c - Pass unsigned char pointer to atoi without cast

Question

On some embedded device, I have passed an unsigned char pointer to atoi without a cast.

unsigned char c[10]="12";
atoi(c);

Question: is it well defined?

I saw somewhere it is ok for string functions, but was not sure about atoi.

Edit: Btw. Some concerns have been expressed on one of the answer below that it might not be OK even for string functions such as strcpy - but if I got right (?) the author meant also it can be that in practice this can be OK.

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Also that I am here, is it ok to do following assignment to unsigned char pointer ok too? Because I used some tool which is complaining about "Type mismatch (assignment) (ptrs to signed/unsigned)"

unsigned char *ptr = strtok(unscharbuff,"-");
    // is assignment also ok to unsigned char? 

Answer 1

No, it's not well defined. It's a constraint violation, requiring a compile-time diagnostic. In practice it's very very likely to work as you expect, but it's not guaranteed to do so, and IMHO it's poor style.

The atoi function is declared in <stdlib.h> as:

int atoi(const char *nptr);

You're passing an unsigned char* argument to a function that expects a char* argument. The two types are not compatible, and there is no implicit conversion from one to the other. A conforming compiler may issue a warning (that counts as a diagnostic) and then proceed to generate an executable, but the behavior of that executable is undefined.

As of C99, a call to a function with no visible declaration is a constraint violation, so you can't get away with it by omitting the #include <stdlib.h>.

C does still permit calls to functions with a visible declaration where the declaration is not a prototype (i.e., doesn't define the number of type(s) of the parameters). So, rather than the usual #include <stdlib.h>, you could add your own declaration:

int atoi();

which would permit calling it with an unsigned char* argument.

This will almost certainly "work", and it might be possible to construct an argument from the standard that its behavior is well defined. The char and unsigned char values of '1' and '2' are guaranteed to have the same representation

But it's far easier to add the cast than to prove that it's not necessary -- or, better yet, to define c as an array of char rather than as an array of unsigned char, since it's intended to hold a string.

unsigned char *ptr = strtok(unscharbuff,"-");

This is also a constraint violation. There is no implicit conversion from unsigned char* to char* for the first argument in the strtok call, and there is no implicit conversion from char* to unsigned char* for the initialization of ptr.