字符串操作函数解析

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在编程的时候我们经常会用到一些库函数来操作字符串，例如strcmp，strlen，strcat, strstr, strcpy, strchar , memcpy, memmov, memset 接下来我们就看一看这些函数。

1 strcmp：

从msdn的解释中可以看出字符串比较这个函数有两个参数string1和string2，而且这俩参数都是常量字符串(const修饰)。返回值是一个int型。当string1小于string2的时候会返回一个小于0的数，当string1等于string2的时候会返回0，当string1小于string2的时候会返回一个大于0的数。我们可以根据返回值来判断这两个字符串的关系。

例如：

#define _CRT_SECURE_NO_DEPRECATE 1#include <stdio.h>    #include <stdlib.h>   #include <string.h>int main(){char str1[]="hello future";char str2[]="hello my future";int ret;ret = strcmp(str1,str2);if(ret>0)printf("str1大于str2");else if(ret=0)printf("str1等于str2");elseprintf("str1小于str2");system("pause");    return 0;}

2 strlen：

从msdn的解释中可以看出这个函数有一个参数 const char *string, 返回值是一个size_t（无符号整形），因为字符串的个数根本就不可能是一个负数。

我们来看看msdn给的例子：

Example/*  STRLEN.C */#include <string.h>#include <stdio.h>#include <conio.h>#include <dos.h>void main( void ){   char buffer[61] = "How long am I?";   int  len;   len = strlen( buffer );   printf( "'%s' is %d characters long\n", buffer, len );}Output'How long am I?' is 14 characters long

3 strcat：

strcat函数实现的功能是把一个字符串连接到另一个字符串的后面。既然是两个字符串连接就要有两个参数即char *strDestination和const char *strSource; 返回值是一个char* 指针，指向strDestination的首元素地址，这样有利于作为链式访问使用。

#define _CRT_SECURE_NO_DEPRECATE 1#include <stdio.h>    #include <stdlib.h>   #include <string.h>int main(){char str1[]="Do you want to ";        char str2[]="play baketball?";printf("%s  \n",strcat(str1,str2));//返回值char * 作为首元素用于实现链式访问system("pause");        return 0;}

当我们使用strcat的时候有一点要注意，不能使用strcat（str1，str1）即給一个字符串自身连接自己本身。否则会出现如下情况。

如果非要自身连接自身请使用strncat.

4 strstr：

这个函数的功能是在一个字符串里面寻找另一个字符串是否存在。这个函数有两个参数即const char *string ,const char *strcharset,返回值是一个char *指针。函数返回指向字符串中第一次出现strcharset，或null如果strcharset不出现在字符串。如果strcharset指向一个字符串的长度为零，函数返回字符串。

我们来看一个msdn中的例子便于理解

Example/* STRSTR.C */#include <string.h>#include <stdio.h>char str[] =    "lazy";char string[] = "The quick brown dog jumps over the lazy fox";char fmt1[] =   "         1         2         3         4         5";char fmt2[] =   "12345678901234567890123456789012345678901234567890";void main( void ){   char *pdest;   int  result;   printf( "String to be searched:\n\t%s\n", string );   printf( "\t%s\n\t%s\n\n", fmt1, fmt2 );   pdest = strstr( string, str );   result = pdest - string + 1;   if( pdest != NULL )      printf( "%s found at position %d\n\n", str, result );   else      printf( "%s not found\n", str );}OutputString to be searched:   The quick brown dog jumps over the lazy fox            1         2         3         4         5   12345678901234567890123456789012345678901234567890lazy found at position 36

5 strcpy：

这个函数实现的是字符串拷贝功能。有两个参数 char *strDestination 和 const char *strSource。从这俩参数我们就可以直接看出是把const char *strSource中的字符串拷贝到char *strDestination中因为是改变char *strDestination中的内容所以要传址。而使用const的修饰char *strSource使之成为常量字符串是为了增加程序的健壮性（便于记忆俩参数的用处，放置俩参数放错引起的不必要的错误）。而返回值char * 指向拷贝完成的字符串首元素。便于链式访问。

看看msdn的例子

Example/* STRCPY.C: This program uses strcpy * and strcat to build a phrase. */#include <string.h>#include <stdio.h>void main( void ){   char string[80];   strcpy( string, "Hello world from " );   strcat( string, "strcpy " );   strcat( string, "and " );   strcat( string, "strcat!" );   printf( "String = %s\n", string );}OutputString = Hello world from strcpy and strcat!

