Re: safely reading large files

James Kanze <>
Wed, 21 May 2008 01:33:31 -0700 (PDT)
On May 21, 4:11 am, Victor Bazarov <> wrote: wrote:

How does C++ safely open and read very large files? For example, say I
have 1GB of physical memory and I open a 4GB file and attempt to read
it like so:

#include <iostream>
#include <fstream>
#include <string>
using namespace std;

int main () {
  string line;
  ifstream myfile ("example.txt", ios::binary);
  if (myfile.is_open())
    while (! myfile.eof() )
      getline (myfile,line);
      cout << line << endl;

  else cout << "Unable to open file";

  return 0;

In particular, what if a line in the file is more than the
amount of available physical memory? What would happen?
Seems getline() would cause a crash. Is there a better way.
Maybe... check amount of free memory, then use 10% or so of
that amount for the read. So if 1GB of memory is free, then
take 100MB for file IO. If only 10MB is free, then just read
1MB at a time. Repeat this step until the file has been read
completely. Is something built into standard C++ to handle
this? Or is there a accepted way to do this?

Actually, performing operations that can lead to running out
of memory is not a simple thing at all.

I'm sure you don't mean what that literally says. There's
certainly nothing difficult about running out of memory. Doing
something reasonable (other than just aborting) when it happens
is difficult, however.

Yes, if you can estimate the amount of memory you will need
over what you right now want to allocate and you know the size
of available memory somehow, then you can allocate a chunk and
operate on that chunk until done and move over to the next
chunk. In the good ol' days that's how we solved large
systems of linear equations, one piece of the matrix at a time
(or two if the algorithm called for it).

And you'd manually manage overlays, as well, so that only part
of the program was in memory at a time. (I once saw a PL/1
compiler which ran in 16 KB real memory, using such techniques.
Took something like three hours to compile a 500 line program,
but it did work.)

Unfortunately there is no single straightforward solution. In
most cases you don't even know that you're going to run out of
memory until it's too late. You can write the program to
handle those situations using C++ exceptions. The pseudo-code
might look like this:

     std::size_t chunk_size = 1024*1024*1024;
     MyAlgorithgm algo;

     do {
         try {
             // if I am here, the chunk_size is OK
         catch (std::bad_alloc & e) {
             chunk_size /= 2; // or any other adjustment
     while (chunk_size > 1024*1024); // or some other threshold

Shouldn't the condition here be "while ( operation not done )",
something like:

    bool didIt = false ;
    do {
        try {
            // your code from the try block
            didIt = true ;
        // ... your catch
    } while ( ! didIt ) ;

That way if your preparation fails, you just restart it using
a smaller chunk, until you either complete the operation or
your chunk is too small and you can't really do anything...

Just a note, but that isn't allways reliable. Not all OS's will
tell you when there isn't enough memory: they'll return an
address, then crash or suspend your program when you try to
access it. (I've seen this happen on at least three different
systems: Windows, AIX and Linux. At least in the case of AIX
and Linux, and probably Windows as well, it depends on the
version, and some configuration parameters, but most Linux are
still configured so that you cannot catch allocation errors: if
the command "/sbin/sysctl vm.overcommit_memory" displays any
value other than 2, then a reliably conforming implementation of
C or C++ is impossible.)

James Kanze (GABI Software)
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