strtok.c

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 Copyright (c) 2021 Walt Elam
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/***************************************************************************//**
 @file strtok.c
 
 @brief Defines the strtok function
*******************************************************************************/
#include <string.h>
 
/*******************************************************************************
 strtok
*//**
 @brief Breaks a string into a sequence of tokens split on a given delimiter
 @param *s1 String to be tokenized
 @param *s2 Separator string used to delimit tokens
 
 A sequence of calls to the strtok functions breaks the string pointed to by
 s1 into a sequence of tokens, each of which is delimited by a character from
 the string pointed to by s2. The first call in the sequence has s1 as its
 first argument, and is followed by calls with a null pointer as their first
 argument. The separator string pointed to by s2 may be different from call
 to call.
 
 The first call in the sequence searches the string pointed to by s1 for the
 first character that is not contained in the current separator string
 pointed to by s2. If no such character is found, then there are no tokens in
 the string pointed to by s1 and the strtok function returns a null pointer.
 If such a character is found, it is the start of the first token.
 
 The strtok function then searches from there for a character that is
 contained in the current separator string. If no such character is found,
 the current token extends to the end of the string pointed to by s1, and
 subsequent searches for a token will return a null pointer. If such a
 character is found, it is overwritten by a null character, which terminates
 the current token. The strtok function saves a pointer to the following
 character, from which the next search for a token will start.
 
 Each subsequent call, with a null pointer as the value of the first
 argument, starts searching from the saved pointer and behaves as described
 above.
 
 The implementation shall behave as if no library function calls the strtok
 function.
 
 @return A pointer to the first character of a token
 @retval NULL No token was found
*******************************************************************************/
char *
strtok(char *s1, const char *s2)
{
 static char *save = NULL;
 
 if (!s2)
 {
 return NULL;
 }
 
 if (!s1)
 {
 if (!save)
 {
 return NULL;
 }
 
 s1 = save;
 }
 
 /* Skip initial delimiters */
 s1 += strspn(s1, s2);
 if ('\0' == *s1)
 {
 return NULL;
 }
 
 /* Find next delimiter */
 save = s1 + strcspn(s1, s2);
 if (save && *save)
 {
 *save++ = '\0';
 }
 
 return s1;
}