argv[argc] ==?

Question

My professor and a couple of students are arguing about whether argv is null terminated or not. My friend wrote a small program and it printed out null but another kid said that he is probably simply reading into blank memory. Can someone solve this discussion?

Jaques

Answer 1

From the Standard:

5.1.2.2.1 Program startup
...
-- argv[argc] shall be a null pointer.

So, yes; argv is null terminated

Sahara

Wow, I didn't know that. Why then the argc anyway? – Yellowstone

I don't know why. But it makes writing some programs a little easier by avoiding having to walk the argv array just to count the arguments. – Sahara

Umm.. why: argc: if(argc <= 3) /* not enough arguments */ I think better than if(sizeof(argv) / sizeof(argv[0]) <= 3) – Mckinnie

@Jack: argv is a pointer; argv[0] is also a pointer. In all likelihood sizeof(argv) / sizeof(argv[0]) evaluates to 1 ... anyway, your idea is similar to what I said (not having to walk to argv array). – Sahara

Still, the Operating System (in case of Linux, the ELF loader) has to walk argv, since it's only passed argv in execve(). – Nib

I have noticed that getopt() can crash if argv[argc] is not a NULL pointer (when using it in the Clide library, github.com/gbmhunter/clide-cpp). – Mackenziemackerel

@Sahara But why is char* argv[argc] a pointer to NULL in general? Isn´t it actually an array of pointers to char? Like char* a[i] is also a pointer to char, not to NULL or void. – Cheryl

@RobertS: independent of the names used, by the c specification char *argv[argc] declares an array named argv capable of holding argc pointers to char. When passed to a function, the array gets converted to a pointer to its 1st element (so a pointer to pointer to char [this is why it is usual to see main(int argc, char **argv)]) losing information about size (char *a[10] has 10 elements; char **a is a pointer --- if the pointer points to an array, there is no way of knowing how many elements the underlying array has). – Sahara

Answer 2

According to the standard, "argv[argc] shall be a null pointer" (5.1.2.2.1).

Ballroom

Why and how did it become standard? Any idea? – Proteus

@Reigel: The man page for exec from 1979 (plan9.bell-labs.com/7thEdMan/v7vol1.pdf) shows that this predates the standard by quite a bit, and contains a possible hint as to why it's this way: "Argv is directly usable in another execv because argv[argc] is 0." – Endoplasm