An Introduction to C Programminggramming

(assuming that you already know Java;this is not an introduction to C++)

Minimal C programgramint main ( int argc, char** argv ) {

return 0; //comment just like Java}

Or

int main ( int argc, char* argv[] ) {return 0; /* comment just like Java */

}

Or (typically, you aren’t going to change them)

int main ( const int argc, const char* const argv[] ) {return 0;

}

What does main look like in Java? What is used as ‘const’ in Java?

Java’s main

public static void main ( String args[] ) {

}

C vs. Java types• C

– doesn’t init vars (with a few exceptions)

– bool, char, short, int, long

– float, double– unsigned char– unsigned short– unsigned int– long long– long double

• Java– does init vars

(specifically objects which are defined to be an instance of a class OR arrays; primitive types are NOT initialized and require initialization)

– boolean, byte, short, int, long

– float, double

ASCII output in C (using the standard I/O library)#include <stdio.h> //needed for defns

int main ( int argc, char* argv[] ) {puts( “hello, world.” );printf( “hello, world.” );printf( “hello, world.\n” );return 0;

}

ASCII output in C#include <stdio.h> //header file w/ definitions

int main ( int argc, char* argv[] ) {printf( “hello, %s. \n”, “fred” );

int i = 12;printf( “I am %d years old. \n”, i );

double d = 12.9;printf( “d is equal to %f (d is not equal to %d). \n”,d, d );

return 0;}

How can d be both %f and %d?

ASCII file output in C (using the standard I/O library)

#include <assert.h>#include <stdio.h>

int main ( int argc, char* argc[] ) {//open the file for output (erase anything already there!)FILE* fp = fopen( “fred.dat”, “wb” );assert( fp!=NULL );//write to the filefprintf( fp, “hello, world.\n” );//close the filefclose( fp );fp = NULL;return 0;

}

Hint: Always open b/binary or binary I/O will act strangely under Windows.

ASCII file I/O format specs• %d – an int• %c – a char• %x – an int in hex• %f – a double• %s – string• Many, many, many others and

variations.

Binary file output in C

• We will discuss binary file I/O after we discuss pointers.

• We will discuss input after we discuss pointers as well.

I/O in C

• printf, scanf, gets, and puts perform buffered I/O (to stdin and stdout).

• fopen, fread, fwrite, fprintf, fseek, fscanf, fgets, fputs, and fclose perform buffered I/O.

• open, read, write, seek, and close perform unbuffered I/O.

C pointers int i=12;

– i is a variable. It exists in some memory location. At that memory location is the value of 12.

– What is the size of i (in bytes)?printf( “An int is %d bytes. \n”, sizeof(int) );printf( “i=%d, i occupies %d bytes. \n”,

i, sizeof(i) );– What is the virtual address of i, i.e.,

where is i in memory?

C pointers

Let’s introduce an operator (&) that yields the address of a variable.

int i = 12;printf( “the address of i is %d. \n”, &i );printf( “the address of i is %x. \n”, &i );

C pointers

How do we declare variables that hold pointers (instead of values)?

int i=12, j=52;int* ptr = &i; //ptr points to iptr = &j; //ptr now points to j

C pointers

Derefencing pointers (getting at what they point to):

int i = 12;int* iptr = &i;printf( “i=%d, *iptr=%d. \n”, i, *iptr );

Is *iptr the same as iptr?

C pointers

Derefencing pointers (getting at what they point to):

int i = 12;int* iptr = &i;*iptr = 52;printf( “i=%d. \n”, i ); //What is i’s value?

C pointers

• C pointers can be treated as arrays.

#define N 100int ray[ N ];int* ptr = ray; //same as ptr=&ray[0]for (int i=0; i<N; i++) {

ptr[i] = 0; //same as ray[i] = 0}

C pointers

• C pointers can be treated as arrays.

#define N 100int ray[ N ];int* ptr = ray; //same as ptr=&ray[0]for (int i=0; i<N; i++) {

*ptr = 0;++ptr;

}

C pointers

• C pointers can be treated as arrays.

#define N 100int ray[ N ];int* ptr = ray; //same as ptr=&ray[0]for (int i=0; i<N; i++) {

*ptr++ = 0;}

C pointers

• C pointers can be treated as arrays (most of the time).

#define N 100int ray[ N ];int* ptr = ray; //same as ptr=&ray[0]for (int i=0; i<N; i++) {

*ptr++ = 0;}

Every time we ++ ptr, how much is added to ptr? 1?

C pointers

• Common pitfall

int* ptr1, ptr2; //what (type) is ptr2?

C pointers

• Common pitfall

int* ptr1, ptr2;

This is actually the same asint *ptr1, ptr2;

Which means that ptr2 is an int (not an int*)!

C pointers and arrays

• Common pitfalls

#define N 100int ray[ N ];int* ptr = ray;

printf( “%d %d \n”, ray[N], ptr[1000] );

C pointers and arrays

• Common pitfalls

#define N 100int ray[ N ];int* ptr = ray;

printf( “%d %d \n”, ray[N], ptr[1000] );

More C pointers

Cast = change from one type to another.

double d = 0.7;int i = (int)(d+0.5); //round dint j = (int)d; //don’t round d

Sometimes we need to change (cast) from one pointer type to another.

ASCII input

Use scanf (from stdin) and fscanf.

printf( “enter two integers: “ );int n1, n2;scanf( “%d %d”, &n1, &n2 );

Why is ‘&’ necessary?

ASCII input

Use fscanf to read from ASCII files.

FILE* fp = fopen( “input.txt”, “rb” );assert( fp!=NULL );int n1, n2;fscanf( fp, “%d %d”, &n1, &n2 );…How do we know if anything was

read?

