Is there a portable way to print a message from the C preprocessor?

printing c-preprocessor

92,865

Solution 1

The warning directive is probably the closest you'll get, but it's not entirely platform-independent:

#warning "C Preprocessor got here!"

AFAIK this works on most compilers except MSVC, on which you'll have to use a pragma directive:

#pragma message ( "C Preprocessor got here!" )

Solution 2

The following are supported by MSVC, and GCC.

#pragma message("stuff")
#pragma message "stuff"

Clang has begun adding support recently, see here for more.

Solution 3

You might want to try: #pragma message("Hello World!")

Solution 4

Most C compilers will recognize a #warning directive, so

 #warning "Got here"

There's also the standard '#error' directive,

 #error "Got here"

While all compilers support that, it'll also stop the compilation/preprocessing.

Solution 5

#pragma message("foo")

works great. Also wouldn't stop compilation even if you use -Werror

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92,865

Author by

Andrew Wagner

I'm currently a Post-Doc researcher at the University of Leuven, Belgium working on launching systems for flying wind turbines.

Updated on January 10, 2020

Comments

Andrew Wagner over 4 years
I would like to be able to do something like
```
#print "C Preprocessor got here!"
```
for debugging purposes. What's the best / most portable way to do this?
Bob Kaufman almost 14 years

This won't print at compile-time, which is what I'm thinking OP is looking for.
Alexander Rafferty almost 14 years

I assumed he meant printing at run-time.
Bryan almost 14 years

Which begs the question, can you put a directive based on a compilation flag to swap "pragma message" and "warning" somehow? For example, something like: #ifdef _LINUX #define #preprocmsg "#warning" else #define #preprocmsg "#pragma message"... I'll have to try that but instinct tells me the answer is no.
Andrew Wagner over 13 years

I was asking about compile-time. Thanks!
Matt Joiner over 13 years

@Bryan: Yes. #define WARNING(msg) _Pragma("message " #msg)
maxschlepzig about 11 years

Just for the record, Solaris Studio 12.3 (Sun C 5.12) does not support this pragma.
Danny S over 9 years

#pragma message () is not supported by older versions of gcc (such as gcc 4.1.2, the default version on RHEL5). I have yet to find an appropriate equivalent for these older versions - #warning is not going to be great, as warnings are treated as errors for us generally, and we'd really like the message to be informational, rather than stop the compilation.
Renan Gemignani over 7 years

Issuing a warning is very inconvenient when your project compiles with -Wall by default. #pragma message doesn't have that problem.
save_jeff over 4 years

Works with Arduino 1.8 using Visual Studio vMicro. Thanks!
ead over 3 years

Supported by clang at least since 3.0 and by gcc since 5.1 (and in a slightly different form since 4.4.7). Even if clang says "1 warning generated", it doesn't fail build when built with -Werror: gcc.godbolt.org/z/xoK6b8
Aaron Franke over 3 years

Is there a way to make this work with numbers or other values? It says it expects a string, but I would like to print out what a numeric #define is calculated to be.
jxramos almost 3 years

is there someway to substitute in macro values, is that possible? Maybe something like #warning "DEBUG="DEBUG
Tzalumen over 2 years

@AaronFranke During the preprocessor step, all defines should be ASCII.