c/c++字符串处理函数

       C语言处理字符串极为不便，幸好库函数提供了还算丰富的处理字符串的函数。有一些函数我们平时不大用的到，不过了解一下总是好的，知道有这么个东西，下次要用的时候再去查一下手册就好了。

        这里介绍的函数主要有：strcat, strcmp, strcasecmp, strcpy, strdup, strlen, strchr, strrchr, strstr, strpbrk, strspn, strcspn, strtok. 在本章的最后还会给出这些库函数的具体实现，以应付一些企业的面试。

       文章共分四个部分，第一部分直接列出了常用的字符串处理函数，由于这些都是大家平时接触的比较多的，在此不再多说；第二部分介绍了不太常用的字符串处理函数，并给出了具体示例，很多函数看名字很难猜到它的具体作用，但是一个小小的例子就能解释得很清楚；第三部分给出了这些函数的具体实现，大部分参考网上个人认为已经不错的实现，部分是查看glibc源代码后，简化得来的程序；第四部分作者非常道德的给出了参考资料。

常用的字符串处理函数：

char *strcat(char *s1, const char *s2);

char *strncat(char *s1, const char *s2);

int strcmp(const char *s1, const char *s2);
int strncmp(const char *s1, const char *s2, size_t length);

int strcasecmp(const char *s1, const char *s2);
int strncasecmp(const char *s1, const char *s2, size_t length);

char *strcpy(char *s1, const char *s2);
char *strncpy(char *s1, const char *s2, size_t length);

size_t strlen(const char *s1);

不太常用的字符串处理函数：

//the strdup() function allocates memory and copies into it the string addressed s1. including the terminating null character. 

//It is the use's responsibility to free the allocated storage by calling free()

char *strdup(const char *s1);

#include <stdio.h>#include <stdlib.h>#include <string.h>int main(int argc, char* argv[]){char str[] = "this is a example";char *p = NULL;if ((p = strdup(str)) != NULL){printf("strlen(p) = %d  \t sizeof(p) = %d\n", strlen(p), sizeof(p));printf("strlen(str) = %d\t sizeof(str) = %d\n", strlen(str), sizeof(str));if ( p == str ){puts("p == str");}else{puts("p != str");}}free(p);p = NULL;return 0;}

output:

lalor@ubuntu:~/temp$ ./a.out 
strlen(p) = 17   sizeof(p) = 4
strlen(str) = 17 sizeof(str) = 18
p != str

//locate first occurrence of character in string

char *strchr(const char *s1, int c); 

#include <stdio.h>#include <string.h>int main(int argc, char* argv[]){char str[] = "This is a sample string";char *pch;pch = strchr(str, 's');while (pch != NULL){printf("found at %d\n", pch - str + 1);pch = strchr(pch + 1, 's');}return 0;}

output:
lalor@ubuntu:~/temp$ ./a.out 
found at 4
found at 7
found at 11
found at 18

// locate last occurrence of character in string

char *strrchr(const char *s1, int c);

#include <stdio.h>#include <string.h>int main(int argc, char* argv[]){char str[] = "This is a sample string";char *pch;pch = strrchr(str, 's');printf("found at %d\n", pch - str + 1);return 0;}

output:
lalor@ubuntu:~/temp$ ./a.out 
found at 18

//locate substring
char *strstr(const char *s1, const char *s2);

#include <stdio.h>#include <string.h>int main(int argc, char* argv[]){char str[] = "This is a simple string. It's really a simple string";char *pch = NULL;pch = strstr(str, "simple");while (pch != NULL){printf("found %d\n", pch - str + 1);pch = strstr(pch+1, "simple");}return 0;}

output:
lalor@ubuntu:~/temp$ ./a.out 
found 11
found 40

//return a pointer to the first occurrence in s1 of any of characters that are
//part of s2 ,or a null pointer if there are no matcher.
char *strpbrk(const char* s1, const char *s2);

#include <stdio.h>#include <string.h>int main(int argc, char* argv[]){char str[] = "This is a simple string.";char key[] = "aeiou";char *pch = NULL;printf("Vowels in '%s' : ", str);pch = strpbrk(str, key);while (pch != NULL){printf("%c  ", *pch);pch = strpbrk(pch + 1, key);}printf("\n");return 0;}

output:
lalor@ubuntu:~/temp$ ./a.out 
Vowels in 'This is a simple string.' : i  i  a  i  e  i 

/*

 * scans s1 for the first occurrence of any of the characters that are part of
 * s2. return the number of characters of s1 read before this first occurrence
 */

size_t strcspn(const char *s1, const char *s2);

#include <stdio.h>#include <string.h>int main (){  char str[] = "fcba73";  char keys[] = "1234567890";  int i;  i = strcspn (str,keys);  printf ("The first number in str is at position %d.\n",i+1);  return 0;}

output:
lalor@ubuntu:~/temp$ ./a.out 
The first number in str is at position 5.

// return the length of the initial portion of s1 containing only characters
// that appear in s2
size_t strspn(const char *s1, const char *s2);

#include <stdio.h>#include <string.h>int main (){  int i;  char strtext[] = "129th";  char cset[] = "1234567890";  i = strspn (strtext,cset);  printf ("The length of initial number is %d.\n",i);  return 0;}

output:

lalor@ubuntu:~/temp$ ./a.out 
The length of initial number is 3.

