strcat的几种实现及性能比较

一  原型说明

strcat()为C语言标准库函数，用于字符串拼接。函数原型声明在string.h头文件中：

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

该函数将参数src所指字符串拷贝到参数dest所指字符串的结尾处(覆盖dest结尾处的'\0')并添加'\0'。其返回值为参数dest所指字符串的起始地址。注意，dest必须有足够的空间来容纳要拷贝的src字符串。

本文将给出strcat函数的几种实现，并比较其执行效率。代码运行环境如下：

二  代码实现

2.1 汇编实现

 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)
         : "" (pszSrc), "" (pszDest), "" (), "" (0xffffffff):"memory");
     return pszDest;
 }

2.2 模拟C库

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

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

     return pszOrigDst;
 }

因strcat的C标准库函数的实现方式未知，故使用SimuStrcat函数模拟标准库实现。

2.3 快速拼接

由strcat函数的原型说明可知，每次调用时都必须扫描整个目的字符串(以寻找结束符)。这会降低该函数的执行效率，尤其是在频繁拼接时。

FastStrcat函数每次返回拼接后的字符串尾部，即指向结束符所在位置。下次调用时便不再需要扫描整串，从而提高执行效率。

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

注意，第二个while循环若不加双层括号则会报”suggest parentheses around assignment used as truth value”的警告。因返回时对pszDest作减法运算，故单次执行时FastStrcat慢于SimuStrcat。

为提高执行速度，本节函数实现均未对指针参数做判空处理。若兼求安全性，可在函数内使用assert断言指针合法性。Gcc编译器还对函数声明提供一种nonnull扩展属性，可在编译时进行有限的判空保护：

 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 ");
     FastStrcat(NULL, "Elizabeth ");
     return ;
 }

其中，__nonnull__(1, 2)指示编译器对FastStrcat函数调用的第一个和第二个参数判空。

编译结果如下：

 [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)

可见，除非将指针参数显式地置空，否则nonnull机制也检测不到。此外，使用nonnull机制时，若打开编译优化选项-O2，则函数内关于指针参数的校验将被优化掉。

三  性能比较

本节采用《Linux用户态程序计时方式详解》一文的TIME_ELAPSED()宏进行程序计时。

 #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()

基于该宏，编写测试代码如下：

 #ifdef strcat //检查标准库strcat是否用宏实现
     #warning strcat has been defined!
 #endif
 int main(void)
 {
     char szCatBuf[];
     szCatBuf[] = '\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';
     TIME_ELAPSED(
         SimuStrcat(szCatBuf, "Abraham, ");
         SimuStrcat(szCatBuf, "Alexander, ");
         SimuStrcat(szCatBuf, "Maximilian, ");
         SimuStrcat(szCatBuf, "Valentine ")
     );
     printf("  [SimuStrcat]szCatBuf = %s\n", szCatBuf);

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

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

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

     return ;
 }

除上节三种strcat实现外，代码还对齐库实现及sprintf方式进行测试。测试结果如下：

 [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

因每次测试仅调用一次待测函数，故计时不太精准。对比另一次测试结果：

 [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

可见，单次执行时，strcat库函数最快，FastStrcat居中，sprintf库函数最慢。

接着，比较批量执行时strcat库函数和FastStrcat的速度。测试代码如下：

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

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

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

     return ;
 }

测试结果如下：

 [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!

可见，批量执行时，FastStrcat远快于strcat库函数。

四  总结

单次拼接时，strcat库函数最快，FastStrcat居中，sprintf库函数最慢(若非格式化需要不建议用于字符串拼接)。

频繁拼接时，FastStrcat相比strcat库函数具有明显的速度优势。