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

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 
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 
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.