C Programming Basics

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C Programming Series
Basics for High-Performance Computing Yaakoub Y.
El Khamra
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Agenda

• Background
• What we will cover
• Why learn C
• When to use C
• The C programming language
• Data types
• Operations
• Control flow

• Advanced C topics
• pointers
• IO, File IO
• System calls
• Signals
• HPC and C
• Notes on Style
• Editors, tools, good practice
• Where to go from here

3
What we will cover

• Introductory session, we cover the basics
• data types, arrays, control statements and
  functions
• paradigms, good practice, writing efficient code
• IDE's, multicore/parallel programming tools

4
Why learn C

• Been around for over 30 years (lots of code to
  learn from, plenty more to use)
• programming language of the Unix OS
• Higher level than assembly and fortran 77, lower
  level than C
• C compilers are on all architectures, from
  supercomputers to PIC MCU's
• It is a common language when all the team knows C
  it is a good idea to develop the project in C
• It is good to understand the C programming
  language design

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When to use/not use C?

• Most libraries have C interfaces
• It is easy to make system calls on nix platform
• For string manipulation, C lacks a lot of useful
  tools
• For programs requiring a lot of abstraction use
  something more high level
• When you need to use a template, go ahead and use
  C, C has no templates

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Agenda

• Background
• What we will cover
• Why learn C
• When to use C
• The C programming language
• Data types
• Operations
• Control flow

• Advanced C topics
• pointers
• IO, File IO
• System calls
• Signals
• HPC and C
• Notes on Style
• Editors, tools, good practice
• Where to go from here

7
But first HelloWorld.c
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Data Types

• All variables must be defined in C.
• data types define variables before they are used
• The definition of a variable will assign storage
  for the variable and define the type of data that
  will be held in the location
• Has the form
• typename variablename
• Examples
• int myInteger
• char myCharacter

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Integer Types

• Byte sizes for Lonestar Ranger.
• How to get the sizes
• char c
• int i
• printf("d,d\n", sizeof (c),
• sizeof (i) )
• Output 1,4
• Common sizes (in bytes)

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Float Types

• The majority of scientific codes will use
  floating-point data in double precision
• float (single precision) 4 bytes
• double - 8 bytes

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Character Types

• char myName10
• Create a string variable called "myName" which
  contains 10 elements
• strcpy(myName, "Evan")
• myName"yye00" /Invalid syntax/

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Assigning Values

• Why can't I assign a string to a value?
• Only scalar (a single value) can be assigned at a
  time. A string is an array of values, and each
  location must be assigned individually
• Values are assigned by the assignment operator
  "".
• Valid syntax
• char myChar
• int x
• x 3
• myChar x'
• notice the use of single quotes (') instead of
  double quotes
• ("), because it is a single character, not a
  string.

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Mathematical Operations

• mathematical operations , -, multiply ,
  divide /
• increment decrement , --
• result
• result result 1
• capture a remainder for integer numbers
• result sum 4 /result is 2/
• when an operation occurs on variables of
  different type, the lesser type is promoted (it
  may be necessary to typecast)

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Program Flow

• Loops
• for(i 0 i lt num i) ...
• while(thisValueIsTrue) ...
• Do ...
• ....while(thisValueisTrue)
• Conditionals (nested if else statements are
  possible)
• if (valueIsTrue) do this..
• else if (AnotherValueIsTrue)do that..
• else do this other thing..

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Switch Statements

• Useful for constructing argument parsing instead
  of using large if-else-if-else statements
• switch(letter)
• case 'A' printf("A, for apple\n")
• break
• case 'B' printf("B, for Bob Garza\n")
• break

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Constructing Logic

• Use in conditional statements (and loops)
• Relational operators ( lt, gt, lt, gt )
  Equality operators ( , ! )
• if (value1 gt value2)
• Logical operators "" (and), "" (or), "!"
  (not)
• if (value1 gt value2 value3)
• Order of evaluation
• depending on the operator precedence is not a
  good idea in any language, use parenthesis
• if((value1 gt value2) value3)

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Advanced Control

• continue
• used in a for() loop will cause the current
  iteration to stop and will start the next loop
  iteration
• break
• used to "break" outside of a loop or conditional
  prematurely.
• exit
• quits the program (please make sure you have only
  one exit point to your code)

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Operator Precedence

• -- Postfix increment and decrement
• () Function call
• Array subscribing
• -- Prefix increment and decrement
• / Multiplication, division, and
  modulus (remainder)
• - Addition and subtraction
• lt lt Relational less than and less than
  or equal to
• gt gt Relational greater than and
  greater than or equal to
• Logical AND
• Logical OR

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Agenda

• Background
• What we will cover
• Why learn C
• When to use C
• The C programming language
• Data types
• Operations
• Control flow

• Advanced C topics
• pointers
• IO, File IO
• System calls
• Signals
• HPC and C
• Notes on Style
• Editors, tools, good practice
• Where to go from here

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Pointers

• Elusive, tricky and hard to master
• Very easy to misuse pointers, this typically
  leads to
• Segmentation faults
• Memory access issues (memory stomping), memory
  corruption, possibly unintended, erratic behavior
  in the code
• Memory leaks
• We include them in this introduction to C because
  they are essential to efficient code

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What is a pointer

• A pointer is a data type whose value refers
  directly to another value stored in memory using
  its address
• A pointer references a value in memory, and to
  retrieve the value is to dereference a pointer
• Pointers to different data types have different
  types, example
• int intPtr /pointer to integer/
• double doublePtr /pointer to double/

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Pointer Example

• What would the output be in this case?

