An Introduction to C Programming

1. An Introduction to C Programming (assuming that you already know Java; this is not an introduction to C++)

2. Minimal C program int 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?

3. Java’s main public static void main ( String args[] ) { }

4. 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 C vs. Java types

5. 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; }

6. 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?

7. 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 file fprintf( fp, “hello, world.\n” ); //close the file fclose( fp ); fp = NULL; return 0; } Hint: Always open b/binary or binary I/O will act strangely under Windows.

8. ASCII file output format specs • %d – an int • %c – a char • %x – an int in hex • %f – a double or a float • %s – string • Many, many, many others and variations.

9. ASCII file input format specs • %d – an int • %c – a char • %x – an int in hex • %f – a float • %lf – a double (that’s % ell eff) • %s – string • Many, many, many others and variations.

10. 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.

11. 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.

12. 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?

13. 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 );

14. C pointers How do we declare variables that hold pointers (instead of values)? int i=12, j=52; int* ptr = &i; //ptr points to i ptr = &j; //ptr now points to j

15. 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?

16. 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?

17. C pointers • C pointers can be treated as arrays. #define N 100 int 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 }

18. C pointers • C pointers can be treated as arrays. #define N 100 int ray[ N ]; int* ptr = ray; //same as ptr=&ray[0] for (int i=0; i<N; i++) { *ptr = 0; ++ptr; }

19. C pointers • C pointers can be treated as arrays. #define N 100 int ray[ N ]; int* ptr = ray; //same as ptr=&ray[0] for (int i=0; i<N; i++) { *ptr++ = 0; }

20. C pointers • C pointers can be treated as arrays (most of the time). #define N 100 int 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?

21. C pointers • Common pitfall int* ptr1, ptr2; //what (type) is ptr2?

22. C pointers • Common pitfall int* ptr1, ptr2; This is actually the same as int *ptr1, ptr2; Which means that ptr2 is an int (not an int*)!

23. C pointers and arrays • Common pitfall #define N 100 int ray[ N ]; int* ptr = ray; printf( “%d %d \n”, ray[N], ptr[1000] );

24. C pointers and arrays • Common pitfall #define N 100 int ray[ N ]; int* ptr = ray; printf( “%d %d \n”, ray[N], ptr[1000] );

25. More C pointers Cast = change from one type to another. double d = 0.7; int i = (int)(d+0.5); //round d int j = (int)d; //don’t round d Sometimes we need to change (cast) from one pointer type to another.

26. ASCII input Use scanf (from stdin) and fscanf. printf( “enter two integers: “ ); int n1, n2; scanf( “%d %d”, &n1, &n2 ); Why is ‘&’ necessary?

27. 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?

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

29. 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) { … }

30. 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>.

31. 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 );

32. 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 ); and fgets( buff, sizeof(buff), stdin );

33. More notes about I/O What’s the difference between: char buff[ 255 ]; gets( buff ); and fgets( buff, sizeof(buff), stdin ); gets: “Reads characters from stdin and stores them as a string into str until a newline character ('\n') or the End-of-File is reached. The ending newline character ('\n') is not included in the string. A null character ('\0') is automatically appended after the last character copied to str to signal the end of the C string. Notice that gets does not behave exactly as fgets does with stdin as argument: First, the ending newline character is not included with gets while with fgets it is. And second, gets does not let you specify a limit on how many characters are to be read, so you must be careful with the size of the array pointed by str to avoid buffer overflows.” from http://www.cplusplus.com/reference/clibrary/cstdio/gets/

34. More C pointers • Dynamic memory allocation (and deallocation). • malloc int* iptr = (int*)malloc( 10*sizeof(int) ); assert( iptr!=NULL ); • free free( iptr ); iptr = NULL; //good idea, not required • Must #include <stdlib.h> at top. cast

35. Binary I/O • Buffered • fread and fwrite • Unbuffered • read and write • We will skip unbuffered I/O (but you are free to use it).

36. 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 100 int buffer[ N ]; int k = fread( buffer, sizeof( int ), N, fp ); • What is k?

37. Binary input: fread size_t fread ( void* ptr, size_t size, size_t nitems, FILE* stream ); • Returns nitems if OK. #define N 100 int buffer[ N ]; int k = fread( buffer, sizeof( int ), N, fp ); k = fread( buffer, 1, sizeof( buffer ), fp ); k = fread( buffer, sizeof( buffer ), 1, fp );

38. 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!

39. 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 100 int buffer[ N ]; int k = fwrite( buffer, sizeof( int ), N, fp ); • What is k?

40. 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.

41. C structs (structures) • Collection of data (only). • precursor to objects • similar to a FORTRAN record • Especially useful for message passing. • Ex. { do this operation, here is the data }

42. C structs (structures) struct message { enum { OP_CALCULATE, OP_RESULT, OP_EXIT }; int operation; int parameters[100]; double result; }; Tip: don’t forget ;

43. C structures int main ( int argc, char* argv[] ) { struct message m; m.operation = m.OP_CALCULATE; return 0; }

44. 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*)?

45. C++ classes (not required for this course) class Simple3D : public DistanceTransform3D { public: //define public members (data) and methods (functions) here . . . private: . . . protected: . . . };

46. Useful header files #include <assert.h> #include <stdio.h> #include <stdlib.h>

47. 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.

48. Compiling and running C programs on Linux g++ 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.

49. Example of compiling C programs g++ 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.

50. 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.