Reading whole files in Lua

Question

I am trying to read a full mp3 file in order to read out the id3 tags. That's when I noticed that file:read("*a") apparently does not read the full file but rather a small part. So I tried to build some kind of workaround in order to get the content of the whole file:

function readAll(file)
    local f = io.open(file, "r")
    local content = ""
    local length = 0

    while f:read(0) ~= "" do
        local current = f:read("*all")

        print(#current, length)
        length = length + #current

        content = content .. current
    end

    return content
end

for my testfile, this shows that 256 reading operations are performed, reading a total of ~113kB (the whole file is ~7MB). Though this should be enough to read most id3 tags, I wonder why Lua behaves in this way (especially because it does not when reading large textbased files such as *.obj or *.ase). Is there any explanation for this behaviour or maybe a solution to reliably read the whole file?

Are you on windows? make sure you open the file in binary mode ("rb" to io.open) — daurnimator, Apr 30 '12 at 16:01
Argh, that was the point. Never had to use binary mode before. Thank you a lot! — Henrik Ilgen, Apr 30 '12 at 16:26
Could someone explain the format ***all** in function `f:read("*all")` ? It seems "*all" is equal to "*a" ? I could not find the document in lua manual https://www.lua.org/manual/5.1/manual.html#pdf-file:read — Liu Hao, Nov 14 '17 at 02:45
@liuhao All characters after the first two (The `*a`) are ignored, according to this answer: https://stackoverflow.com/questions/47277203/what-is-the-all-format-in-lua-fileread-means I guess I got this from this chapter in pil back in the days: https://www.lua.org/pil/21.1.html, the advantage being (I guess) that you don't have to remember what `*a`, `*n` and `*l` stand for, because you can instead write it out. — Henrik Ilgen, Nov 14 '17 at 10:55

score 71 · Accepted Answer · edited Nov 21 '17 at 08:32

71

I must be missing something but I fail to see why a loop is needed. This should work (but you'd better add error handling in case the file cannot be opened):

function readAll(file)
    local f = assert(io.open(file, "rb"))
    local content = f:read("*all")
    f:close()
    return content
end

edited Nov 21 '17 at 08:32

VasiliNovikov

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answered Apr 30 '12 at 17:32

lhf

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as can be seen in the comments to my question, @daurnimator already figured out that the problem was the missing b, but thanks for your answer anyway :) edit: I marked your answer as accepted in order to show people my question has been solved. – Henrik Ilgen Apr 30 '12 at 19:43

Reading whole files in Lua

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