strstr.c

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 Copyright (c) 2021 Walt Elam
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/***************************************************************************//**
 @file strstr.c
 
 @brief Defines the strstr function
*******************************************************************************/
#include <string.h>
 
/*******************************************************************************
 strstr
*//**
 @brief Locate the first occurrence of a string in a another string
 @param *s1 String being searched
 @param *s2 String being found
 
 The strstr function locates the first occurrence in the string pointed to by
 s1 of the sequence of characters (excluding the terminating null character)
 in the string pointed to by s2.
 
 @return A pointer to the located string
 @retval NULL The sequence in s2 was not found in s1
*******************************************************************************/
char *
strstr(const char *s1, const char *s2)
{
 size_t s2_len = 0;
 
 if (!s1 || !s2)
 {
 return NULL;
 }
 
 s2_len = strlen(s2);
 if (0 == s2_len)
 {
 return (char *) s1;
 }
 
 /* A longer string will never be found */
 if (s2_len > strlen(s1))
 {
 return NULL;
 }
 
 while (*s1)
 {
 /* Jump to the next match on the first character */
 s1 = strchr(s1, *s2);
 if (!s1)
 {
 break;
 }
 
 if (0 == strncmp(s1, s2, s2_len))
 {
 break;
 }
 
 s1++;
 }
 
 return (char *) s1;
}