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IO

• printf, the all-in-one output function for C.
• printf ("hello world\n")
• Output hello world
• char myName10yye00
• printf("my name is s \n", myName)
• Output my name is yye00

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Conversion Characters

• Printing values of all data types is possible
  with conversion characters. Each conversion
  character tells printf what kind of data is
  expected to follow.
• Common Characters
• d, i, u integer
• x hexadecimal
• f floating point number
• s string
• c character

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More printf Examples

• printf("My name is s and I am d yrs old\n",
• myName, myAge)
• Carriage control is provided by the backslash
• \n, newline
• \t tab
• \b backspace
• \\ produce a single backslash "\"

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Advanced Output

• printf() is a highly robust function that can
  parse output effectively
• printf("\n\nHOST\t\tR15s\tR1m\tR15m\tPA
  GES\t\tMEM\tSWAP\tTEMP\n")
• printf("s\t5.1f\t5.1f\t5.1f\t5.1fP/s\t4.fM\t
  4.fM\t4.fM\n", hostsi.hostName,hostsi.liR15S
  ,hostsi.liR1M,hostsi.liR15M,hostsi.liP
  G,hostsi.liMEM,hostsi.liSWP,hostsi.liT
  MP)
• Output
• HOST R15s R1m R15m PAGES
  MEM SWAP TEMP
• c21-208 1.0 1.0 1.0 2.1P/s
  6468M 2021M 55776M

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Input

• Input can be handled with the scanf function
  which behaves similarly to printf
• int MyInteger
• float MyFloat
• scanf ("d f", MyInteger, MyFloat)
• Variables must be handed to scanf with the
  address operator "". In C the address operator
  gives a variable's address in memory, (a
  pointer), not the value itself.

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Working with scanf

• scanf will parse standard input and match cases
  that it finds.
• gets (string)
• grabs all of the string input on the screen and
  will save it in the array string
• Later in the presentation we will see a better
  way to do input processing

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Working with Files

• STDIN and STDOUT are streams (for input and
  output)
• Files are also streams, have similar functions
  to scanf and printf
• / definitions /
• define NAMELIMIT 40
• FILE fptr
• char filenameNAMELIMIT
• / in code /
• fptr fopen(filename, "r")
• if ( fptr NULL)
• perror("error opening file")
• fclose (fptr)

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Working with File Pointers

• File pointers can be manipulated in the same
  fashion as I/O from the keyboard and to the
  screen.
• Example fscanf reads input from the file pointer
• int item
• fscanf (fptr, "d", item) / read /
• printf( "d\n", item) / print /

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fprintf() and fscanf()

• Usage is exactly the same as printf and
  scanfexcept specify file pointer
• fprintf(fptr, " .")
• Incidentally, all input and output are file
  pointers that can be manipulated. STDOUT is the
  screen pointer. (STDIN is the input pointer)
• fprintf(STDOUT, "Hi there!")
• printf("Hi there!") /equivalent
  statement/

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fopen() arguments
FILE fptr fptr fopen( "myfile",
"r") - Open file for reading
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Putting I/O together
According to the MAN page for gets() BUGS
Never use getS(). Because it is impossible to
tell without knowing the data in advance how
many characters gets() will read, and
because gets() will continue to store
characters past the end of the buffer, it is
extremely dangerous to use. It has been used to
break computer security. Use fgets() instead
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System Calls
system() Call a UNIX system program
externally example system(clear) / clears
the screen / system(command) is equivalent
to /bin/sh -c command System calls are expensive
computationally. Most system calls can be
accomplished by a similar C library function
chmod, chdir,rename (see section 2 and 3 or 3p
man pages)
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Retrieving Environment Variables
include ltstdio.hgt include ltstdlib.hgt / To
shorten example, not using argp / int main (int
argc, char argv, char envp) char home,
host home getenv("HOME") host
getenv("HOSTNAME") printf ("Your home
directory is s on s.\n", home, host) return
0
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Signals

• Programs are given signals by the UNIX system to
  quit or exit, and they also create signals when
  errors occur
• It is useful to be able to "catch" signals in
  order to execute actions when bugs occur
• extremely advanced programming methods (read the
  manual before attempting this)

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Continuing after Signals

• Sample code (modified) to illustrate catching an
  interrupt signal
• Code will run indefinitely and when we hit Ctrl-C
  it will go to the handler function

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Power of Signals

• "Graceful" exits when things go wrong
• Program receives a kill signal while files are
  open
• print useful information such as an error code
• point fault locations such as functions or
  procedures

