snprintf与strncpy效率对比

一直以为strncpy效率优于snprintf. 无意中在网上看到snprintf效率大于strncpy,颇感震惊. 网址如下:

http://blog.csdn.net/wavemoon/archive/2009/12/06/4952904.aspx

后自己编码测试了下,发现情况复杂,并不是如此.

现在帖上我的测试:

环境: linux

编译器:g++

代码:

#include <iostream>
#include <string.h>
#include <stdlib.h>
#include "sys/time.h"


const int LOOP_COUNT = 10000;

class RecTime
{
    public:
        RecTime()
        {
            gettimeofday(&m_stBegin, NULL);
        }
        ~RecTime()
        {
            gettimeofday(&m_stEnd, NULL);

            int iUsedMs = (m_stEnd.tv_sec - m_stBegin.tv_sec)*1000 +
                            (m_stEnd.tv_usec-m_stBegin.tv_usec + 999) / 1000;

            std::cout << "time used: " << iUsedMs << std::endl;
        }
    private:
        struct timeval m_stBegin;
        struct timeval m_stEnd;
};

void test_strncpy(char* pDst, int iDstLen, char* pSrc)
{
    RecTime _time_record;
    for(int i=0; i<LOOP_COUNT; i++)
    {
        if(pDst != strncpy(pDst, pSrc, iDstLen))
        {
            std::cout << "fatal error, strncpy failed.";
            exit(1);
        }
    }
    return ;
}

void test_strncpy_withnull(char* pDst, int iDstLen, char* pSrc)
{
    RecTime _time_record;
    for(int i=0; i<LOOP_COUNT; i++)
    {
        if(pDst != strncpy(pDst, pSrc, iDstLen))
        {
            std::cout << "fatal error, strncpy failed.";
            exit(1);
        }

        pDst[iDstLen - 1] = 0;
    }
    return ;
}

void test_snprintf(char* pDst, int iDstLen, char* pSrc)
{
    RecTime _time_record;
    for(int i=0; i<LOOP_COUNT; i++)
    {
        if(0 > snprintf(pDst, iDstLen, "%s", pSrc))
        {
            std::cout << "fatal error, snprintf failed.";
            exit(1);
        }
    }
    return ;
}

int main(int argc, char* argv[])
{
    ////////////////////////////////////////////////////////////////////////////////////////////////////////
    char szDst[1024] = {0};
    char szSrc[1024] = {0};

         memset(szSrc, '1', sizeof(szSrc)-1);
         szSrc[sizeof(szSrc)-1] = 0;

    char* pDst = szDst;
    int iDstSize = sizeof(szDst);
    char* pSrc = szSrc;
    int iSrcSize = sizeof(szSrc);

    std::cout << "copy " << iSrcSize << " to "<< iDstSize << std::endl;
    std::cout << "snprintf ";
    test_snprintf(pDst, iDstSize, pSrc);
    std::cout << "strncpy ";
    test_strncpy(pDst, iDstSize, pSrc);
    /////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

    return 0;
}

#include <iostream>   
#include <string.h>   
#include <stdlib.h>   
#include "sys/time.h"      
  
  
const int LOOP_COUNT = 10000;   
  
class RecTime      
{      
    public:      
        RecTime()      
        {         
            gettimeofday(&m_stBegin, NULL);      
        }         
        ~RecTime()      
        {         
            gettimeofday(&m_stEnd, NULL);   
               
            int iUsedMs = (m_stEnd.tv_sec - m_stBegin.tv_sec)*1000 +   
                            (m_stEnd.tv_usec-m_stBegin.tv_usec + 999) / 1000;   
               
            std::cout << "time used: " << iUsedMs << std::endl;    
        }         
    private:      
        struct timeval m_stBegin;   
        struct timeval m_stEnd;      
};      
     
void test_strncpy(char* pDst, int iDstLen, char* pSrc)      
{      
    RecTime _time_record;      
    for(int i=0; i<LOOP_COUNT; i++)      
    {         
        if(pDst != strncpy(pDst, pSrc, iDstLen))      
        {         
            std::cout << "fatal error, strncpy failed.";   
            exit(1);      
        }   
    }         
    return ;      
}      
     
void test_strncpy_withnull(char* pDst, int iDstLen, char* pSrc)      
{      
    RecTime _time_record;      
    for(int i=0; i<LOOP_COUNT; i++)      
    {         
        if(pDst != strncpy(pDst, pSrc, iDstLen))      
        {         
            std::cout << "fatal error, strncpy failed.";   
            exit(1);      
        }   
  
        pDst[iDstLen - 1] = 0;   
    }         
    return ;      
}    
  
void test_snprintf(char* pDst, int iDstLen, char* pSrc)      
{      
    RecTime _time_record;      
    for(int i=0; i<LOOP_COUNT; i++)      
    {      
        if(0 > snprintf(pDst, iDstLen, "%s", pSrc))   
        {      
            std::cout << "fatal error, snprintf failed.";   
            exit(1);      
        }      
    }      
    return ;      
}      
  
int main(int argc, char* argv[])      
{      
    ////////////////////////////////////////////////////////////////////////////////////////////////////////   
    char szDst[1024] = {0};   
    char szSrc[1024] = {0};   
  
         memset(szSrc, '1', sizeof(szSrc)-1);      
         szSrc[sizeof(szSrc)-1] = 0;      
  
    char* pDst = szDst;   
    int iDstSize = sizeof(szDst);   
    char* pSrc = szSrc;   
    int iSrcSize = sizeof(szSrc);   
  
    std::cout << "copy " << iSrcSize << " to "<< iDstSize << std::endl;   
    std::cout << "snprintf ";   
    test_snprintf(pDst, iDstSize, pSrc);      
    std::cout << "strncpy ";   
    test_strncpy(pDst, iDstSize, pSrc);      
    /////////////////////////////////////////////////////////////////////////////////////////////////////////////////////   
  
    return 0;      
}   

输出:

copy 102400 to 102400
snprintf time used: 455
strncpy time used: 938

 

copy 10240 to 10240
snprintf time used: 42
strncpy time used: 94

copy 1024 to 1024
snprintf time used: 6
strncpy time used: 10

copy 64 to 64
snprintf time used: 2
strncpy time used: 1

copy 8 to 8
snprintf time used: 2
strncpy time used: 1

copy 1024 to 10240
snprintf time used: 6
strncpy time used: 104

copy 10240 to 1024
snprintf time used: 1234
strncpy time used: 10

copy 24 to 1024
snprintf time used: 2
strncpy time used: 11

copy 1024 to 24
snprintf time used: 136
strncpy time used: 1

分析:

1. 在源串长度和目标串长度一样大的情况下,效率几乎相同. 在一个数量级上. 因为在linux下snprintf末尾会有'\0'.所以这时推荐使用snprintf

2. 源串长度 < 目标串长度    snprintf效率领先一个级别, 这时推荐使用snprintf

3. 源串长度 > 目标串长度     strncpy效率领先一个级别, 这时推荐使用strncpy

结论:

1.在源串长度 远大于 目标串长度(至少几倍). 且效率重要的时候. 考虑使用strncpy(这时别忘了在末尾加'\0')

2.其他情况推荐使用snprintf