ifstream not working with dirent.h - c++

Question

I'm testing optimizations for dijkstra algorithm and to make it easier to open files I used "dirent.h" to get all the test files in the running path and then ifstream to open this file.
the readDirec method reads all the files in the directory and ignores folder and puts those files names in a vector called files.
void selectDirec(){
files.clear();
DIR *dir;
struct dirent *ent;
if ((dir = opendir (".")) != NULL) {
while ((ent = readdir (dir)) != NULL) {
if(opendir(ent->d_name) == NULL){
files.push_back(ent->d_name);
}
}
closedir (dir);
} else {
cout<<"directory error"<<endl;
}
}
after that I uses a function called selectFile which assigns the name of the file the user chooses to a variable called fileName.
void selectFile(){
selectDirec();
for(int i = 0 ; i < files.size() ; i++){
cout<<i+1<<" : "<<files[i]<<endl;
}
int choice = 0;
do{
cout<<"enter file number"<<endl;
cin>>choice;
}while(choice > files.size());
choice--;
fileName = files[choice];
cout<<fileName<<":"<<endl;
}
after that I enter my readGraph function which opens the file and continue graph operations
void readGraph(){
ifstream ifile; ifile.open(fileName);
if(!ifile.is_open()){
cout<<"no file with the name specified"<<endl;
eflag = true;
return;
}
...
...
}
initialization:
vector<char *> files;
char * fileName ;
now I have those 5 files to test which I got from here http://algs4.cs.princeton.edu/44sp/:
tinyEWD.txt contains 8 vertices and 15 edges [140B]
mediumEWD.txt contains 250 vertices and 2,546 edges[40KB]
1000EWG.txt contains 1,000 vertices and 16,866 edges[313KB]
10000EWG.txt contains 10,000 vertices and 123,462 edges[2.4MB]
NYC.txt . contains 264346 vertices and 733846 edges[12.7MB].
but there's a weird problem with those 3 files:
'mediumEWD' , '10000EWD.txt' , 'NYC.txt'
when I choose any of them the code shows me "no file with the name specified" that in the else statement in readGraph.
but when I enter their name manually and comment selectDirec and selectFile the program opens them successfully.
P.S. I checked the file name and spacing and everything.
P.S.2 currently running this code on ubuntu 14.04 LTS.
thanks in advance.

Answer 1

if(opendir(ent->d_name) == NULL){
files.push_back(ent->d_name);
}
What is files? I suspect that you are using a std::vector<const char *>, or something along the same lines.
This won't work. d_name is a part of the dirent structure. Immediately afterwards, and certainly after the closedir(), that pointer is no longer valid, and points to deallocated memory.
Looks to me like you then proceed and attempt to use the no-longer valid pointer as the filename parameter to std::ifstream.
You should use a std::vector<std::string> to store the filenames, and use the c_str() member function to extract a pointer to a C-style string, for the open() call.
You can't be using a vector of std::strings here, this must be a vector of raw character pointers. That's because you're assigning one of its values to fileName, whatever it is, and then passing it directly to open() without using c_str(). So it can't be a vector of strings.