const char ** argument

[Alberto =?iso-8859-1?Q?Gim=E9nez?=]

Hello. I have a doubt about argument conversion when passing arguments
to a function. I have a function like:

struct packet *pkt_interest(const char **usrs);

struct packet is defined elsewhere (it has no interest to show it here).
With that "const" modifier I want to state that usrs pointer will not
be changed inside the function.

Then, from the main program, I have this:

char *users[] = {
"alberto",
"juan",
"pepito",
NULL
};

struct packet *pkt = pkt_interest(users);

compiler throws the warning:
test_proto.c:23: warning: passing arg 1 of `pkt_interest' from
incompatible pointer type.

And I have to modify the users declaration to be "const char *users" or
make an explicit cast in the function call.

I though the const modifier in the funcion declaration was just a "hey,
i won't change your pointer, trust me" and could use a plain char **, in
a similar way as we use strcpy function (the second argument is a const
char *, but we can use a non-const char * with no problems).

¿Where's the difference with the const in strcpy and the const in my
pkt_interest?

TIA

 

[Michael Mair]

Hello. I have a doubt about argument conversion when passing arguments
to a function. I have a function like:

struct packet *pkt_interest(const char **usrs);

struct packet is defined elsewhere (it has no interest to show it here).
With that "const" modifier I want to state that usrs pointer will not
be changed inside the function.

Then, from the main program, I have this:

char *users[] = {
"alberto",
"juan",
"pepito",
NULL
};

struct packet *pkt = pkt_interest(users);

compiler throws the warning:
test_proto.c:23: warning: passing arg 1 of `pkt_interest' from
incompatible pointer type.

And I have to modify the users declaration to be "const char *users" or
make an explicit cast in the function call.

I though the const modifier in the funcion declaration was just a "hey,
i won't change your pointer, trust me" and could use a plain char **, in
a similar way as we use strcpy function (the second argument is a const
char *, but we can use a non-const char * with no problems).

¿Where's the difference with the const in strcpy and the const in my
pkt_interest?

In the second level of indirection.

Read the FAQ 11.10 on this topic
"Why can't I pass a char ** to a function which expects a
const char **?"

Search the usenet archives for comp.lang.c if this does not suffice.

Cheers
Michael

 

[Steve Checkoway]

[...]
Then, from the main program, I have this:

char *users[] = {
"alberto",
"juan",
"pepito",
NULL
};

The real issue is here. Try this:
char *users[] = {
"foo",
"bar",
NULL
};

users[1][2] = 'z';

This will crash since users[1] points to a read-only string (if the
compiler didn't warn you about that, it's a compiler bug). This is
really the difference between:

char *s1 = "foo"; /* a mistake */
and
char s2[] = "foo"; /* correct */

Given that you cannot modify "alberto", "juan", or "pepito", you should
have:

const char *users[] = { /* ... */ }

- Steve

 

[Michael Mair]

[...]
Then, from the main program, I have this:

char *users[] = {
"alberto",
"juan",
"pepito",
NULL
};

The real issue is here. Try this:
char *users[] = {
"foo",
"bar",
NULL
};

users[1][2] = 'z';

This will crash since users[1] points to a read-only string (if the
compiler didn't warn you about that, it's a compiler bug).

Nope. String literals should not be modified as they could reside in
some sort of read-only storage but they are not guaranteed to be
non-modifiable.
In C, "a string" is not array 9 of const char but array 9 of char.

This is
really the difference between:

char *s1 = "foo"; /* a mistake */

^^^^^^^^^^
This is not a mistake -- however, it is a Bad Idea.

and
char s2[] = "foo"; /* correct */

Given that you cannot modify "alberto", "juan", or "pepito", you should
have:

const char *users[] = { /* ... */ }

Yep. Depending on what you want to achieve, you could even declare
users as
const char *const users[]....


Cheers
Michael