세그먼테이션이 일어납니다.

      1 #include <cstdio>
      2 #include <cstring>
      3
      4 char* ClarifyNumber( char *str, char *buf )
      5 {
      6     int len;
      7     char *p = NULL;
      8
      9     if ( *str == '\0' )
     10     {
     11         *buf = '\0';
     12
     13         return( (char *)NULL );
     14     }
     15
     16     /* strip space backward */
     17     p = str + strlen( str ) - 1;
     18
     19     for ( ; *p == ' ' ; p-- ) ;
     20
     21     *( ++p ) = '\0';
     22
     23     /* strip space forward */
     24     for ( ; *str == ' ' || *str == '0'; str++ );
     25
     26     p = buf;
     27
     28     if ( *str == '+' ) *str++;
     29     if ( *str == '-' ) *buf++ = *str++;
     30
     31     if ( ( len = strlen( str ) ) <= 0 )
     32     {
     33         *buf = '\0';
     34         return( (char *)NULL );
     35     }
     36
     37     while ( ( *buf++ = *str++ ) )
     38     {
     39         if ( --len && ( len % 3 ) == 0 )
     40             *buf++= ',';
     41     }
     42
     43     *buf = '\0';
     44
     45     return( p );
     46 }
     47
     48 int main ( void )
     49 {
     50     char* s = "1234";
     51     char buf[ 1024 ] = { 0, };
     52
     53     char* abc = ClarifyNumber( s, buf );
     54     printf( "%s\n", abc );
     55
     56     return 0;
     57 }

21라인에서 세그먼테이션이 일어나는데...왜그러지요?