我正在尝试逐行比较读取文件的性能。第一种情况是getline for string和istream,第二种情况是getline for char *和FILE *。我想知道:
考虑下面的输出(ifstream first):
Lines count: 10628126
ifstream getline: 43.2684
Lines count: 10628126
fopen getline: 1.06217
FILE * first:
Lines count: 10628126
fopen getline: 1.96065
Lines count: 10628126
ifstream getline: 43.0428
我用于测试的代码:
#include <fstream>
#include <iostream>
#include <string>
#include <sys/time.h>
#include <stdio.h>
using namespace std;
double gettime()
{
double result = 0;
struct timeval tv = {0};
struct timezone tz = {0};
gettimeofday(&tv, &tz);
result = tv.tv_sec + (1.0 * tv.tv_usec / 1000000);
return result;
}
void read_cpp(const char * filename)
{
ifstream ifile(filename);
string line;
unsigned int i = 0;
while(getline(ifile, line)) i++;
cout << "Lines count: " << i << endl;
}
void read_c(const char * filename)
{
FILE * ifile = fopen(filename, "r");
size_t linesz = 4096+1;
char * line = new char[linesz];
unsigned int i = 0;
while(getline(&line, &linesz, ifile) > 0) i++;
delete[] line;
cout << "Lines count: " << i << endl;
fclose(ifile);
}
int main(int argc, char * argv[])
{
double tmstart;
tmstart = gettime();
read_cpp(argv[1]);
cout << "ifstream getline: " << (gettime() - tmstart) << endl;
tmstart = gettime();
read_c(argv[1]);
cout << "fopen getline: " << (gettime() - tmstart) << endl;
}
P.S。我尝试交换read_cpp和read_c几乎没有差别。
更新
看起来@Galik和@geza无法使用g ++编译器重现问题,所以我检查了linux环境下的代码, C 和 C ++之间几乎没有区别< / strong>实施。所以这似乎是一个环境问题。最初我使用Mac OS X和默认的C ++编译器测量时间clang
(对我来说很惊讶):
$ g++ -v
Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 8.1.0 (clang-802.0.42)
Target: x86_64-apple-darwin16.7.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin
但所有这些事情从未发生过真正的g++
:
$ g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.9/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ...
Thread model: posix
gcc version 4.9.2 (Debian 4.9.2-10)
很抱歉给您带来不便。
UPDATE2
我找到了相关主题clang++ fstreams 10X slower than g++。作者还面临clang编译的代码的性能下降。要解决此问题,可以使用不同的stdlib
实施(-stdlib=stdlibc++
)而不是默认的-stdlib=libc++
)。在这种情况下,clang
将显示弃用警告:
clang: warning: libstdc++ is deprecated; move to libc++ [-Wdeprecated]
但性能会更好(即使没有优化):
Lines count: 10628126
fopen getline: 1.02899
Lines count: 10628126
ifstream getline: 1.67594
答案 0 :(得分:1)
c ++版本执行更多边界检查,区域设置解释和iostream状态管理。它非常强大。
c版本极简主义且更加脆弱。
安全和实用是有代价的。
那个价格就是时间。
更新
c readline期望使用malloc和free,而不是new和delete。
以下是更正后的版本:
#include <cstdlib>
#include <cstdio>
#include <iostream>
void read_c(const char * filename)
{
FILE * ifile = fopen(filename, "r");
size_t linesz = 0;
char * line = nullptr;
unsigned int i = 0;
while(getline(&line, &linesz, ifile) > 0) i++;
free(line);
std::cout << "Lines count: " << i << std::endl;
fclose(ifile);
}