2016-17 Programmi C

C language - No Libraries

The following programs do not use library functions (only printf, just to have a feedback of the results).

arrays, pointers and structs

#include <stdio.h>

char *spoint="hello";
char sarr[]="hello";
struct strs {
  char s[6];
} sstruct = {"hello"};

void foo(char *s) {
  s[4]=0;
}

void bar(struct strs s) {
  s.s[4]=0;
  printf("from bar %s\n", s.s);
}

int main(int argc, char *argv[]) {
  printf("%s %s %s\n", spoint, sarr, sstruct.s);
  foo(sarr);
  printf("%s %s %s\n", spoint, sarr, sstruct.s);
  bar(sstruct);
  printf("%s %s %s\n", spoint, sarr, sstruct.s);
  // test the following statements, one at a time
  //spoint = sarr;
  //sarr = spoint;
  foo(spoint);
  printf("%s %s %s\n", spoint, sarr, sstruct.s);
}

iteration and recursion

#include <stdio.h>

struct elem {
  int val;
  struct elem *next;
};

struct elem *head = NULL;

struct elem *rinsert(struct elem *new, struct elem *head) {
  if (head == NULL || new->val < head->val) {
    new->next = head;
    return new;
  } else {
    head->next = rinsert(new, head->next);
    return head;
  }
}

struct elem *iinsert(struct elem *new, struct elem *head) {
  struct elem **pnext;
  for (pnext = &head;
      *pnext != NULL && new->val > (*pnext)->val;
      pnext = &((*pnext)->next))
    ;
  new->next = *pnext;
  *pnext = new;
  return head;
}

void rprint(struct elem *this) {
  if (this) {
    printf("%d ",this->val);
    rprint(this->next);
  }
}

void iprint(struct elem *this) {
  for ( ; this != NULL; this = this->next)
    printf("%d ",this->val);
}

struct elem test[]={{5},{3},{9},{1},{7}};
#define NELEM (sizeof(test) / sizeof(struct elem))

int main(int argc, char *argv[]) {
  int i;
  for (i = 0; i < NELEM; i++)
    head = rinsert(&test[i], head);
  rprint(head);
  printf("\n");
  iprint(head);
  printf("\n");
  head = NULL;
  for (i = 0; i < NELEM; i++)
    head = iinsert(&test[i], head);
  rprint(head);
  printf("\n");
  iprint(head);
  printf("\n");
}

comma operator

#include <stdio.h>

int slen(char *s) { /* in real programs use strlen instead */
  size_t rval;
  for (rval = 0; *s != 0; s++, rval++)
    ;
  return rval;
}

int ispal(char *s) {
  int i,j;
  for (i=0, j=slen(s)-1; i < j; i++, j--) {
    if (s[i] != s[j])
      return 0;
  }
  return 1;
}

void reverse(char *s) {
  int i,j;
  for (i=0, j=slen(s)-1; i < j; i++, j--)
    s[j] ^= s[i] ^= s[j] ^= s[i];
}

int main(int argc, char *argv[1]) {
  for ( ;argc > 1; argv++, argc--) {
    printf("\"%s\" is%s palindrome\n", argv[1],
        ispal(argv[1])?"":"n't");
    reverse(argv[1]);
    printf("\"%s\"\n", argv[1]);
  }
}

string by value

#include <stdio.h>

int slen(char *s) { /* in real programs use strlen instead */
  int rval;
  for (rval = 0; *s != 0; s++, rval++)
    ;
  return rval;
}

void printxvowel(char v, char *s) {
  int s_len = slen(s);
  char locals[s_len];
  int i;
  for (i=0; i<s_len; i++) {
    switch (s[i]) {
      case 'a':
      case 'e':
      case 'i':
      case 'o':
      case 'u':
        locals[i] = v;
        break;
      default:
        locals[i] = s[i];
        break;
    }
  }
  printf("-> %s\n<- %s\n",s,locals);
}

int main(int argc, char *argv[]) {
  for ( ; argc > 1; argc--, argv++) {
    printxvowel('a',argv[1]);
    printxvowel('e',argv[1]);
    printxvowel('i',argv[1]);
    printxvowel('o',argv[1]);
    printxvowel('u',argv[1]);
  }
  return 0;
}

test it using "Garibaldi fu ferito, fu ferito ad una gamba, Garibaldi che comanda, che comanda il battaglion" as argv[1].

tables and preprocessor tricks

#include <stdio.h>

#define rows_of(X) (sizeof(X) / sizeof((X)[0]))

#define printTable(X) do { \
    int i; \
    printf("TABLE " #X ": size of element %d\n" \
        "(printed by the line %d of source file %s)\n",  \
        sizeof(*(table ## X)), __LINE__, __FILE__); \
    for (i = 0; i < rows_of(table ## X); i++)  \
      printf(#X " %02d %s\n",i,table ## X [i]); \
    } while (0);

char tableA[][50] = {"Sempre caro mi fu quest'ermo colle,",
  "e questa siepe, che da tanta parte",
  "dell’ultimo orizzonte il guardo esclude."};
char *tableB[] = {"Sempre caro mi fu quest'ermo colle,",
  "e questa siepe, che da tanta parte",
  "dell’ultimo orizzonte il guardo esclude."};

int main(int argc, char *argv[1]) {
  int i;
  printTable(A);
  printf("\n");
  printTable(B);
}

arrays and pointers

#include <stdio.h>

int slen(char *s) { /* in real programs use strlen instead */
  size_t rval;
  for (rval = 0; *s != 0; s++, rval++)
    ;
  return rval;
}

void echoargs(int argc, char *argv[]) {
  int i;
  for (i = 0; i < argc; i++)
    printf("argv[%d] = \"%s\"\n",i,argv[i]);
  printf("\n");
}

enum state {SPACE, CHAR};

int splitargv(char *s, char **argv) {
  enum state state = SPACE;
  int count = 0;
  for (; *s != 0; s++) {
    if (*s == ' ' || *s == '\t' || *s == '\n') {
      if (state != SPACE)
        count++;
      if (argv != NULL)
        *s = 0;
      state = SPACE;
    } else {
      if (state == SPACE && argv != NULL)
        *argv++ = s;
      state = CHAR;
    }
  };
  if (state != SPACE)
    count++;
  if (argv != NULL)
    *argv = NULL;
  return count;
}

void splitargs(char *args) {
  int newargc = splitargv(args, NULL);
  char *newargv[newargc + 1];
  splitargv(args, newargv);
  echoargs(newargc, newargv);
}

int main(int argc, char *argv[1]) {
  echoargs(argc, argv);
  for ( ; *argv != NULL; argv++) {
    printf("Split \"%s\"\n",*argv);
    splitargs(*argv);
  }
}

This is a simplified version of the idea used in the libexecs library. Test this program using args like:

./a.out "ciao               mare" "a      b c"

void * and function pointers

#include <stdio.h>

typedef void (*voidfun) (void *arg);

void printint(void *arg) {
  int *iarg = arg;
  printf("int %d\n", *iarg);
}

void printstring(void *arg) {
  char *sarg = arg;
  printf("int %s\n", sarg);
}

void printpointer(void *arg) {
  printf("pointer %p\n", arg);
}

void printfun(void *arg) {
  voidfun fun = arg;
  fun((void *) 0x42);
}

void launch(voidfun f, void *opaque) {
  f(opaque);
}

int main(int argc, char *argv[1]) {
  int v = 235;
  char *s = "Lasciate ogni speranza, o voi ch'entrate";
  launch(printint, &v);
  launch(printstring, s);
  launch(printfun, printpointer);
  return 0;
}

This technique is used to implement callbacks with opaque args.