Check if a file exists without opening it

Question

How can I check if a file exists in my directory before continuing with my program? I have read answers that try opening the file using various methods but my issue is that most of the time, the file I am checking for will be corrupt and cant be opened. This happens in the error checking part of my program and will only be triggered when an error in the preceding code has occurred. I want to check if the file exists, if so then ask to delete it, otherwise just print out some message.

How can I go about this?

(Just deleting and accepting the errors would work, but I'm doing this to learn, so I want to do it properly...)

Edit:

I have downloaded Boost to use the filesystem library and have compiled it, seemingly with no errors, but when I try to compile my program, I get this response:

g++ program.cpp -I <path to>/boost_1_54_0 -o output

Undefined symbols for architecture x86_64:
"boost::filesystem::detail::status(boost::filesystem::path const&, boost::system::error_code*)", referenced from:
  boost::filesystem::exists(boost::filesystem::path const&)in cc1XX8rD.o
"boost::system::system_category()", referenced from:
  __static_initialization_and_destruction_0(int, int)in cc1XX8rD.o
"boost::system::generic_category()", referenced from:
  __static_initialization_and_destruction_0(int, int)in cc1XX8rD.o
ld: symbol(s) not found for architecture x86_64
collect2: ld returned 1 exit status

The only places I use boost in the program are:

boost::filesystem::path my_file(s4);
if (boost::filesystem::exists(my_file)){ ...

Answer 1

Use stat() or access():

#include <unistd.h>

int res = access(path, R_OK);
if (res < 0) {
    if (errno == ENOENT) {
         // file does not exist
    } else if (errno == EACCES) {
         // file exists but is not readable
    } else {
         // FAIL
    }
}

Answer 2

A file that exists but contains corrupt data can be opened with no ill effect. As long as you don't try to read the contents of the file.

However, any "check if it exists" is subject to TOCTUI and is quite unnecessary in the first place. Just trying to delete the file and accept that deletion may not always work is a better approach.

Answer 3

If, by any chance, you're using Boost in your project, you could go with Boost.Filesystem:

#include <boost/filesystem.hpp>

boost::filesystem::path my_file("some/path/some_file");

if (boost::filesystem::exists(my_file))
{
    boost::filesystem::remove(my_file);
}
else
{
    std::cout << "File does not exist!" << std::endl;
}

This solution is portable across many systems Boost.Filesystem Implementation.

Answer 4

You want to do that without try to open. If you change your mind and try to open the file:

bool file_exists(std::string filename){
   ifstream ifile(filename);
   return ifile;
}

Somebody says, that this will close automatically. I don't know.

And, if you don't want to open the file, I think there is a lots of opportunities. I can tell a slow, oldschool and ugly solution, maybe others will tell you better solutions. Here it is:

system("dir > something.abc");

After that, you have to open something.abc file, and parse / interpreter the lines. You have to open something.abc file, but, you don't have to open that file, which's you want to analize.

This solution will be very slow (0.5 seconds, this is not good for example with 100 files), you have to write a lots of code, and this will be very complecated to parse / interpreter the "dir" command's answer.