strcat在某种特定条件下的优化

strcat是C语言一个基本的字符串操作函数，它的源代码一般是这样的。

 

 

char *strcat(char *dest, const char *src){char *tmp = dest;while (*dest) dest++;while ((*dest++ = *src++) != '/0');return tmp;}

 

由此可见，strcat调用时，先移动目标字符串的指针到其尾部，再进行复制。这种做法对于下标比较大的数组重复调用时，效率比较低。想象一下，第一次调用strcat时，指针由0数到100，只不过复制了几个字符，第二次调用strcat时，指针又从0数到108，无论调用多少次，指针总是从0数起，就会知道这个时候是多么浪费系统资源了！

 

我找到一个办法，字符串追加时，事先给出目标字符串结尾所在的位置，追加时，也就不用从头开始计算其长度了，复制的过程中，目标字符串的结尾也随之移动，下一次再追加也就可以使用它了。以下就是优化过的string_append，与strcat相比，增加了一个整形指针以传递目标字符串长度的地址。

 

 

 

/* * optimizer for strcat when appending to a large array again and again */char *string_append(char *dest, int *end, const char *src) {if ( *end >= 0 && dest && src ) {char *p = dest + *end;while ( *p++ = *src++ ) (*end)++;}return dest;}

 

经试验，string_append在大数组重复追加内容的情形下，优势非常明显。其它情形下，使用原来的strcat也就足够了。

 

#include <stdio.h>#include <string.h>#include <time.h>#define BUFF_SIZE 4096/* * optimizer for strcat when appending to a large array again and again */char *string_append(char *dest, int *end, const char *src) {if ( *end >= 0 && dest && src ) {char *p = dest + *end;while ( *p++ = *src++ ) (*end)++;}return dest;}int main() {int i = 0, j = 0;int retry = 100000;int field = 100;char output1[BUFF_SIZE], output2[BUFF_SIZE];time_t time1 = time(NULL);for ( i = 0; i < retry; i++ ) {memset(output1, 0, BUFF_SIZE);int length = 0;string_append(output1, &length, "header/n");for ( j = 0; j < field; j++ ) {string_append(output1, &length, "/tcall detail record ");char c[8];sprintf(c, "%d", j);string_append(output1, &length, c);string_append(output1, &length, "/n");}string_append(output1, &length, "trailer/n");}time_t time2 = time(NULL);printf("It takes %d seconds to show the performance of string_append()/n", time2 - time1);time1 = time(NULL);for ( i = 0; i < retry; i++ ) {memset(output2, 0, BUFF_SIZE);strcat(output2, "header/n");for ( j = 0; j < field; j++ ) {strcat(output2, "/tcall detail record ");char c[8];sprintf(c, "%d", j);strcat(output2, c);strcat(output2, "/n");}strcat(output2, "trailer/n");}time2 = time(NULL);printf("It takes %d seconds to show the performance of strcat()/n", time2 - time1);if ( strcmp(output1, output2) )printf("They are NOT equal/n");elseprintf("They are equal/n");return 0;}

 

-bash-3.2$ ./string_append_demo

It takes 2 seconds to show the performance of string_append()

It takes 11 seconds to show the performance of strcat()

They are equal