strcat的幾種實現及性能比較

 

一  原型說明

     strcat()為C語言標准庫函數，用於字符串拼接。函數原型聲明在string.h頭文件中：

char *strcat(char *dest, const char *src);

     該函數將參數src所指字符串拷貝到參數dest所指字符串的結尾處(覆蓋dest結尾處的'\0')並添加'\0'。其返回值為參數dest所指字符串的起始地址。注意，dest必須有足夠的空間來容納要拷貝的src字符串。

     本文將給出strcat函數的幾種實現，並比較其執行效率。代碼運行環境如下：

 

二  代碼實現

2.1 匯編實現

char *AssemStrcat(char *pszDest, const char *pszSrc)
{
    int d0, d1, d2, d3;
    __asm__ __volatile__(
        "repne\n\t"
        "scasb\n\t"
        "decl %1\n"
        "1:\tlodsb\n\t"
        "stosb\n\t"
        "testb %%al,%%al\n\t"
        "jne 1b"
        : "=&S" (d0), "=&D" (d1), "=&a" (d2), "=&c" (d3)
        : "0" (pszSrc), "1" (pszDest), "2" (0), "3" (0xffffffff):"memory");
    return pszDest;
}

2.2 模擬C庫

char *SimuStrcat(char *pszDest, const char *pszSrc)
{
    char *pszOrigDst = pszDest;

    while(*pszDest)
        pszDest++;
    while((*pszDest++ = *pszSrc++) != '\0')
        ;

    return pszOrigDst;
}

     因strcat的C標准庫函數的實現方式未知，故使用SimuStrcat函數模擬標准庫實現。

2.3 快速拼接

     由strcat函數的原型說明可知，每次調用時都必須掃描整個目的字符串(以尋找結束符)。這會降低該函數的執行效率，尤其是在頻繁拼接時。

     FastStrcat函數每次返回拼接后的字符串尾部，即指向結束符所在位置。下次調用時便不再需要掃描整串，從而提高執行效率。

char *FastStrcat(char *pszDest, const char* pszSrc)
{
    while(*pszDest)
        pszDest++;
    while((*pszDest++ = *pszSrc++));
    return --pszDest;
}

     注意，第二個while循環若不加雙層括號則會報”suggest parentheses around assignment used as truth value”的警告。因返回時對pszDest作減法運算，故單次執行時FastStrcat慢於SimuStrcat。

     為提高執行速度，本節函數實現均未對指針參數做判空處理。若兼求安全性，可在函數內使用assert斷言指針合法性。Gcc編譯器還對函數聲明提供一種nonnull擴展屬性，可在編譯時進行有限的判空保護：

char *FastStrcat(char *pszDest, const char* pszSrc)__attribute__((__nonnull__(1, 2)));
char *NullPtr(void){return NULL;}
int main(void)
{
    char *pszBuf = NULL;
    FastStrcat(pszBuf, "Elizabeth ");
    FastStrcat(NullPtr(), "Elizabeth ");
 8 FastStrcat(NullPtr()?pszBuf:NULL, "Elizabeth ");  9     FastStrcat(NULL, "Elizabeth ");
    return 0;
}

     其中，__nonnull__(1, 2)指示編譯器對FastStrcat函數調用的第一個和第二個參數判空。

     編譯結果如下：

[wangxiaoyuan_@localhost test1]$ gcc -Wall -o test test.c
test.c: In function 'main':
test.c:8: warning: null argument where non-null required (argument 1)
test.c:9: warning: null argument where non-null required (argument 1)

     可見，除非將指針參數顯式地置空，否則nonnull機制也檢測不到。此外，使用nonnull機制時，若打開編譯優化選項-O2，則函數內關於指針參數的校驗將被優化掉。

 

三  性能比較

     本節采用《Linux用戶態程序計時方式詳解》一文的TIME_ELAPSED()宏進行程序計時。

#define TIME_ELAPSED(codeToTime) do{ \
    struct timeval beginTime, endTime; \
    gettimeofday(&beginTime, NULL); \
    {codeToTime;} \
    gettimeofday(&endTime, NULL); \
    long secTime  = endTime.tv_sec - beginTime.tv_sec; \
    long usecTime = endTime.tv_usec - beginTime.tv_usec; \
    printf("[%s(%d)]Elapsed Time: SecTime = %lds, UsecTime = %ldus!\n", __FILE__, __LINE__, secTime, usecTime); \
}while(0)

     基於該宏，編寫測試代碼如下：

#ifdef strcat //檢查標准庫strcat是否用宏實現
    #warning strcat has been defined!
#endif 
int main(void)
{
    char szCatBuf[1000];
    szCatBuf[0] = '\0'; //字符串快速初始化

    char *pszBuf = szCatBuf;
    TIME_ELAPSED(
        pszBuf = FastStrcat(pszBuf, "Abraham, ");
        pszBuf = FastStrcat(pszBuf, "Alexander, ");
        pszBuf = FastStrcat(pszBuf, "Maximilian, ");
        pszBuf = FastStrcat(pszBuf, "Valentine ")
    );
    printf("  [FastStrcat]szCatBuf = %s\n", szCatBuf);

