What does this program do ?

What does this program do ? #include <stdio.h> int main(int argc, char* argv[]) { int i; printf("%d arguments\n", argc); for(i = 0; i < argc; i++) printf(" %d: %s\n", i, argv[i]); return 0; }

Command line arguments • main() can be called with arguments • argc (argument count) : Number of command line arguments program is invoked with • argv (argument vector) : Array of pointers to character strings (input arguments with main()) .

Arrays of pointers • int a[10][20] • int *b[10]; • a[3][4], b[3][4] both valid references. • sizeof(a) = ? • sizeof(b) = ? • Any other advantage ?

Most useful in storing character strings of different lengths. • char *name[] = {“Illegal month”, “Jan”}; • char aname[][15] = {“Illegal month”, “Jan”}; • Similarly : char *argv[]

#include <stdio.h> void f(char *p[]) { void *q; q = *p; *p = *(p+1); *(p+1) = q; } main(int argc, char **argv) { f(argv); printf("%s,%c\n",argv[0],argv[0][1]); } • What does f() do. • What is the output of the program if we type: • $ gcc f.c • ./a.out 23 45

Pointer to functions #include <stdio.h> #include <malloc.h> int * funct2(int a) { int b = ++a * 10; return &b; } int main(void) { int *(*fp)(int); int *c, d= 6; c = (int *) malloc(sizeof(int)); *c = 30; fp = funct2; d = 2; c = fp(d); printf("c = %d, d = %d\n",*c,d); return 0; }

Automatic and static variables • malloc: variables allocated memory at runtime • dynamic memory allocation. • Resides in a heap. • Has to be freed explicitly.

Convert to English • static int c = 1; • const int * pc; • int * const cp; • int *a[10]; • int **ip; • int *f(void *) • int (*f) (void *)

Convert to C Structure person has three components: name (a pointer to a structure that contains two components fname (a character string), and lname (a character string)); dob (an array of 3 integers), and parent (a pointer to a person). Declare variable employees as an array of 10 pointers to structure person. Function p that takes person as a call-by-reference argument, and returns a generic pointer. Procedure map that takes a pointer fp to a function that returns an integer pointer. Show how you would invoke the function referenced by fp, and print out it’s result.

char *f0 (char *a[][3]) { return (**a); } char *f1 (char *a[][3]) { return (*(*(a+1)+2)); } int main (void) { char *(*funcs[2]) (char *array[][3]); static char *array[][3] = {{"0","1","2"}, {"3","4","5"}, {"6","7","8"}}; funcs[0] = &f0; funcs[1] = &f1; printf("%s\n", (**funcs)(array)); printf("%s\n", (**(funcs+1))(array+1)); return (0); }

What does this program do ?