int strlen(const char * s)
{ int i;
  for (i = 0; 1; i += 1)
  { if (s[i] == '\0')
      return i; } }

char * strcpy(char * dest, const char * src)
{ int i;
  for (i = 0; 1; i += 1)
  { dest[i] = src[i];
    if (dest[i] == '\0')
      break; }
  return dest; }

void strcpy(char * dest, const char * src)
{ while (*dest++ = *src++) ; }

int strcmp(const char * a, const char * b)
{ int i;
  for (i = 0; ; i += 1)
  { char ca = a[i], cb = b[i];
    int diff = ca - cb;
    if (diff != 0)
      return diff;
    if (ca == 0)
      return 0; } }

char * strcat(char * dest, const char * src)
{ int lendest = strlen(dest);
  int i;
  for (i = 0; ; i += 1)
  { dest[lendest + i] = src[i];
    if (src[i] == '\0')
      return dest; } }

#include <ctype.h>

int strcasecmp(const char * s, const char * t)
{ int i;
  for (i = 0; ; i += 1)
  { char ca = tolower(a[i]), cb = tolower(b[i]);
    int diff = ca - cb;
    if (diff != 0)
      return diff;
    if (ca == 0)
      return 0; } }

char tolower(char c)
{ if (c >= 'A' && c <= 'Z')
    return c + ('a' - 'A');
  return c; }

char TL[256] = { '\0', '\001', \002', ...., 'a', 'b', 'c', ... 'a', 'b', 'c', ... '~', 0, 0, 0, ... }

char tolower(unsigned char c)
{ return TL[c]; }

char * strncpy(char * dest, const char * src, int N)
int strncmp(const char * a, oonst char * b, int N)
char * strncat(char * dest, const char * src, int N)

char * strstr(const char * s, const char * t)
// finds where t appears as a substring of s

printf("the sum %d + %d is %d\n", a, b, a+b);

char s[] = "one two three four five six\n";

char * r = strtok(s, " \n");
while (r != NULL)
{ printf("%s\n", r);
  r = strtok(NULL, " \n"); }

void bzero(void * ptr, int N)
  set N bytes starting from ptr to zero

void bcopy(void * dst, void * src, int N)

{ int A[1000];
  bzero(A, 1000 * sizeof(int));
  ...