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Alek Richter
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Enlightened
Asked: 2022-01-13T10:56:01+00:00

Fastest way to check if a file exist using standard C++/C++11,14,17/C?

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I would like to find the fastest way to check if a file exist in standard C++11,14,17, or C. I have thousands of files and before doing something on them I need to check if all of them exist. What can I write instead of /* SOMETHING */ in the following function?

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

1 Answer

  2. Enlightened

    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>
#include <fstream>

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 Time
    exists_test0 (ifstream) 0.485s
    exists_test1 (FILE fopen) 0.302s
    exists_test2 (posix access()) 0.202s
    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.

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