[Minor request] keyword flags when opening.

[ryofurue]

NCDatasets uses "c":

NCDatasets("filename.nc", "c")

to create the file, whereas Julia's open function doesn't have a "c" flag. It seems that "w" is the right flag for Julia'sopen function, but it's not clear to me whether this is always equivalent to "create".

This situation is confusing enough for me so that I never use the "w" flag for Julia's open function and I instead use

open("filename"; create = true)

( https://docs.julialang.org/en/v1/base/io-network/#General-I/O )

So, my request would be: It would be nice if the NCDataset function faithfully mirrored open's keywords.

(Purely out of curiosity, where did "c" come from? Julia's "w", "r", . . . seem to be the same as those of fopen() the C library function.)

Environment

• operating system: macOS 13.4.1 (c)
• Julia version: julia 1.9.2 from juliaup
• Output of the julia command versioninfo()

Julia Version 1.9.2
Commit e4ee485e909 (2023-07-05 09:39 UTC)
Platform Info:
  OS: macOS (arm64-apple-darwin22.4.0)
  CPU: 10 × Apple M2 Pro
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-14.0.6 (ORCJIT, apple-m1)
  Threads: 1 on 6 virtual cores

• NCDatasets version: v0.12.17
• Output of using Pkg; Pkg.status(mode=PKGMODE_MANIFEST)

Full output
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[Alexander-Barth]

The option "c" comes from the NC_CLOBBER:
https://docs.unidata.ucar.edu/netcdf-c/current/group__datasets.html#gaccbdb128a1640e204831256dbbc24d3e

see also:
https://en.wikipedia.org/wiki/Clobbering

I guess that the NetCDF authors want to make it clear that existing content will be overwritten.

I agree that the boolean parameters (read, write, create, append) are easier to read.
For some combinations (write = false, append = true) we would need to raise an error.

For reference here is the logic for the default values in base julia:
https://github.com/JuliaLang/julia/blob/e4ee485e90961018b7e53ce14b8b99335953e176/base/io.jl#L344

[ryofurue]

The option "c" comes from the NC_CLOBBER:

I see! I thought it was "create".

https://docs.unidata.ucar.edu/netcdf-c/current/group__datasets.html#gaccbdb128a1640e204831256dbbc24d3e

Hmm . . . This makes me wonder what happens if you nc_open an existing netCDF file with NC_CLOBBER and immediately close it. Will the file remain intact or be truncated to become zero-sized? It's not clear from the documentation.

If you are familiar with Unix-shell programming, you know "clobber" means "truncate" & "write", but the description in the netCDF documentation, "overwrite existing file", can mean "if you write something into it, the existing contents will be overwritten" and it's not clear whether "truncate" is implied or not. Did the netCDF developers adopt the shell's clobbering?

If you say "create", it would be clear that the existing file will be truncated.

So, among the following three methods:

NCDatasets: "r", "w", "c"
julia's mode strings: "r", "w", "a", . . .
julia's boolean keywords: write, truncate, create, append . . .

the last set is the clearest. The julia code you linked to (thanks!) serves as a "flow chart" of the logic but even without that, what will happen is fairly clear from the names "truncate", "append", "create", etc.

Anyway, this is just a very minor issue.