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Agenda

• Background
• What we will cover
• Why learn C
• When to use C
• The C programming language
• Data types
• Operations
• Control flow

• Advanced C topics
• pointers
• IO, File IO
• System calls
• Signals
• HPC and C
• Notes on Style
• Editors, tools, good practice
• Where to go from here

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HPC and C

• C is a lingua franca amongst HPC packages
• Most HPC libraries have C interfaces to their
  functions
• Typically, for in-house parallel code, you will
  use either OpenMP or MPI, rarely anything else
• OpenMP and MPI can be used in C effectively, a
  lot of MPI implementations are in fact written in
  C

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HPC and C

• Before jumping into OpenMP and MPI, understand
  the problem you are trying to solve and the
  parallelism model
• Chances are somebody already wrote a library to
  solve a lot of the problems you encounter, (reuse
  reuse reuse)
• Eventually, you will need to learn OpenMP or MPI
• TACC hosts OpenMP and MPI tutorials on a regular
  basis

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MPI Code
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OpenMPCode
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Agenda

• Background
• What we will cover
• Why learn C
• When to use C
• The C programming language
• Data types
• Operations
• Control flow

• Advanced C topics
• pointers
• IO, File IO
• System calls
• Signals
• HPC and C
• Notes on Style
• Editors, tools, good practice
• Where to go from here

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Sadly, this is valid C code
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Code Style

• Give your variables useful, meaningful names
  whenever possible, iterators and temp values can
  be excluded
• Give your functions meaningful names func is
  not a good name, and chances are other
  programmers already use func in their code and
  that will cause conflicts
• Be consistent pick a style and stick to it
• Do not be hesitant to add comments, but when you
  add them, make sure they actually say something
  useful / here we do this / is quite silly to
  add in code

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Code Style

• If you have more than one version of your code,
  time to move to a version control system (check
  TACC tutorials page for more info)
• When you have to type more than five words to
  compile your code, time to use a make or a build
  system
• When you have a file that is over 1k lines, time
  to break it down into many smaller more
  manageable files
• When your main function is over 200 lines of code
  long, time to read more about functions and start
  using them. If you know a software engineer, ask
  them about design patterns
• And finally, and most importantly, read your own
  code before you show it to others, and make sure
  you do show it to others for them to read. Code
  reviews save you time, effort and frustration

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Different Styles

• Kernighan Ritchie

• GNU Style (my personal preference)

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Variable/Function naming

• We used to have the Hungarian Notation, do not
  use aforementioned notation!
• Matter of preference but keep names short and
  meaningful use underscores and capitalization
  this_is_a_variable or ThisIsAVariable
• Make sure variable names are not misleading, if
  it does not confuse you it will confuse somebody
  else
• When in doubt something is not sufficiently
  clear, add more documentation

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Agenda

• Background
• What we will cover
• Why learn C
• When to use C
• The C programming language
• Data types
• Operations
• Control flow

• Advanced C topics
• pointers
• IO, File IO
• System calls
• Signals
• HPC and C
• Notes on Style
• Editors, tools, good practice
• Where to go from here

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Advanced Editors

• Vi/Vim is good, ubiquitous, easy to use but not
  terribly productive
• While you can add plugins to vi/vim such as
  tabby, you cannot browse the code structure
• This lead to the rise of integrated development
  environments (emacs does not count)
• Most of IDE's are a personal choice, you have to
  use the tools that you are comfortable with and
  provide the features you require

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Advanced Editors (cont)

• On Linux
• Eclipse, Geany, Kdevelop, Nedit, Scite
• On Windows
• Eclipse, Scite, UltraEdit32, MS Developer Studio
• On Mac
• Eclipse, Scite, Xcode
• Many more IDE's exist, but the IDE I use is
  Eclipse

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Code Tools

• indent indents your code and makes it look
  pretty
• splint, uno, cppcheck statically check C
  programs for security vulnerabilities and common
  programming mistakes
• gdb debug your code
• ddd debug your code with a nice graphical
  interface
• valgrind Memory checker for your code

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Good Practice

• Exactly that, practice write sort algorithms
  repeatedly, think of them as Katas
• Read your own code, and have others read it too
• Write code in teams one man on keyboard another
  looking over the shoulder (Agile Code
  development)
• Read good references mythical man month,
  practical C programming, write great code

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Agenda

• Background
• What we will cover
• Why learn C
• When to use C
• The C programming language
• Data types
• Operations
• Control flow

• Advanced C topics
• pointers
• IO, File IO
• System calls
• Signals
• HPC and C
• Notes on Style
• Editors, tools, good practice
• Where to go from here

56
Where to go from here

• The C programming language by Kernighan and
  Ritchie (aka the C bible)
• Practical C Programming by Steve Oualline
• Write Great Code book series
• http//www.ioccc.org/
• Tune in to more tutorials at TACC for everything
  from development, using the various HPC packages
  (everything from PDEs, linear algebra, comp.
  Chemistry etc...) to debugging, profiling,
  optimization and visualization