Is it possible to change argv or do I need to create an adjusted copy of it?

Question

My application has potentially a huge number of arguments passed in and I want to avoid the memory of hit duplicating the arguments into a filtered list. I would like to fi

Answer 1

Once argv has been passed into the main method, you can treat it like any other C array - change it in place as you like, just be aware of what you're doing with it. The contents of the array don't have an effect on the return code or execution of the program other than what you explicitly do with it in code. I can't think of any reason it wouldn't "be advisable" to treat it specially.

Of course, you still need to take care about accidentally accessing memory beyond the bounds of argv. The flip side of it being accessible like a normal C array is that it's also prone to access errors just like any other normal C array. (Thanks to all who pointed this out in comments and other responses!)

Answer 2

The only time I would say that directly manipulating argv is a bad idea would be when an application changes its behavior depending on the contents of argv[0].

However, changing a program's behavior depending on argv[0] is in itself a very bad idea where portability is a concern.

Other than that, you can treat it just like you would any other array. As Jonathan said, GNU getopt() permutes the argument list non-destructively, I've seen other getopt() implementations that go as far as serializing and even hashing the arguments (useful when a program comes close to ARG_MAX).

Just be careful with your pointer arithmetic.

Answer 3

The original allocation of argv is left as a compiler/runtime choice. So it may not be safe to modify it willy-nilly. Many systems build it on the stack, so it is auto-deallocated when main returns. Other build it on the heap, and free it (or not) when main returns.

It is safe to change the value of an argument, as long as you don't try to make it longer (buffer overrun error). It is safe to shuffle the order of the arguments.

To remove arguments you've preprocessed, something like this will work:

( lots of error conditions not checked for, "--special" other that first arg not checked for, etc. This is, after all, just a demo-of-concept. )

int main(int argc, char** argv)
{
    bool doSpecial = false; // an assumption
    if (0 == strcmp(argv[1], "--special"))
    {
        doSpecial = true; // no longer an assumption
        // remove the "--special" argument
        //  but do copy the NULL at the end.
        for(int i=1; i<argc; ++i)
            argv[i]  = argv[i+1];
        --argc;
    }
    // all normal processing with "--special" removed.
    // the doSpecial flag is available if wanted.
    return 0;
}

But see this for full manipulation: (the part of the libiberty library that is used to manipulates argv style vectors)

http://www.opensource.apple.com/source/gcc/gcc-5666.3/libiberty/argv.c

It is licensed GNU LGPL.

Answer 4

The operating system push the argv and argc into the applicaton stack before executing it, and you can treat them like any other stack variables.

Answer 5

The C99 standard says this about modifying argv (and argc):

The parameters argc and argv and the strings pointed to by the argv array shall be modifiable by the program, and retain their last-stored values between program startup and program termination.

Answer 6

The latest draft of the C standard (N1256) states that there are two allowed forms of the main function:

int main (void);
int main (int argc, char* argv[]);

but the crux is the clause "or in some other implementation-defined manner". This seems to me to be a loophole in the standard large enough to drive a semi-trailer through.

Some people specifically use "const char *" for the argv to disallow changes to the arguments. If your main function is defined that way, you are not permitted to change the characters that argv[] points to, as evidenced by the following program:

pax> cat qq.c
#include <stdio.h>
int main (int c, const char *v[]) {
    *v[1] = 'X';
    printf ("[%s]\n", v[1]);
    return 0;
}

pax> gcc -o qq qq.c
qq.c: In function `main':
qq.c:3: error: assignment of read-only location

However, if you remove the "const", it works fine:

pax> cat qq2.c
#include <stdio.h>
int main (int c, char *v[]) {
    *v[1] = 'X';
    printf ("[%s]\n", v[1]);
    return 0;
}

pax> gcc -o qq2 qq2.c ; ./qq2
[Xello]

I think this is also the case for C++. The current draft states:

All implementations shall allow both of the following definitions of main:
    int main();
    int main(int argc, char* argv[]);

but it doesn't specifically disallow other variants so you could presumably accept a "const" version in C++ as well (and, in fact, g++ does).

The only thing you need to be careful of is trying to increase the size of any of the elements. The standards do not mandate how they're stored so extending one argument may (probably will) affect others, or some other unrelated data.