[原创]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.youkuaiyun.com/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; }

 

 

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