Command Line arguments

1
Command Line arguments

• Main function int main(argc, char argv)
• argc is the count of command line arguments
• argv is an array of strings
• argv0 is the name of the program
• argv1 is the first argument from the command
  line
• argvk is the kth argument from the command
  line
• Command line example gtprog 123 abc
• argc 3
• argv0 "prog", argv1 "123", argv2
  "abc"
• If you expect two argumentsif (argc!3)
  printf("usage prog ltarg1gt ltarg2gt) exit(1)
  

2
Inputting Problem

• scanf User can crash the program by typing too
  much
• fgets User cannot crash the program
• Example
• includeltstdio.hgt
• int main(void)
• char input101
• char result
• printf("Enter some text\n")
• result fgets(input, sizeof(input), stdin)
• if (result ! NULL) printf("You entered
  s", result) else printf("Error processing
  fgets()\n")
• return 0

3
typedef

• typedef assigns a label to a declaration
• Syntax typedef ltsome typegt label
• Then you can declare using the label
• Examples
• Declare a new typetypedef char Stringtypedef
  int Boolean
• Declare an instance of that typevoid
  myFunc(String data, Boolean flag)

4
struct

• struct is a collection of data types
• Examplestruct int age char name100
  1 float salary
• Useful to create a new complex typetypedef
  struct int age char name1001 float
  salary Person
• Instantiate Person bill

Note A complex type is like Java class, but
without methods
5
Accessing struct data

• Declare Person person100
• Access fields JLJprintf("His name is s\n",
  person0.name)for (int i0 ilt100 i)
  printf("s is d years old with salary f\n"
  , personi.name , personi.age
  , personi.salary)

6
Using pointers for functions

• void myFunc(const Person first,
• Person second)
• second-gtname first.name
• second-gtage first.age
• second-gtsalary first.salary
• Note The arrow indirectly addresses access data
  in structs
• Note Pass by value copies the entire structure
  to the stack. This can be inefficient, so most
  programmers always use pointers in functions

7
Reading a text file

• include ltstdio.hgt
• int main(void)
• char data256
• FILE file
• file fopen("someFile.txt","r") if
  (fileNULL) return 2
• while (!feof(file))
• int cSizesizeof(char) // normally 1
• int dSizesizeof(data) // 256 cSize
• int size dSize/cSize // Array byte size
• fgets(data, size,file)
• printf("s",data)
• fclose(file)
• return 0

8
Writing to a text file

• include ltstdio.hgt
• int main()
• FILE file char data2561
• fgets(data, sizeof(data), stdin)
• file fopen("someFile.txt", "w")
  if(fileNULL) fprintf(stderr, "Can't create
  output file\n") return 2 fprintf(file,
  "s\n", data)
• fclose(file)
• return 0

Question Why not use printf instead of fprintf
with stderr
9
fscanf and fgets to read files

• fscanf(File fp, const char temp, args )
• Works just like scanf but is insecure
• Stops on white space
• No protection for buffer size
• Better approach
• Use fgets reads till newline or n-1 characters
  read
• Automatically puts the null at the end of the
  string
• Returns null if error or end of file
• If you have to parse, use sscanf after the data
  is in memory

10
sscanf

• Examplechar str "30 20 10"int a, b,
  cif (sscanf(str, "d d d", a, b, c) ! 3)
  fprintf(stderr, "What happened?")
• sscanf is a very useful tool for parsing input or
  data read from files
• Note the strstr function is useful for finding
  the first occurrence of a little string in a big
  one. See lab 10.char strstr (const char
  haystack, const char needle)

11
Buffered Output

• Data does NOT go dirctly to a file
• Why? It is more efficient for the operating
  system to decide when to write data.
• What happens? Data goes to an operating system
  buffer
• When does the data write? When the buffer is full
  or manually flushed
• How does it manually get flushed? Use the call
  fflush(file)
• What about stdout? call fflush(stdout)

12
Final Notes

• Portability
• Operating Systems end lines differently
• C always uses '\n' to end a line
• stdio functions translate for the OS in question
• EOF stdio functions use this constant
• include lt stdio.hgt
• void main()
• int c, nc 0
• while ( (c getchar()) ! EOF ) nc
• printf("Characters in file d\n",nc)
• GDB The set args command is useful before
  running the program. It sets the command line
  arguments as if you typed them from the command
  line

13
Make

• Utility to build systems composed of many source
  and data files
• Purpose
• To minimize management errors of incorrect builds
  made after a minor change
• To only compile and build those things that were
  changed since the previous build
• To create source for creating systems geared for
  certain configurations and locals

14
Installations

• Typical installations of applications from source
• Obtain tar.gz bundle and uncompress
• Execute ./configure script to set platform-based
  parameters. The script may launch a set of C
  based programs to inquire about the linux
  configuration
• Execute make to compile and create executables
• The linux command to execute make
• make -f custom make file buildType
• Note If no custom make file, the command looks
  for a file named makefile
• Note buildType tells the make script the kind of
  build desired.

15
Make Command Script Example

• all hello
• hello main.o factorial.o hello.o
• g main.o factorial.o hello.o -o hello
• main.o main.cpp
• g -c main.cpp
• factorial.o factorial.cpp
• g -c factorial.cpp
• hello.o hello.cpp
• g -c hello.cpp
• clean
• rm -rf o hello

Syntax target dependencies tab
system commandCreate target if the dependencies
are met
16
Using Make Variables

• CCg
• CFLAGS-c -Wall
• all hello
• hello main.o factorial.o hello.o
• (CC) main.o factorial.o hello.o -o hello
• main.o main.cpp
• (CC) (CFLAGS) main.cpp
• factorial.o factorial.cpp
• (CC) (CFLAGS) factorial.cpp
• hello.o hello.cpp
• (CC) (CFLAGS) hello.cpp
• clean
• rm -rf o hello

Use g for the compiler.
Use G option flags compile and all warnings
17
A More Complex Example

• CCg
• CFLAGS-c -Wall
• LDFLAGS
• SOURCESmain.cpp hello.cpp factorial.cpp
  OBJECTS(SOURCES.cpp.o)
• EXECUTABLEhello
• all (SOURCES) (EXECUTABLE)
• (EXECUTABLE) (OBJECTS)
• (CC) (LDFLAGS) (OBJECTS) -o _at_
• .cpp.o
• (CC) (CFLAGS) lt -o _at_
• http//www.gnu.org/software/make/manual/make.pdf