How do we know if anything was read?int scanf ( const char *format, ... );

int fscanf ( FILE *stream,const char *format, ... );

ASCII input

Use fscanf to read from ASCII files.

FILE* fp = fopen( “input.txt”, “rb” );assert( fp!=NULL );int n1, n2, count;count=fscanf( fp, “%d %d”, &n1, &n2 );if (count!=2) {

…}

Notes about I/O

• stdin, stdout, and stderr are predefined for you and available for every program to use.– You do not need to define them

and you should not open them.– They are already defined for you

when you #include <stdio.h>

More notes about I/O

fprintf( stdout, “hello \n” );is the same as

printf( “hello \n” );

scanf( “%d”, &n1 );is the same as

fscanf( stdin, “%d”, &n1 );

More notes about I/O

• Read in a line of input.char *gets ( char *s );

char *fgets ( char *s, int size, FILE *stream );

• What’s the difference between:char buff[ 255 ];gets( buff );

fgets( buff, sizeof(buff), stdin );

More C pointers

• Dynamic memory allocation (and deallocation).– malloc

int* iptr = (int*)malloc( 10*sizeof(int) );assert( iptr!=NULL );

– freefree( iptr );iptr = NULL; //good idea, not required

• Must #include <stdlib.h> at top.

cast

Binary I/O

• Buffered– fread and fwrite

• Unbuffered– read and write– We will skip unbuffered I/O (but

you are free to use it).

Binary input: fread

size_t fread ( void* ptr, size_t size,size_t nitems, FILE* stream );

• Returns nitems if OK.FILE* fp = fopen( “junk.in”, “rb” );#define N 100int buffer[ N ];int k=fread( buffer, sizeof( int ), N, fp );• What is k?

Binary input: fread

size_t fread ( void* ptr, size_t size,size_t nitems, FILE* stream );

• Returns nitems if OK.#define N 100int buffer[ N ];int k=fread( buffer, sizeof( int ), N, fp );k = fread( buffer, 1, sizeof( buffer ), fp );k = fread( buffer, sizeof( buffer ), 1, fp );

Binary input: fread

int buffer[ N ];

int k=fread( buffer, 1, sizeof( buffer ), fp );

k = fread( buffer, sizeof( buffer ), 1, fp );

This (sizeof(buffer)) is an example of where arrays and pointers are NOT the same in C.

Binary output: fwrite

size_t fwrite ( void* ptr, size_t size,size_t nitems, FILE* stream );

• Returns nitems if OK.FILE* fp = fopen( “junk.in”, “wb” );#define N 100int buffer[ N ];int k=fwrite( buffer, sizeof( int ), N, fp );• What is k?

0utput in C (using the standard C++ library)#include <iostream>

using namespace std;

int main ( int argc, char* argc[] ) {cout << “hello, world.” << endl;return 0;

}

You are free to use the standard C++ library but we won’t discuss it in this class.

C structures

• Collection of data (only).– precursor to object– similar to a FORTRAN record

• Especially useful for message passing.– Ex. { do this operation, here is the

data }

C structures

struct message {enum { OP_CALCULATE,

OP_RESULT,OP_EXIT };

int operation;int parameters[100];double result;

};Tip: don’t forget ;

C structures

int main ( int argc, char* argv[] ) { struct message m; m.operation = m.OP_CALCULATE;

return 0;}

C structures (w/ pointers)

int main ( int argc, char* argv[] ) { struct message* m; m = (struct message*) malloc (sizeof(struct message) ); assert( m!=NULL );

m->operation = m->OP_CALCULATE; //most often used

(*m).operation = (*m).OP_CALCULATE; //same thing

return 0;} Why not sizeof(struct

message*)?

C++ classes (not required for this course)

class Simple3D : public DistanceTransform3D { public:

//define public members (data) and methods (functions) here...

private:...

protected:...

};

Useful header files

#include <assert.h>#include <stdio.h>#include <stdlib.h>

Compiling C/C++ programs on Linux• g++ compiles both C and C++

– Always use g++ for C or C++.• (Don’t use gcc. It only compiles C and C

programs that compile with gcc may not compile with g++.)

• g++ won’t compile 1978 K&R C code but you shouldn’t be coding in that anyway.

• File name extensions– Always use .cpp for C or C++.

• (Don’t use .c. It is only for C programs which may compile with gcc but not with g++.)

• Enter ‘man g++’ for help.

Compiling and running C programs on Linuxg++ command options

-g, -O, -O2, -O3• Optimization level – debug to optimized.

-c• Compile (making a .o file) but don’t link.

-o• Specify output file name.

-l• (el=lower case L) Specify link library name.

Note: use mpic++ to compile mpi-based C/C++ programs.

Example of compiling C programsg++ junk.cpp

• Compile and link producing a.out.

g++ -c junk.cpp• Compile (but don’t link) producing

junk.o.

g++ -o junk.exe junk.o• Link junk.o and produce junk.exe.

g++ -o junk.exe junk.cpp• Compile and link producing junk.exe.

Example of compiling C programs

g++ -g -o junk.exe junk.cpp• Compile and link producing junk.exe w/ debug

information.

g++ -O3 -o junk.exe junk.cpp• Compile and link producing an optimized version

called junk.exe.

g++ -O -o fred.exe junk1.cpp junk2.o \ junk3.cpp -lm

• Compile junk.1pp and junk3.cpp (but not junk2.o) and link producing fred.exe which is somewhat optimized and linked to the math library.

Handling C command g C command line argumentsline arguments• argc

– count/number of arguments• argv

– array of strings (the actual arguments)• Java doesn’t have argc. Why?• In Java, argv[0] is the first argument.

In C, argv[0] is the name of the program/command and argv[1] is the first argument.

End of C for Java programmers