字符串操作函数解析
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");
else
printf("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
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 );
}
Output
String to be searched:
The quick brown dog jumps over the lazy fox
1 2 3 4 5
12345678901234567890123456789012345678901234567890
lazy found at position 36
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 );
}
Output
String = Hello world from strcpy and strcat!
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" );
}
Output
String to be searched:
The quick brown dog jumps over the lazy fox
1 2 3 4 5
12345678901234567890123456789012345678901234567890
Search char: r
Result: first r found at position 12
Result: last r found at position 30
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 of
dest.）
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 ) );
}
Output
Function: memcpy without overlap
Source: fox
Destination: dog jumps over the lazy fox
Result: The quick brown fox jumps over the lazy fox
Length: 43 characters
Function: memmove with overlap
Source: quick brown fox jumps over the lazy dog
Destination: brown fox jumps over the lazy dog
Result: The quick quick brown fox jumps over the lazy dog
Length: 49 characters
Function: memcpy with overlap
Source: quick brown dog jumps over the lazy fox
Destination: brown dog jumps over the lazy fox
Result: The quick quick brown dog jumps over the lazy fox
Length: 49 characters
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 overlap
Source: fox
Destination: dog jumps over the lazy fox
Result: The quick brown fox jumps over the lazy fox
Length: 43 characters
Function: memmove with overlap
Source: quick brown fox jumps over the lazy dog
Destination: brown fox jumps over the lazy dog
Result: The quick quick brown fox jumps over the lazy dog
Length: 49 characters
Function: memcpy with overlap
Source: quick brown dog jumps over the lazy fox
Destination: brown dog jumps over the lazy fox
Result: The quick quick brown dog jumps over the lazy fox
Length: 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 function
After: **** is a test of the memset function
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。