June 29, 2008

OCaml’s Unix module and ARGV

Filed under: OCaml, Tech — Chris @ 9:10 pm

Be warned: the string array argument to Unix.create_process et al. represents the entire argument vector: the first element should be the command name. I didn’t expect this, since there is a separate prog argument to create_process, and ended up with weird behavior* like,

# open Unix;;
# create_process "sleep" [|"10"|] stdin stdout stderr;;
10: missing operand
Try `10 --help' for more information.
- : int = 22513

This can be a bit insidious—in many cases skipping the first argument will only subtly change the behavior of the child process.

Note that the prog argument is what matters in terms of invoking the sub-process—the first element of the argument vector is what just what is passed into the process. Hence,

# create_process "gcc" [|"foo";"--version"|] stdin stdout stderr;;
- : int = 24364
foo (GCC) 4.2.3 (Ubuntu 4.2.3-2ubuntu7)

* Actually, this “weird behavior” is the test that finally made me realize what was going on. The emergent behavior of my app was much more mysterious…

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