Originální popis anglicky: 
index, rindex - locate character in string
Návod, kniha: Linux Programmer's Manual
#include <strings.h>
 
char *index(const char *s, int c);
 
char *rindex(const char *s, int c);
The 
index() function returns a pointer to the first occurrence of the
  character 
c in the string 
s.
The 
rindex() function returns a pointer to the last occurrence of the
  character 
c in the string 
s.
The terminating NULL character is considered to be a part of the strings.
The 
index() and 
rindex() functions return a pointer to the matched
  character or NULL if the character is not found.
BSD 4.3
memchr(3), 
strchr(3), 
strpbrk(3), 
strrchr(3),
  
strsep(3), 
strspn(3), 
strstr(3), 
strtok(3)