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0 pointsajross7y ago0 comments

> If you want to tell the user how to invoke the program again, argv[0] is the right thing

Some pedantry: it's actually not. The argv array is a completely arbitrary thing, passed by the caller as an array of strings and packed by the kernel into some memory at the top of the stack on entry to main(). It doesn't need to correspond to anything in particular, the use of argv[0] as the file name of the program is a side effect of the way the Bourne shell syntax works. The actual file name to be executed is a separate argument to execve().

In fact there's no portable way to know for sure exactly what file was mapped into memory by the runtime linker to start your process. And getting really into the weeds there may not even be one! It would be totally possible to write a linker that loaded and relocated an ELF file into a bunch of anonymous mappings, unmapped itself, and then jumped to the entry point leaving the poor process no way at all to know where it had come from.

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LukeShu7y ago

Sure, argv[0] is a just string, that the caller can set when they call execve(). That doesn't have mean that it doesn't have meaning. You are correct, there is no way to know how your executable was passed as the first argument to execve(). But, argv[0] is specified to mean roughly "welcome to the world, you are argv[0]", and to tell the program what it is. Sure, you could lie to the program, and tell it that it's something it's not by passing a different string to execve(), you can even do this from a Bash script with `exec -a`.

I stand by my original statement: If you want to tell the user how to invoke the program again, argv[0] is the right thing. I didn't say that running argv[0] will necessarily actually invoke the program again, I said that it's the right thing to tell the user. If the caller set argv[0] to something else, it's because they wanted your program to believe that is its identity, so that's what it should represent its identity as to the user.

tjoff7y ago

Surely the user already knows how to invoke the program, since he/she did it literally two seconds ago.

What if I invoke it from a distant path, do I want my 73 character long path to be prepended in the --help ?

majewsky7y ago

Especially on something like NixOS, where /bin/bash is actually /nix/store/3508wrguwrgu3h5y9354rhfgw056y-bash-5.0/bin/bash.

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int_19h7y ago

argv[0] is defined to be the "program name" by ISO C. What exactly this means is obviously platform dependent, and of course the caller can ignore it altogether and put something random there, but at that point they're the ones misusing the API in a way that breaks the spec.

Note that this is ISO C, not even POSIX. So ELF, Bourne shell etc are implementation details that are out of scope on this level.

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