snprintf variable list doesnt convert enum to char*

Question

I have been doing a task to change the current logging mechanism in my CPP codebase.

The current one is something like a printf in which we can write

MYLOGGING(("Example Log :%s, Example Num: %d", String, Number));

Now as part of modifying it to the new logging mechanism, I want to get this complete output to a string using the snprintf(). For that I used a variable list for this.

The code was like

#define MYLOGGING(log_string,...) do { \
char buff[1024]; \
memset(buff, 0, sizeof(buff)); \
snprintf(buff, sizeof(buff), log_string, ##__VA_ARGS__); \
MYNEW_LOG(NewlogParams, buff); \
} while(0)

Now, I am facing an issue. The existing logging had many instances in which some enum values are printed using simple %d. In such cases, with this new code, it is throwing the following error.

error: cannot convert ‘enumType’ to ‘const char*’ for argument ‘3’ to ‘int snprintf(char*, size_t, const char*, ...)’

One obvious option to get rid of this is to give integer type casting for enum prints.

As that could be a tedious process, i am curious to know whether I can fix this in some other way?

Please share your views.

Thanks in advance.

glampert · Accepted Answer · 2014-09-15 15:46:34Z

2

By appending the ## operator to the __VA_ARGS__, you are turning the format parameters into a const char *:

snprintf(buff, sizeof(buff), log_string, ##__VA_ARGS__);

Change that to:

snprintf(buff, sizeof(buff), log_string, __VA_ARGS__);

and the conversion should work.

Actually, after looking at your question more carefully, I've noticed another problem. The ## operator is indeed unusual and probably incorrect, however, the main cause of your problem seems to be the extra parens in the call to MYLOGGING

MYLOGGING(("Example Log :%s, Example Num: %d", String, Number));

If you compile a call to that macro in GCC with the -E flag to output the preprocessed source, you'll get this line expanded from the macro:

do { char buff[1024]; memset(buff, 0, sizeof(buff)); snprintf(buff, sizeof(buff), ("Example Log :%s, Example Num: %d", String, Number)); ....

You can see that the () are still there, messing up the call to snprintf. So remove both ## in the macro and the extra parens in the call site.

edited Sep 15, 2014 at 15:46

answered Sep 15, 2014 at 15:11

glampert

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2 Comments

Dhanesh Over a year ago

Thank you glampert. Things are fine when i changed it to snprintf(buff, sizeof(buff), "log_string",## VA_ARGS); Now i have a confusion what is the diff between my old code and the one with "" for log_string. Could you give an answer?

glampert Over a year ago

Read about the token concatenation operator: gcc.gnu.org/onlinedocs/cpp/Concatenation.html

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