（1）strchr

const char * strchr ( const char * str, int character );
      char * strchr (       char * str, int character );

Locate first occurrence of character in string

Returns a pointer to the first occurrence of character in the C string str.

The terminating null-character is considered part of the C string. Therefore, it can also be located in order to retrieve a pointer to the end of a string.

Return: 第一次出现位置，出错则返回NULL

strch.c源码

char *
strchr (s, c_in)
     const char *s;
     int c_in;
{
  const unsigned char *char_ptr;
  const unsigned long int *longword_ptr;
  unsigned long int longword, magic_bits, charmask;
  unsigned char c;

  c = (unsigned char) c_in;

  /* Handle the first few characters by reading one character at a time.
     Do this until CHAR_PTR is aligned on a longword boundary.  */
  for (char_ptr = (const unsigned char *) s;
       ((unsigned long int) char_ptr & (sizeof (longword) - 1)) != 0;
       ++char_ptr)
    if (*char_ptr == c)
      return (void *) char_ptr;
    else if (*char_ptr == ‘\0‘)
      return NULL;

  /* All these elucidatory comments refer to 4-byte longwords,
     but the theory applies equally well to 8-byte longwords.  */

  longword_ptr = (unsigned long int *) char_ptr;

  /* Bits 31, 24, 16, and 8 of this number are zero.  Call these bits
     the "holes."  Note that there is a hole just to the left of
     each byte, with an extra at the end:

总结： 1. strchr查找是以循环比较所查找字符

　　　 2. 字符 char默认是 1 个字节，若是宽字符则需令处理