standard c library <string.h>

1

char *strchr( const char *s ,int c)

{

const char ch = c;

for(; *s!=ch;++s)

if(*s!='\0')

return (NULL);

return ((char*)s);

}

2

char * strrchr(const char *s, int c)

{

const char ch = c;

const char *sc;

for(sc=NULL; ; ++s)

{

if(*s==ch)

sc=s;

if(*s == '\0')

return ((char*)sc);

}

}

3　　

char *strstr(const char *s1, const char *s2)
{
if(*s2=='\0')
return (char*)s1;
for(; ( s1 = strchr(s1, *s2) ) != NULL ; ++s1)
{
const char *sc1, *sc2;
for(sc1 = s1, sc2= s2; ;)
{
if(*++sc2 == '\0')/* come to the end ---- all match*/
return (char *)s1;
else if(*++sc1 != *sc2)
break;
}
}
return (NULL);
}

4

char * strcpy(char *s1, const char *s2)
{
char *s = s1;
for(s = s1; (*s++ = *s2++ ) !='\0' ;)

return (s1);}