Reading through null pointer in #define with condition

Question

I have definition of macro printerr

#define CLR_R "\x1b[1;31m" 
#define CLR_N "\x1b[0m"
#define printerr(caller,msg) (caller==NULL)?printf(CLR_R "%s" CLR_N "\n",msg) \
   : printf(CLR_R "%s:" CLR_N " %s\n",caller,msg)

And get warning

warning: reading through null pointer (argument 2), when calling printerr(NULL,"error");.

How to suppress this warning?

#include <stdio.h>
#define printerr(caller,msg) (caller==NULL)?printf("%s\n",msg) : \
    printf("%s: %s\n",caller,msg)
void main() {
    printerr("Error","error occurred"); //will be ok
    printerr(NULL,"error"); //Warning: is caller even checked here?
}

Just char* constants #define CLR_R "\x1b[1;31m" #define CLR_N "\x1b[0m" — pfoof
– pfoof, Commented Mar 29, 2017 at 17:18
@SouravGhosh Why does it matter? I'm guessing they're escape sequences for switching to red and back to no color. — Barmar
– Barmar, Commented Mar 29, 2017 at 17:18
@Barmar ummm... maybe because it's better to have minimal reproducible example? — Sourav Ghosh
– Sourav Ghosh, Commented Mar 29, 2017 at 17:19
1) Read How to Ask and provide a minimal reproducible example. 2) Don't use a macro where a function will do as well. — too honest for this site
– too honest for this site, Commented Mar 29, 2017 at 17:19

Petr Skocik · Accepted Answer · 2017-03-29 17:39:52Z

1

This works (gcc, clang):

#include <stdio.h>
int main()
{
#define CLR_R "\x1b[1;31m" 
#define CLR_N "\x1b[0m"

#define printerr(caller,msg)  \
    (caller==NULL)? printf(CLR_R "%s" CLR_N "\n",msg) : \
    printf(CLR_R "%s:" CLR_N " %s\n", ((caller)?(caller):""),msg)


    printerr(NULL,"msg");
    printerr("caller","msg");
}

I'm using ((caller)?(caller):"")to suppress the warning. It will never evaluate to "".

You could also use an inline function to the same effect:

#define CLR_R "\x1b[1;31m" 
#define CLR_N "\x1b[0m"
static inline int inl_printerr(char const *caller, char const *msg)
{
    if (caller)
        return printf(CLR_R "%s:" CLR_N " %s\n", caller,msg);
    else
        return printf(CLR_R "%s" CLR_N "\n",msg);
}

(Recommended as it avoids the typesafety and double-evaluation issues that macros have).

edited Mar 29, 2017 at 17:39

answered Mar 29, 2017 at 17:26

Petr Skocik

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2 Comments

Petr Skocik Over a year ago

I'm curious as to why it's been also marked "not useful", given that it is demonstrably useful to the OP. :(

Eugene Sh. Over a year ago

Because the DV button is rather used for "I don't like it for a generic reason or having a bad mood".

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