seekg, relative or absolute position?

Question

I need to read an array from a file. The array is not ordered continuously in the file, have to jump "offset" bytes to get the next element. What is more efficient, assuming that I read a very large file.

1) Use an incremental relative position.
2) Use an absolute position.

option 1:

int var[N];
seekg(0);
for (int i=0; i<N; i++) {
  file.read( (char*) var+i, sizeof(int))
  seekg(offset,ios_base::cur);
}

option 2:

int var[N];
for (int i=0; i<N; i++) {
  file.seekg(offset*i);
  read( (char*) var+i, sizeof(int))
}

Kerrek SB · Answer 1 · 2013-07-06T18:24:11.460

2

read will already advance the position, so you don't need to seek inside the loop. Moreover, arrays are laid out contiguously in memory, so you can just say:

std::vector<int> var(N);

auto res = file.read(reinterpret_cast<char*>(var.data()), sizeof(int) * var.size());

Just make sure to check the value of res and of file afterwards:

if (!file || res != sizeof(int) * var.size())
{
     // an error occurred
}

If you're reading from random parts of the file, it makes no difference how you seek (files are essentially "random access"). But be sure to run the above test after every single read to catch errors.

edited Jul 06 '13 at 18:24

answered Jul 06 '13 at 18:07

Kerrek SB

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And to answer the original question: A relative position must always be turned into an absolute position (by a simple add), so there will be no measurable difference between using relative vs absolute positioning. – BraveNewCurrency Jul 06 '13 at 18:13
Rephrased my question: The array is not ordered continuously in the file – Barak Jul 06 '13 at 18:18
@Barak: OK, in that case you can seek, and it makes absolutely no difference how. Just bear in mind when you seek relatively that `read` already advances the cursor, and make sure to check for errors *after every single extraction*. – Kerrek SB Jul 06 '13 at 18:23

score 0 · Answer 2 · answered Jul 06 '13 at 18:16

I'm 99.9% sure that it will make no difference at all (aside from correctness in terms of offset needs to be correctly adjusted for the fact that you've moved sizeof(int) bytes forward in the relative case, and not in the absolute case. In both cases, you do a seek, which will move the current position in the file. The actual code in the filesystem that deals with that will ultimately move to an absolute position by calculating it from the current one in the case of ios_base::cur).

If it's REALLY important for you to know which is better, then benchmark the two options. But I'm pretty certain that it makes absolutely no difference at all inside the actual seek function in the filesystem. It's just a large integer (probably 64 bits) keeping track of where in the file you are reading (or writing) next.

seekg, relative or absolute position?

2 Answers2