#Best way to read a file into a buffer?

1 messages · Page 1 of 1 (latest)

latent dagger
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I was doing this before with just read(&buf) but found it wasn't very clean. I'd like to use readToEndAlloc() instead and have the following questions:

  1. If I use FixedBufferAllocator for alloc, will it return a buffer that is sized to the same length as the file input?
  2. Does it matter what allocator I use for this?
vale temple
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not sure what you mean by the first one

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for the second one, it doesn't matter, no

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and by the way, you can specify the maximum amount to read from the file

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so you don't end up allocating bajillions

latent dagger
vale temple
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yes

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std.fs.File.seekFromEnd(0);

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std.fs.File.getPos();

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should tell you the exact size in bytes of a file's contents

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that's only after you've opened it though

stuck pollen
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you can also stat() it

vale temple
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yes actually I just see that getEndPos() does that

stuck pollen
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didn't realize there was a dedicated function for that

vale temple
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you can just do const sizeInBytes = try file.getEndPos(); then

latent dagger
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    const cwd: std.fs.Dir = std.fs.cwd();
    const file = try cwd.openFile("input.txt", .{});
    defer file.close();
    // const fba = std.heap.FixedBufferAllocator.init(&mem);
    var gpa = std.heap.GeneralPurposeAllocator(.{}){};
    const alloc = gpa.allocator();
    defer gpa.deinit();
    const buf = try file.readToEndAlloc(alloc, try file.getEndPos());
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It's erroring out on the defer

vale temple
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well what's the error

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and there's 2 defers

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you probably need to do defer _ = gpa.deinit();

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since I am guessing the error is a compile error for an unused return value

latent dagger
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yeah that was it

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it now compiles, but I get a runtime memory leak

vale temple
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such is the way of using low level code

latent dagger
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idk what happened though, if the code does what I think it does then the buffer should just be the file contents and nothing more

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so idk why it would leak memory. maybe I need to free the iterator?

vale temple
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you need to free the buf

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defer allocator.free(buf);