Portability Issues

1. Portability Issues CIS*2450 Advanced Programming Concepts

2. Reference • The Practice of Programming by Brian W. Kernighan and Rob Pike, Addison-Wesley, 1999.

3. Portability Issues • Ideally you should be able to move your program to a different compiler, processor or operating system. • In practice, one strives for code that takes only a few revisions to make it work on another system.

4. Portability Issues • Why worry about portability? • successful programs are expected to do more than what was originally planned • environments change • portable programs are better: better design, better construction and better testing

5. Troubles on the Road to Portability

6. Sizes of Data Types • In C, data type sizes are not defined. #include < stdio.h > int main ( int argc, char *argv[] ) { printf("char = %d, short = %d, int = %d, long = %d\n", sizeof(char),sizeof(short),sizeof(int),sizeof(long)); printf("float = %d, double = %d, pointer = %d\n", sizeof(float),sizeof(double),sizeof(void *)); }

7. Sizes of Data Types • In C, data type sizes are not defined. > testsize char = 1, short = 2, int = 4, long = 4 float = 4, double = 8, pointer = 4

8. Sizes of Data Types • It is not even required that a pointer value fit into an int! • The only rules that you can follow are sizeof(char) <= sizeof(short) <= sizeof(int) <= sizeof(long) sizeof(float) <= sizeof(double) • char must have at least 8 bits, short and int at least 16 and long at least 32

9. Sizes of Data Types • Always use sizeof. Used sizeof Didn’t use sizeof!

10. Alignment of Structure Members • The alignment of items within a structure is not defined except that the order of items in the declaration is observed. • What will the following program print out?

11. Alignment of Structure Members int main ( int argc, char *argv[] ) { struct X { char c; int i; }; printf("Sizeof c = %d and sizeof i = %d and sizeof X = %d\n", sizeof(char), sizeof(int), sizeof(struct X)); }

12. Alignment of Structure Members int main ( int argc, char *argv[] ) { struct X { char c; int i; }; printf(“sizeof c = %d and sizeof i = %d and sizeof X = %d\n", sizeof(char), sizeof(int), sizeof(struct X)); } sizeof c = 1 and sizeof i = 4 and sizeof X = 8

13. Alignment of Structure Members • Never assume that the elements of a structure occupy contiguous memory. • Most machines require that n-byte primitive data types be stored at an n-byte boundary. • The compiler may force different alignments for performance reasons. • Optimization options can affect packing, too.

14. Order of Evaluation • In C, the order of evaluation of operands, side effects, and function arguments is not defined. • What is evaluated first in the following statements and does the order matter?

15. Order of Evaluation ptr[count] = name[++count]; printf("%c %c\n",getchar(),getchar()); printf("%f %s\n",log(-1.23),strerror(errno));

16. Order of Evaluation int main ( int argc, char *argv[] ) { int i; int count = 0; int count2 = 0; int name[10], ptr[10], ptr2[10]; for ( i=0; i < 10; i++ ) { name[i] = i; ptr[i] = ptr2[i] = 0; }

17. Order of Evaluation int main ( int argc, char *argv[] ) { int i; int count = 0; int count2 = 0; int name[10], ptr[10], ptr2[10]; for ( i=0; i < 10; i++ ) { name[i] = i; ptr[i] = ptr2[i] = 0; } for ( i=0; i < 5; i++ ) { ptr[count] = name[++count]; ptr2[count2] = name[count2++]; }

18. Order of Evaluation int main ( int argc, char *argv[] ) { int i; int count = 0; int count2 = 0; int name[10], ptr[10], ptr2[10]; for ( i=0; i < 10; i++ ) { name[i] = i; ptr[i] = ptr2[i] = 0; } for ( i=0; i < 5; i++ ) { ptr[count] = name[++count]; ptr2[count2] = name[count2++]; } for ( i=0; i < 5; i++ ) printf("%d %d %d\n",name[i],ptr[i],ptr2[i]); printf("%c %c\n",getchar(),getchar()); }

19. Order of Evaluation > Testorder 0 10ptr[count] = name[++count];ptr2[count2] = name[count2++]; 1 21 2 32 3 43 4 54 ab input b a printf("%c %c\n",getchar(),getchar());

20. Unambiguous Code – I int main ( int argc, char *argv[] ) { int i, count = 0; int name[10], ptr[10]; for ( i=0; i < 10; i++ ) name[i] = i; for ( i=0; i < 5; i++ ) { ptr[count] = name[count]; count++; } for ( i=0; i < 5; i++ ) printf("%d %d\n",name[i],ptr[i]); printf("%c ",getchar()); printf("%c\n",getchar()); }

21. Unambiguous Code – I > goodorder 0 0 1 1 for ( i=0; i < 5; i++ ) { 2 2 ptr[count] = name[count]; 3 3 count++; 4 4 } ab a b printf("%c ",getchar()); printf("%c\n",getchar());

22. Unambiguous Code – II int main ( int argc, char *argv[] ) { int i, count = 0; int name[10],ptr[10]; for ( i=0; i < 10; i++ ) name[i] = i; for ( i=0; i < 5; i++ ) { ptr[count] = name[count+1]; count++; } for ( i=0; i < 5; i++ ) printf("%d %d\n",name[i],ptr[i]); }

23. Unambiguous Code – II > good2order 0 1 1 2 2 3 3 4 4 5

24. Hints • Always use sizeof. • Say exactly what you mean -- make sure that you have not introduced ambiguities because of sloppy coding.

25. Hints • Study the features and problems of the language that you are working in. • Never assume that a structure is organized contiguously.

26. Hints • Do not write code that relies on a certain compiler interpretation for correctness. • Use the simplest data and control structures that you can.

27. Hints • Listen to all compiler warnings! (-Wall) • Study the function libraries that you are using. Make sure that you understand what every function is returning and what it expects as parameters.