6 strchr：

函数功能：在一个字符串里面寻找一个字符。这个函数有两个参数 const修饰的常量字符串char *string，和int型变量c。c是用来定位这个字符在常量字符串string中的位置的。 char *指向的是要找的这个字符第一次出现在常量字符串string中的位置（可能存在多个该字符）。如果不存在则表示不存在。

看看例子

Example/* STRCHR.C: This program illustrates searching for a character * with strchr (search forward) or strrchr (search backward). */#include <string.h>#include <stdio.h>int  ch = 'r';char string[] = "The quick brown dog jumps over the lazy fox";char fmt1[] =   "         1         2         3         4         5";char fmt2[] =   "12345678901234567890123456789012345678901234567890";void main( void ){   char *pdest;   int result;   printf( "String to be searched: \n\t\t%s\n", string );   printf( "\t\t%s\n\t\t%s\n\n", fmt1, fmt2 );   printf( "Search char:\t%c\n", ch );   /* Search forward. */   pdest = strchr( string, ch );   result = pdest - string + 1;   if( pdest != NULL )      printf( "Result:\tfirst %c found at position %d\n\n",               ch, result );   else      printf( "Result:\t%c not found\n" );   /* Search backward. */   pdest = strrchr( string, ch );   result = pdest - string + 1;   if( pdest != NULL )      printf( "Result:\tlast %c found at position %d\n\n", ch, result );   else      printf( "Result:\t%c not found\n" );}OutputString to be searched:       The quick brown dog jumps over the lazy fox               1         2         3         4         5      12345678901234567890123456789012345678901234567890Search char:   rResult:   first r found at position 12Result:   last r found at position 30

7 memcpy：

功能：从缓存中拷贝字符。参数 void *dest （需要拷贝到的目的地） const void *src（拷贝源）

size_t （需要拷贝的个数）返回值 void * （memcpy returns the value ofdest.）

例子：msdn给的这个例子很长但是实现的功能确实非常简单的，看一眼就能明白。

Example/* MEMCPY.C: Illustrate overlapping copy: memmove * handles it correctly; memcpy does not. */#include <memory.h>#include <string.h>#include <stdio.h>char string1[60] = "The quick brown dog jumps over the lazy fox";char string2[60] = "The quick brown fox jumps over the lazy dog";/*                           1         2         3         4         5 *                  12345678901234567890123456789012345678901234567890 */void main( void ){   printf( "Function:\tmemcpy without overlap\n" );   printf( "Source:\t\t%s\n", string1 + 40 );   printf( "Destination:\t%s\n", string1 + 16 );   memcpy( string1 + 16, string1 + 40, 3 );   printf( "Result:\t\t%s\n", string1 );   printf( "Length:\t\t%d characters\n\n", strlen( string1 ) );   /* Restore string1 to original contents */   memcpy( string1 + 16, string2 + 40, 3 );   printf( "Function:\tmemmove with overlap\n" );   printf( "Source:\t\t%s\n", string2 + 4 );   printf( "Destination:\t%s\n", string2 + 10 );   memmove( string2 + 10, string2 + 4, 40 );   printf( "Result:\t\t%s\n", string2 );   printf( "Length:\t\t%d characters\n\n", strlen( string2 ) );   printf( "Function:\tmemcpy with overlap\n" );   printf( "Source:\t\t%s\n", string1 + 4 );   printf( "Destination:\t%s\n", string1 + 10 );   memcpy( string1 + 10, string1 + 4, 40 );   printf( "Result:\t\t%s\n", string1 );   printf( "Length:\t\t%d characters\n\n", strlen( string1 ) );}OutputFunction:   memcpy without overlapSource:      foxDestination:   dog jumps over the lazy foxResult:      The quick brown fox jumps over the lazy foxLength:      43 charactersFunction:   memmove with overlapSource:      quick brown fox jumps over the lazy dogDestination:   brown fox jumps over the lazy dogResult:      The quick quick brown fox jumps over the lazy dogLength:      49 charactersFunction:   memcpy with overlapSource:      quick brown dog jumps over the lazy foxDestination:   brown dog jumps over the lazy foxResult:      The quick quick brown dog jumps over the lazy foxLength:      49 characters

