains the first appearance of its value, so print it. Otherwise, E's value was already printed when some previous element was processed, so do not print it again.
Consider the next E, if any (go back to step 1).
I have learnt that using getchar_unlocked is fast way of reading input. I have seen the code to read at many places but was unable to understand. Can anyone please help me understand how to read using getchar_unlocked ?
Thanks in Advance.
void scanint(int &x)
{
register int c = getchar_unlocked();
x = 0;
for(;(c<48 || c>57);c = getchar_unlocked())
;
for(;c>47 && c<58;c = getchar_unlocked())
{
x = (x<<1) + (x<<3) + c - 48;
}
}
I have seen many other codes as well. I dont particularly understand the purpose of shifting the number. Any help regarding that is appreciated
getch_lock reads a character at a time. here in the given code we are trying to read an integer. the purpose of first for loop is to read digit character if any present and neglect it. The second for loop reads a char which must be digit and performs
n=n*10+c
As C is in Ascii we have subtracted 48 ie Ascii code of '0' . To make code faster instead of using multiplication shift is used.
n*10=n*(8+2)=n*8+n*2=n<<3+n<<1
getchar_unlocked() is like getchar() except that it does not check for multi-thread locks.
So, it is faster, but it is not thread-safe.
I think you might have the wrong idea of the purpose of getchar_unlocked(). Really.
When doing single-character I/O from a human user, it's fantastically hard to believe that you need to focus on being "fast", since the human will be very slow.
The function you included looks like it's reading an integer using getchar_fast(), and is written in a pretty horrible style. It certainly doesn't look like part of a solution to anything in particular. It's also totally broken in its handling of the x pointer variable.
In short, your question is not very clear.
I'm searching for few bytes in a char array. The problem is that on slower machines the process gets up to 90%+ cpu usage. How to prevent that? My code is:
for(long i = 0; i < size - 5; ) {
if (buff[++i] == 'f' && buff[++i] == 'i' && buff[++i] == 'l' && buff[++i] == 'e') {
printf("found at: %d\n", i);
}
}
EDIT:
The string "file" is not null-terminated.
This looks like an attempt at very naive string search, I'd suggest you use either the standard functions provided for this purpose (like strstr) and/or research string search algorithms like Boyer-Moore.
The linked Wikipedia article on Boyer-Moore shows quite well why moving along one character at a time on a mismatch (like you do) is not necessary - it's an interesting read.
EDIT: also look at this page, it has a nice animated presentation that shows how BM does its job.
EDIT2: regarding the string not being nullterminated: either you
buff[size] = 0;
terminate it yourself, and use strstr, or you have a look at the BM code from the page I linked, that works with lengths, ie it will work with strings without terminating 0.
There is nothing wrong with getting 90% utilisation, since the algorithm is CPU-bound. But...
Unless you expect the search term to be on a 32-bit word boundary, the code is broken. If the word 'file' begins on the second character of the buffer, you will simply skip over it. (EDIT: Short-circuit eval means the code is correct as it stands. My mistake.)
Don't roll your own code for this; use strstr.
Try just storing a list of values where 'file' is found and print them out after the loop. It will prevent context switches and will enable the CPU to use the cache better. Also put i in a register.
I am trying to modify a value in an array using the C programming language and I seem to be hitting a blank wall with this seemingly easy operation. Please see code snippet below:
while(1) {
printf("Current prime candidate is %i\n",nextPrimeCandidate);
int innerSieve;//=2;
int currentPrimeCandidate=0;
for (innerSieve=2;innerSieve<SIEVELIMIT;innerSieve++) {
currentPrimeCandidate = nextPrimeCandidate * innerSieve;
//printf("Inner Sieve is b4 funny place %i,%i\n",innerSieve,currentPrimeCandidate);
//initArray[currentPrimeCandidate]=5;
//VERY UNIQUE LINE
myArray[currentPrimeCandidate] = 0;
//printf("Inner Sieve after funny place is %i,%i \n",innerSieve,currentPrimeCandidate);
}
nextPrimeCandidate=getNextPrimeCandidate(myArray,++nextPrimeCandidate);
if ((nextPrimeCandidate^2) > SIEVELIMIT ) break;
}
The problem is with the line highlighted with the VERY UNIQUE LINE comment. For some reason, when the innerSieve variable reaches 33 and gets to that line, it sets the contents of the innerSieve variable to the value of that line ( which currently is 0) and basically forces the loop into an infinite loop( the SIEVELIMIT variable is set at 50). It seems that there is some funny stuff going on in the registers when I checked using the Eclipse Debug facility but I am not too sure what I should be looking for.
If you need the whole code listing, this can be provided.( with a particular variable which is not yet initialised in the code being initialised at the precise point that the innerSieve variable hits 32)
Any help will be greatly appreciated.
Guessing that currentPrimeCandidate is greater than the maximum index of myArray, and you're overwriting innerSieve (which likely follows myArray on the stack).
#ruslik hit on it in the comment. The problem is this line:
if ((nextPrimeCandidate^2) > SIEVELIMIT ) break;
In C, the ^ operator is not the power operator, it is the bitwise xor operator. You're iterating far too many times than you intend, which is resulting in an array-index-out-of-bounds error, so you're overwriting random memory and getting strange results.
There is no power operator in C (though there is the pow function). Since you're just squaring a number, the simplest fix is to multiply the number by itself:
if ((nextPrimeCandidate * nextPrimeCandidate) > SIEVELIMIT ) break;
What is the meaning of == and how does it differ from =?
How do I know which one to use?
== is a test for equality. = is an assignment.
Any good C book should cover this (fairly early on in the book I would imagine).
For example:
int i = 3; // sets i to 3.
if (i == 3) printf("i is 3\n"); // prints it.
Just watch out for the heinous:
if (i = 4) { }
which is valid C and frequently catches people out. This actually assigns 4 to the variable i and uses that as the truth value in the if statement. This leads a lot of people to use the uglier but safer:
if (4 == i) {}
which, if you accidentally use = instead of ==, is a compile-time error rather than something that will bite you on the backside while your program is running :-)
The logical-or operator is two vertical bar characters, one after the other, not a single character. Here it is lined up with a logical-and, and a variable called b4:
||
&&
b4
No magic there.
a == b is a test if a and b are equal.
a = b is called an assignment, which means to set the variable a to having the same value as b.
(You type | with Shift-\ in the US keyboard layout.)
== tests equality
= assigns a value
neither are related to ||
I might add that in Finnish and Swedish keyboards. Pipe symbol; |; of OR is AltGr (the right alt) and < key. IF you are using Mac on the other hand it is Alt-7 key.
Gave me a lot of sweat when I first started typing on these keyboards.
Now that you know the difference between '==' and '=", let me put you some words of caution. Although '==' is used as a standard test of equality between comparable variables and '=' used as an internally type-casted assignment, the following programming error is quiet common.
In the below example and similar codes, '=' is know as "Always true" conditional operator.
#include<stdio.h>
int main()
{
int i = 10, j = 20;
if ( i = j )
printf("Equal\n");
else
printf("NOT Equal\n");
return 0;
}
So, the word of caution is "Never use '=' in if statements, unless you have something evil in your mind."