strcat是C語言一個基本的字符串操作函數，它的源代碼一般是這樣的。

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

由此可見，strcat調用時，先移動目標字符串的指針到其尾部，再進行復制。這種做法對于下標比較大的數組重復調用時，效率比較低。想象一下，第一次調用strcat時，指針由0數到100，只不過復制了幾個字符，第二次調用strcat時，指針又從0數到108，無論調用多少次，指針總是從0數起，就會知道這個時候是多么浪費系統資源了！

我找到一個辦法，字符串追加時，事先給出目標字符串結尾所在的位置，追加時，也就不用從頭開始計算其長度了，復制的過程中，目標字符串的結尾也隨之移動，下一次再追加也就可以使用它了。以下就是優化過的string_append，與strcat相比，增加了一個整形指針以傳遞目標字符串長度的地址。飛鴿傳書：http://www.freeeim.com/，strcat在某種特定條件下的優化。

?

/*
?* 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;
}

經試驗，string_append在大數組重復追加內容的情形下，優勢非常明顯。其它情形下，使用原來的strcat也就足夠了。

?

#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");
??? else
??????? printf("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

總結

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