    szCatBuf[0] = '\0';
    TIME_ELAPSED(
        SimuStrcat(szCatBuf, "Abraham, ");
        SimuStrcat(szCatBuf, "Alexander, ");
        SimuStrcat(szCatBuf, "Maximilian, ");
        SimuStrcat(szCatBuf, "Valentine ")
    );
    printf("  [SimuStrcat]szCatBuf = %s\n", szCatBuf);

    szCatBuf[0] = '\0';
    TIME_ELAPSED(
        AssemStrcat(szCatBuf, "Abraham, ");
        AssemStrcat(szCatBuf, "Alexander, ");
        AssemStrcat(szCatBuf, "Maximilian, ");
        AssemStrcat(szCatBuf, "Valentine ")
    );
    printf("  [AssemStrcat]szCatBuf = %s\n", szCatBuf);

    szCatBuf[0] = '\0';
    TIME_ELAPSED(
        strcat(szCatBuf, "Abraham, ");
        strcat(szCatBuf, "Alexander, ");
        strcat(szCatBuf, "Maximilian, ");
        strcat(szCatBuf, "Valentine ")
    );
    printf("  [LibStrcat]szCatBuf = %s\n", szCatBuf);

    szCatBuf[0] = '\0';
    TIME_ELAPSED(
        sprintf(szCatBuf, "%s%s%s%s","Abraham, ", "Alexander, ", "Maximilian, ", "Valentine ")
    );
    printf("  [LibSprintf]szCatBuf = %s\n", szCatBuf);

    return 0;
}

     除上節三種strcat實現外，代碼還對齊庫實現及sprintf方式進行測試。測試結果如下：

[wangxiaoyuan_@localhost test1]$ gcc -Wall -o test test.c
[wangxiaoyuan_@localhost test1]$ ./test
[test.c(15)]Elapsed Time: SecTime = 0s, UsecTime = 2us!
  [FastStrcat]szCatBuf = Abraham, Alexander, Maximilian, Valentine 
[test.c(24)]Elapsed Time: SecTime = 0s, UsecTime = 2us!
  [SimuStrcat]szCatBuf = Abraham, Alexander, Maximilian, Valentine 
[test.c(33)]Elapsed Time: SecTime = 0s, UsecTime = 1us!
  [AssemStrcat]szCatBuf = Abraham, Alexander, Maximilian, Valentine 
[test.c(42)]Elapsed Time: SecTime = 0s, UsecTime = 1us!
  [LibStrcat]szCatBuf = Abraham, Alexander, Maximilian, Valentine 
[test.c(48)]Elapsed Time: SecTime = 0s, UsecTime = 6us!
  [LibSprintf]szCatBuf = Abraham, Alexander, Maximilian, Valentine

     因每次測試僅調用一次待測函數，故計時不太精准。對比另一次測試結果：

[wangxiaoyuan_@localhost test1]$ ./test
[test.c(15)]Elapsed Time: SecTime = 0s, UsecTime = 4us!
  [FastStrcat]szCatBuf = Abraham, Alexander, Maximilian, Valentine 
[test.c(24)]Elapsed Time: SecTime = 0s, UsecTime = 4us!
  [SimuStrcat]szCatBuf = Abraham, Alexander, Maximilian, Valentine 
[test.c(33)]Elapsed Time: SecTime = 0s, UsecTime = 3us!
  [AssemStrcat]szCatBuf = Abraham, Alexander, Maximilian, Valentine 
[test.c(42)]Elapsed Time: SecTime = 0s, UsecTime = 2us!
  [LibStrcat]szCatBuf = Abraham, Alexander, Maximilian, Valentine 
[test.c(48)]Elapsed Time: SecTime = 0s, UsecTime = 15us!
  [LibSprintf]szCatBuf = Abraham, Alexander, Maximilian, Valentine

     可見，單次執行時，strcat庫函數最快，FastStrcat居中，sprintf庫函數最慢。

     接着，比較批量執行時strcat庫函數和FastStrcat的速度。測試代碼如下：

int main(void)
{
#define ROUND    (unsigned short)1000
    char szCatBuf[sizeof("Elizabeth ")*ROUND];
    szCatBuf[0] = '\0';

    char *pszBuf = szCatBuf;
    TIME_ELAPSED(
        unsigned short wIdx = 0;
        for(; wIdx < ROUND; wIdx++)
            pszBuf = FastStrcat(pszBuf, "Elizabeth ");
    );

    szCatBuf[0] = '\0';
    TIME_ELAPSED(
        unsigned short wIdx = 0;
        for(; wIdx < ROUND; wIdx++)
            strcat(szCatBuf, "Elizabeth ");
    );

    return 0;
}

     測試結果如下：

[wangxiaoyuan_@localhost test1]$ gcc -Wall -o test test.c
[wangxiaoyuan_@localhost test1]$ ./test
[test.c(12)]Elapsed Time: SecTime = 0s, UsecTime = 99us!
[test.c(19)]Elapsed Time: SecTime = 0s, UsecTime = 3834us!

     可見，批量執行時，FastStrcat遠快於strcat庫函數。

 

四  總結

     單次拼接時，strcat庫函數最快，FastStrcat居中，sprintf庫函數最慢(若非格式化需要不建議用於字符串拼接)。

     頻繁拼接時，FastStrcat相比strcat庫函數具有明顯的速度優勢。