C ++ ofstream指针无法写入磁盘

Question

I am having trouble writing to ofstream pointer and this is quite perplexing as I really don't see anything that is missing anymore. Note, this is a follow up from this question: C++ vector of ofstream, how to write to one particular element

My code is as follows:

std::vector<shared_ptr<ofstream>> filelist;

void main()
{
  for(int ii=0;ii<10;ii++)
  {
     string filename = "/dev/shm/table_"+int2string(ii)+".csv";
     filelist.push_back(make_shared<ofstream>(filename.c_str()));
  }

  *filelist[5]<<"some string"<<endl;
  filelist[5]->flush();
  exit(1);

}

This does doesn't write anything to the output file but it does create 10 empty files. Does anybody know what might possibly be wrong here?

EDIT: I ran some further tests. I let the code run without exit(1) until completion, over all files until all callbacks are finished. It turns out that some files are not empty, while others that should have data are empty.

There is plenty of disk space, and I know I have more file descriptors than are necessary for this. Any explanation for why some of the files would be written properly while others are not?

Answer 1

I'd try: (*filelist[5])<<"some string\\n"; .

I'd guess, however, that you probably meant to write to the files inside a loop -- as-is, you're writing to only one file.

Oh, and in C++, you don't want to use exit .

Edit: Here's a quick (tested) standalone demo:

#include <fstream>
#include <string>
#include <vector>

std::vector<std::ofstream *> filelist;

int main() {
  for(int ii=0;ii<3;ii++)
  {
    char *names[] = {"one", "two", "three"};
     std::string filename = "c:\\trash_";
     filename += names[ii];
     filename += ".txt";
     filelist.push_back(new std::ofstream(filename.c_str()));
  }

  for (int i=0; i<filelist.size(); i++) {
    (*filelist[i])<<"some string\n";
    filelist[i]->close();
  }
}

Note, however, that the file name this generates is for Windows, whereas the original was (apparently) intended for something Unix-like. For a Unix-like OS, you'll need/want a different file name string.

Answer 2

Try closing the file before you call exit with filelist[5]->close(); . You've aborted a process with an open file which means your write may not have made it to the OS buffer or was discarded upon process exit. You could also remove the exit call it would probably fix the problem. The results of IO on a process that is aborted are tricky to nail down, so it's best to try avoiding aborts with active IO or to assume any active IO will fail upon abort.