//split string into tokens

char *strtok( char *str, const char *delimiters);

#include <stdio.h>#include <string.h>int main(int argc, char* argv[]){char str[] = " - this, a sample string.";char *pch = NULL;printf("Splitting string \" %s \" into tokens:\n", str);pch = strtok(str, " ,.-");//first callwhile (pch != NULL){printf("%s\n", pch);pch = strtok(NULL, " ,.-");//later call, the second time use strtok, first argument must be NULL}//note: strtok will modify the first stringprintf("strlen(str) = %d\tsizeof(str)/sizeof(char) = %d\n", strlen(str),sizeof(str)/sizeof(char));return 0;}

output:

lalor@ubuntu:~/temp$ ./a.out 
Splitting string "  - this, a sample string. " into tokens:
this
a
sample
string
strlen(str) = 7 sizeof(str)/sizeof(char) = 26

函数源码：

char *strcat(char *s1, const char *s2);

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 *s1, const char *s2);

char* strncat(char *dest, const char*str, size_t n){size_t dest_len = strlen(dest);size_t i;for (i = 0; i < n && src[i] != '\0'; i++) {dest[dest_len + i] = src[i];}dest[dest_len + i] = '\0';return dest;}

int strcmp(const char *s1, const char *s2);

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 *s1, const char *s2, size_t length);

int strcmp(const char *s, const char *t, size_t n){assert((s != NULL) && (t != NULL) && n > 0)while (*s && *t && *s == *t && n--) {s++;t++;}return (*s - *t);}

int strcasecmp(const char *s1, const char *s2);

int strcmp(const char *s, const char *t){assert((s != NULL) && (t != NULL))char c1, c2;while (*s && *t ) {c1 = TOLOWER(*s);c2 = TOLOWER(*t);if (c1 != c2) {break;}else{s++;t++;}}return (c1 - c2);}

int strncasecmp(const char *s1, const char *s2, size_t length);

int strcmp(const char *s, const char *t, size_t n){assert((s != NULL) && (t != NULL) && n > 0)char c1, c2;while (*s && *t && n--) {c1 = TOLOWER(*s);c2 = TOLOWER(*t);if (c1 != c2) {break;}else{s++;t++;}}return (c1 - c2);}

char *strcpy(char *s1, const char *s2);

char *strncpy(char *s1, const char *s2, size_t length);
The strncpy() function is similar, except that at most n bytes  of  src
 are  copied.  Warning: If there is no null byte among the first n bytes
of src, the string placed in dest will not be null-terminated.


       If the length of src is less than n, strncpy() pads  the  remainder  of
       dest with null bytes.

char *   strncpy(char *dest, const char *src, size_t n)           {               size_t i;               for (i = 0; i < n && src[i] != '\0'; i++)                   dest[i] = src[i];               for ( ; i < n; i++)                   dest[i] = '\0';               return dest;           }

size_t strlen(const char *s1);

int strlen(const char *str){assert(str != NULL);int len = 0;while (*str++ != '\0') {len++;}return len;}

char *strdup(const char *s1);

/* Duplicate S, returning an identical malloc'd string.  */char *__strdup (const char *s){  size_t len = strlen (s) + 1;  void *new = malloc (len);  if (new == NULL)    return NULL;  return (char *) memcpy (new, s, len);}

char *strchr(const char *s1, int c); 

char * strchr(const char *str, int c){assert(str != NULL);for ( ; *str != (char)c; ++str){if (*str == '\0') {return NULL;}}return str;}

char *strrchr(const char *s1, int c); 重复使用strchr，直至找到c的最后一次出现

/* Find the last occurrence of C in S.  */char *strrchr (const char *s, int c){  register const char *found, *p;  c = (unsigned char) c;  /* Since strchr is fast, we use it rather than the obvious loop.  */  if (c == '\0')    return strchr (s, '\0');  found = NULL;  while ((p = strchr (s, c)) != NULL)    {      found = p;      s = p + 1;    }  return (char *) found;}

char *strstr(const char *s1, const char *s2);


char *strpbrk(const char* s1, const char *s2);

/* Find the first occurrence in S of any character in ACCEPT.  */char *strpbrk (s, accept)     const char *s;     const char *accept;{  while (*s != '\0')    {      const char *a = accept;      while (*a != '\0')if (*a++ == *s)  return (char *) s;      ++s;    }  return NULL;}

size_t strcspn(const char *s1, const char *s2); //实现非常简单，又好理解，cool

/* Return the length of the maximum initial segment of S   which contains no characters from REJECT.  */size_tstrcspn (s, reject)     const char *s;     const char *reject;{  size_t count = 0;  while (*s != '\0')    if (strchr (reject, *s++) == NULL)      ++count;    else      return count;  return count;}

size_t strspn(const char *s1, const char *s2);//为什么不像strcspn一样，用strchr来实现……

/* Return the length of the maximum initial segment   of S which contains only characters in ACCEPT.  */size_tstrspn (s, accept)     const char *s;     const char *accept;{  const char *p;  const char *a;  size_t count = 0;  for (p = s; *p != '\0'; ++p)    {      for (a = accept; *a != '\0'; ++a)if (*p == *a)  break;      if (*a == '\0')return count;      else++count;    }  return count;}

char *strtok(char *str, const char *delimiters);

参考资料：

1.c语言手册http://www.cplusplus.com/reference/clibrary/cstring/strcmp/

2. glibc源代码，下载地址：ftp://ftp.gnu.org/gnu/glibc

3. 周磊的整理的面试笔试题目http://www.cnblogs.com/v-July-v/