Question

I’m trying to compile a C codebase as C++, tweaking some of the includes. It uses strchr() on unsigned char pointers, e.g.:

#include <string.h>
#include <stdio.h>

// Cannot modify this file with any non-C-isms, but let's say an include can
// be changed (although all things being equal I'd rather not)
#include <SomethingICanChange.h>

int main(int argc, char* argv[]) {
    unsigned char b = 'B';
    unsigned char const * str = "ABC";
    unsigned char const * pos = strchr(str, b);
    if (pos) {
        printf("position is %d\n", pos - str);
    }
    return 0;
}

That causes an error in C++ (for reasons explained elsewhere)…even with -fpermissive.

test.c: In function ‘int main(int, char**)’:
test.c:6:33: warning: invalid conversion from ‘const char*’ to ‘const unsigned char*’ [-fpermissive]
test.c:7:46: error: call of overloaded ‘strchr(const unsigned char*&, unsigned char&)’ is ambiguous
test.c:7:46: note: candidates are:
In file included from test.c:1:0:
/usr/include/string.h:215:14: note: char* strchr(char*, int) <near match>
/usr/include/string.h:215:14: note:   no known conversion for argument 1 from ‘const unsigned char*’ to ‘char*’
/usr/include/string.h:217:22: note: const char* strchr(const char*, int) <near match>
/usr/include/string.h:217:22: note:   no known conversion for argument 1 from ‘const unsigned char*’ to ‘const char*’

Usually when faced with this kind of thing I go “strchr, yuck, just get rid of it entirely”. But I don’t want to modify these files myself, and this is C code that is being kept very portable to rather old platforms. They wouldn’t be happy putting casts at callsites to appease C++…though if a “struchr” existed and was standard they’d probably use it.

I can also hack around it one way or another, like in SomethingICanChange.h I could add:

unsigned char const * strchr(unsigned char const * s, int c) {
    return (unsigned char const *)strchr((char const *)s, c);
}

That works with -fpermissive. But I wouldn’t be able to check that overload into the C codebase (well, without an #ifdef). Just wondering if anyone has a better idea, I will pitch adding struchr if that’s what people in this situation wind up doing (or whatever name is common if there’s a common one).

Answer 1

Initial note: I am aware that this is also quite an ugly thing to do, but it might help you with a (hopefully) better idea!

How about using a macro:

#define strchr(s, c) ((unsigned char const *)strchr((char const *)s, c))

You could include it as a compile flag (-D) from you Makefile/build script. I suppose you have a custom Makefile/build script for your C++ project, so you do not need to add this macro to the C code-base.

To make it slightly better, you can use an additional file, that you include from your Makefile/build script, which adds these macros in a (more) structured way to CPPFLAGS