Vincent Vincent - 2 months ago 13
C Question

Fastest way to check if a file exist using standard C++/C++11/C?

I would like to find the fastest way to check if a file exist in standard c++11(or)c++(or)c (I have thousands of files and before doing something on it I need to check if all of them exist). What to write instead of

/* SOMETHING */
in the following function ?

inline bool exist(const std::string& name)
{
/* SOMETHING */
}

Answer

Well I threw together a test program that ran each of these methods 100,000 times, half on files that existed and half on files that didn't.

#include <sys/stat.h>
#include <unistd.h>
#include <string>

inline bool exists_test0 (const std::string& name) {
    ifstream f(name.c_str());
    return f.good();
}

inline bool exists_test1 (const std::string& name) {
    if (FILE *file = fopen(name.c_str(), "r")) {
        fclose(file);
        return true;
    } else {
        return false;
    }   
}

inline bool exists_test2 (const std::string& name) {
    return ( access( name.c_str(), F_OK ) != -1 );
}

inline bool exists_test3 (const std::string& name) {
  struct stat buffer;   
  return (stat (name.c_str(), &buffer) == 0); 
}

Results for total time to run the 100,000 calls averaged over 5 runs,

Method exists_test0 (ifstream): **0.485s**
Method exists_test1 (FILE fopen): **0.302s**
Method exists_test2 (posix access()): **0.202s**
Method exists_test3 (posix stat()): **0.134s**

The stat() function provided the best performance on my system (Linux, compiled with g++), with a standard fopen call being your best bet if you for some reason refuse to use POSIX functions.