C语言字符串库函数的实现
［来源］ 达内 ［编辑］ 达内 ［时间］2012-09-08
C语言字符串库函数的实现也是笔试题常考的题目，以下代码没有严格测试，只是简单的实现
C语言字符串库函数的实现也是笔试题常考的题目，以下代码没有严格测试，只是简单的实现：
< div style="margin: 5px 0px; padding: 5px; background-color: rgb(245, 245, 245); font-family: 'Courier New'; font-size: 12px; border: 1px solid rgb(204, 204, 204); overflow: auto; color: rgb(0, 0, 0); font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; orphans: 2; text-align: left; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; " class="cnblogs_code">// 字符串长度 int strlen(const char * str) { assert(str != NULL); int len = 0; while (*str ++ != ' \0' ) ++ len; return len; } //字符串拷贝 char *strcpy(char *to, const char *from) { assert((to != NULL) && (from != NULL)); char * result = to; while( (*to++ = *from ++) != '\0 ') NULL; return result; } // strncpy()，如果from指向的字符个数少于count，则用'\0'补齐char *strncpy(char *to, const char *from , size_t count) { assert((to != NULL) && (from != NULL)); char * result = to; while(count--) { if(*from != '\0 ') { *to++ = *from++ ; } else { *to++ = '\0 '; } } return result; } // memcpy(), 拷贝不重叠的内存块 void * memcpy(void * to, const void * from , size_t count) { assert((to != NULL) && (from != NULL)); void * result = to; char * pto = (char *)to; char * pfrom = (char *)from ; assert(pto < pfrom || pto > pfrom + count -1 ); while (count--) { *pto++ = *pfrom++ ; } return result; } // memmove(), 拷贝重叠或者是不重叠的内存块 void * memmove(void * to, const void * from , size_t count) { assert((to != NULL) && (from != NULL)); void * result = to; char * pto = (char *)to; char * pfrom = (char *)from ; // to与from没有重叠 if (pto < pfrom || pto > pfrom + count -1 ) { while(count--) { *pto++ = *pfrom++; } } // to与from有重叠，从后向前move else { pto = pto + count -1 ; pfrom = pfrom + count -1; while(count-- ) { *pto-- = *pfrom--; } } return result; } // memset():把指定内存区域的前count个字节设置成字符c void * memset(void * buffer, int c, size_t count) { assert(buffer != NULL); char * p = (char *)buffer; while(count--) *p++ = (char)c; return buffer; } // 查找字符串s中首次出现字符c的位置 char *strchr(char *str, int c) { assert(str != NULL); for (; *str != (char)c; ++ str) if (*str == ' \0' ) return NULL; return str; } // 字符串比较 int strcmp(const char *s, const char *t) { assert(s != NULL && t != NULL); while (*s && *t && *s == *t) { ++ s; ++ t; } return (*s - *t); } int strncmp(const char *s, const char *t, unsigned int count) { assert((s != NULL) && (t != NULL)); while (*s && *t && *s == *t && count --) { ++ s; ++ t; } return (*s - *t); } //字符串连接 char *strcat(char *strDes, const char *strSrc) { assert((strDes != NULL) && (strSrc != NULL)); char *address = strDes; while (*strDes != '\0 ' ) ++ strDes; while ((*strDes ++ = *strSrc ++) != ' \0' ) NULL; return address; } char *strncat(char *strDes, const char *strSrc, unsigned int count) { assert((strDes != NULL) && (strSrc != NULL)); char *address = strDes; while (*strDes != ' \0' ) ++ strDes; while (count -- && *strSrc != ' \0' ) *strDes ++ = *strSrc ++ ; *strDes = '/0 '; return address; } // 查找字符串第一次出现的位置char *strstr(const char *strSrc, const char * str) { assert(strSrc != NULL && str != NULL); const char *s = strSrc; const char *t = str; for (; *strSrc != '\0 '; ++ strSrc) { for (s = strSrc, t = str; *t != ' \0' && *s == *t; ++s, ++t) NULL; if (*t == '\0 ') return (char *) strSrc; } return NULL; } // 将字符串拷贝到新的位置 char *strdup_(char *strSrc) { if (strSrc!=NULL) { char *start=strSrc; int len=0; while(*strSrc++!=' \0' ) len++ ; char *address=(char *)malloc(len+1 ); assert(address != NULL); while ((*address++=*start++)!='\0 '); return address-(len+1 ); } return NULL; }
char * __cdecl strcat (char * dst, const char * src) { char * cp = dst; while( *cp ) cp ++; /* find end of dst */ while( *cp++ = *src++ ) ; /* Copy src to end of dst */ return( dst ); /* return dst */ } int __cdecl strcmp (const char * src,const char * dst) { int ret = 0 ; while( ! (ret = *(unsigned char *)src - *(unsigned char *)dst) && *dst) ++src, ++dst; if ( ret < 0 ) ret = -1 ; else if ( ret > 0 ) ret = 1 ; return ( ret ); } size_t __cdecl strlen ( const char * str) { const char *eos = str; while( *eos++ ) ; return( (int )(eos - str - 1) ); } char * __cdecl strncat (char * front,const char * back,size_t count) { char *start = front; while (*front++) ; front-- ; while (count--) if (!(*front++ = *back++ )) return (start); *front = '\0 '; return (start); } int __cdecl strncmp (const char * first,const char * last,size_t count) { if (!count) return (0); while (--count && *first && *first == * last) { first++; last ++ ; } return ( *(unsigned char *)first - *(unsigned char * )last ); } /* Copy SRC to DEST. */ char * strcpy (char * dest,const char * src) { reg_char c; char *__unbounded s = (char * __unbounded) CHECK_BOUNDS_LOW (src); const ptrdiff_t off = CHECK_BOUNDS_LOW (dest) - s - 1 ; size_t n; do { c = *s++; s[off] = c; } while (c != ' \0' ); n = s - src; ( void) CHECK_BOUNDS_HIGH (src + n); ( void) CHECK_BOUNDS_HIGH (dest + n); return dest; } char * __cdecl strncpy (char * dest,const char * source,size_t count) { char *start = dest; while (count && (*dest++ = *source++)) /* copy string */ count --; if (count) /* pad out with zeroes */ while (-- count) *dest++ = ' \0 '; return(start); }