8 memmov：

函数功能：缓冲区数据移动。参数 void *dest （需要移动到的目的地） const void *src（需要移动的数据源） size_t count （需要拷贝的类型字节数）返回值 void * （memmove returns the value of dest.）

例子：

Example

/* MEMCPY.C: Illustrate overlapping copy: memmove * handles it correctly; memcpy does not. */#include <memory.h>#include <string.h>#include <stdio.h>char string1[60] = "The quick brown dog jumps over the lazy fox";char string2[60] = "The quick brown fox jumps over the lazy dog";/*                           1         2         3         4         5 *                  12345678901234567890123456789012345678901234567890 */void main( void ){   printf( "Function:\tmemcpy without overlap\n" );   printf( "Source:\t\t%s\n", string1 + 40 );   printf( "Destination:\t%s\n", string1 + 16 );   memcpy( string1 + 16, string1 + 40, 3 );   printf( "Result:\t\t%s\n", string1 );   printf( "Length:\t\t%d characters\n\n", strlen( string1 ) );   /* Restore string1 to original contents */   memcpy( string1 + 16, string2 + 40, 3 );   printf( "Function:\tmemmove with overlap\n" );   printf( "Source:\t\t%s\n", string2 + 4 );   printf( "Destination:\t%s\n", string2 + 10 );   memmove( string2 + 10, string2 + 4, 40 );   printf( "Result:\t\t%s\n", string2 );   printf( "Length:\t\t%d characters\n\n", strlen( string2 ) );   printf( "Function:\tmemcpy with overlap\n" );   printf( "Source:\t\t%s\n", string1 + 4 );   printf( "Destination:\t%s\n", string1 + 10 );   memcpy( string1 + 10, string1 + 4, 40 );   printf( "Result:\t\t%s\n", string1 );   printf( "Length:\t\t%d characters\n\n", strlen( string1 ) );}

Output

Function:   memcpy without overlapSource:      foxDestination:   dog jumps over the lazy foxResult:      The quick brown fox jumps over the lazy foxLength:      43 charactersFunction:   memmove with overlapSource:      quick brown fox jumps over the lazy dogDestination:   brown fox jumps over the lazy dogResult:      The quick quick brown fox jumps over the lazy dogLength:      49 charactersFunction:   memcpy with overlapSource:      quick brown dog jumps over the lazy foxDestination:   brown dog jumps over the lazy foxResult:      The quick quick brown dog jumps over the lazy foxLength:      49 characters

9 memset ：

功能:把指定的缓存空间设置为指定的字符。参数 void *dest （需要设置的目的地） c:需要设置的字符类型 size_t count （需要设置的指定类型的个数）返回值 void * （memmove returns the value of dest.）

例子：比起之前几个的繁琐相当精简

Example

/* MEMSET.C: This program uses memset to * set the first four bytes of buffer to "*". */#include <memory.h>#include <stdio.h>void main( void ){   char buffer[] = "This is a test of the memset function";   printf( "Before: %s\n", buffer );   memset( buffer, '*', 4 );   printf( "After:  %s\n", buffer );}

Output

Before: This is a test of the memset functionAfter:  **** is a test of the memset function

在使用这些字符串操作函数的时候我们需要注意的地方是：

1 各个参数的意义，我们可以通过const修饰减少一些不必要的错误引起的失。

2 巧妙的利用返回值，使程序更加的巧妙。

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