C字符串函數(shù)底層實(shí)現(xiàn).doc

C字符串函數(shù)底層實(shí)現(xiàn)1 /*2 * linux/lib/string.c3 *4 * Copyright (C) 1991, 1992 Linus Torvalds5 */6 7 /*8 * stupid library routines. The optimized versions should generally be found9 * as inline code in 10 *11 * These are buggy as well.12 */13 14 #include 15 16 char * _strtok = NULL;17 18 char * strcpy(char * dest,const char *src)19 20 char *tmp = dest;21 22 while (*dest+ = *src+) != 0)23 /* nothing */;24 return tmp;25 26 27 char * strncpy(char * dest,const char *src,size_t count)28 29 char *tmp = dest;30 31 while (count- & (*dest+ = *src+) != 0)32 /* nothing */;33 34 return tmp;35 36 37 char * strcat(char * dest, const char * src)38 39 char *tmp = dest;40 41 while (*dest)42 dest+;43 while (*dest+ = *src+) != 0)44 ;45 46 return tmp;47 48 49 char * strncat(char *dest, const char *src, size_t count)50 51 char *tmp = dest;52 53 if (count) 54 while (*dest)55 dest+;56 while (*dest+ = *src+) 57 if (-count = 0)58 break;59 60 61 62 return tmp;63 64 65 int strcmp(const char * cs,const char * ct)66 67 register signed char _res;68 69 while (1) 70 if (_res = *cs - *ct+) != 0 | !*cs+)71 break;72 73 74 return _res;75 76 77 int strncmp(const char * cs,const char * ct,size_t count)78 79 register signed char _res = 0;80 81 while (count) 82 if (_res = *cs - *ct+) != 0 | !*cs+)83 break;84 count-;85 86 87 return _res;88 89 90 char * strchr(const char * s,char c)91 92 for(; *s != c; +s)93 if (*s = 0)94 return NULL;95 return (char *) s;96 97 98 size_t strlen(const char * s)99 100 const char *sc;101 102 for (sc = s; *sc != 0; +sc)103 /* nothing */;104 return sc - s;105 106 107 size_t strnlen(const char * s, size_t count)108 109 const char *sc;110 111 for (sc = s; *sc != 0 & count-; +sc)112 /* nothing */;113 return sc - s;114 115 116 size_t strspn(const char *s, const char *accept)117 118 const char *p;119 const char *a;120 size_t count = 0;121 122 for (p = s; *p != 0; +p) 123 for (a = accept; *a != 0; +a) 124 if (*p = *a)125 break;126 127 if (*a = 0)128 return count;129 +count;130 131 132 return count;133 134 135 char * strpbrk(const char * cs,const char * ct)136 137 const char *sc1,*sc2;138 139 for( sc1 = cs; *sc1 != 0; +sc1) 140 for( sc2 = ct; *sc2 != 0; +sc2) 141 if (*sc1 = *sc2)142 return (char *) sc1;143 144 145 return NULL;146 147 148 char * strtok(char * s,const char * ct)149 150 char *sbegin, *send;151 152 sbegin = s ? s : _strtok;153 if (!sbegin) 154 return NULL;155 156 sbegin += strspn(sbegin,ct);157 if (*sbegin = 0) 158 _strtok = NULL;159 return( NULL );160 161 send = strpbrk( sbegin, ct);162 if (send & *send != 0)163 *send+ = 0;164 _strtok = send;165 return (sbegin);166 167 168 void * memset(void * s,char c,size_t count)169 170 char *xs = (char *) s;171 172 while (count-)173 *xs+ = c;174 175 return s;176 177 178 char * bcopy(const char * src, char * dest, int count)179 180 char *tmp = dest;181 182 while (count-)183 *tmp+ = *src+;184 185 return dest;186 187 188 void * memcpy(void * dest,const void *src,size_t count)189 190 char *tmp = (char *) dest, *s = (char *) src;191 192 while (count-)193 *tmp+ = *s+;194 195 return dest;196 197 198 void * memmove(void * dest,const void *src,size_t count)199 200 char *tmp, *s;201 202 if (dest = src) 203 tmp = (char *) dest;204 s = (char *) src;205 while (count-)206 *tmp+ = *s+;207 208 else 209 tmp = (char *) dest + count;210 s = (char *) src + count;211 while (count-)212 *-tmp = *-s;213 214 215 return dest;216 217 218 int memcmp(const void * cs,const void * ct,size_t count)219 220 const unsigned char *su1, *su2;221 signed char res = 0;222 223 for( su1 = cs, su2 = ct; 0 count; +su1, +su2, count-)224 if (res = *su1 - *su2) != 0)225 break;226 return res;227 228 229 /*230 * find the first occurrence of byte c, or 1 past the area if none231 */232 void * memscan(vo