How to disable GCC warnings for a few lines of code

Question

In Visual C++, it's possible to use #pragma warning (disable: ...). Also I found that in GCC you can override per file compiler flags. How can I do this for "next line", or with push/pop semantics around areas of code using GCC?

Answer 1

It appears this can be done. I'm unable to determine the version of GCC that it was added, but it was sometime before June 2010.

Here's an example:

#pragma GCC diagnostic error "-Wuninitialized"
    foo(a);         /* error is given for this one */

#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wuninitialized"
    foo(b);         /* no diagnostic for this one */
#pragma GCC diagnostic pop

    foo(c);         /* error is given for this one */
#pragma GCC diagnostic pop 

    foo(d);         /* depends on command line options */

Answer 2

To net everything out, this is an example of temporarily disabling a warning:

#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wunused-result"
    write(foo, bar, baz);
#pragma GCC diagnostic pop

You can check the GCC documentation on diagnostic pragmas for more details.

Answer 3

TL;DR: If it works, avoid, or use specifiers like _Noreturn, [[nodiscard]], __attribute__, otherwise _Pragma.

This is a short version of my blog article Suppressing Warnings in GCC and Clang.

Consider the following Makefile,

CPPFLAGS:=-std=c11 -W -Wall -pedantic -Werror

.PHONY: all
all: puts

for building the following puts.c source code:

#include <stdio.h>

int main(int argc, const char *argv[])
{
    while (*++argv)
        puts(*argv);
    return 0;
}

It will not compile because argc is unused, and the settings are hardcore (-W -Wall -pedantic -Werror).

There are five things you could do:

• Improve the source code, if possible
• Use an attribute, like [[maybe_unused]]
• Use a declaration specifier, like __attribute__
• Use _Pragma
• Use #pragma
• Use a command line option.

Improving the source

The first attempt should be checking if the source code can be improved to get rid of the warning. In this case we don't want to change the algorithm just because of that, as argc is redundant with !*argv (NULL after last element).

Using an attribute, like [[maybe_unused]]

#include <stdio.h>

int main([[maybe_unused]] int argc, const char *argv[])
{
    while (*++argv) puts(*argv);
    return 0;
}

If you're lucky, the standard provides an attribute for your situation, like [[maybe_unused]]. Attributes are a new feature of C2x. So far, C2x defines four attributes, [[deprecated]], [[fallthrough]], [[maybe_unused]], and [[nodiscard]].

Using a declaration specifier, like __attribute__

#include <stdio.h>

int main(__attribute__((unused)) int argc, const char *argv[])
{
    while (*++argv) puts(*argv);
    return 0;
}

If you're lucky, the standard provides a specifier for your situation, like _Noreturn.

__attribute__ is proprietary GCC extension (supported by Clang and some other compilers like armcc as well) and will not be understood by many other compilers. Put __attribute__((unused)) inside a macro if you want portable code.

_Pragma operator

_Pragma can be used as an alternative to #pragma.

#include <stdio.h>

_Pragma("GCC diagnostic push")
_Pragma("GCC diagnostic ignored \"-Wunused-parameter\"")

int main(int argc, const char *argv[])
{
    while (*++argv)
        puts(*argv);
    return 0;
}
_Pragma("GCC diagnostic pop")

The main advantage of the _Pragma operator is that you could put it inside macros, which is not possible with the #pragma directive.

Downside: It's almost a tactical nuke, as it works line-based instead of declaration-based.

The _Pragma operator was introduced in C99.

#pragma directive.

We could change the source code to suppress the warning for a region of code, typically an entire function:

#include <stdio.h>

#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wunused-parameter"
int main(int argc, const char *argv[])
{
    while (*++argc) puts(*argv);
    return 0;
}
#pragma GCC diagnostic pop

Downside: It's almost a tactical nuke, as it works line-based instead of declaration-based.

Note that a similar syntax exists in Clang.

Suppressing the warning on the command line for a single file

We could add the following line to the Makefile to suppress the warning specifically for puts:

CPPFLAGS:=-std=c11 -W -Wall -pedantic -Werror

.PHONY: all
all: puts

puts.o: CPPFLAGS+=-Wno-unused-parameter

This is probably not want you want in your particular case, but it may help other readers who are in similar situations.

Answer 4

I know the question is about GCC, but for people looking for how to do this in other and/or multiple compilers…

TL;DR

You might want to take a look at Hedley, which is a public-domain single C/C++ header I wrote which does a lot of this stuff for you. I'll put a quick section about how to use Hedley for all this at the end of this post.

Disabling the warning

#pragma warning (disable: …) has equivalents in most compilers:

• MSVC: #pragma warning(disable:4996)
• GCC: #pragma GCC diagnostic ignored "-W…" where the ellipsis is the name of the warning; e.g., #pragma GCC diagnostic ignored "-Wdeprecated-declarations.
• Clang: #pragma clang diagnostic ignored "-W…". The syntax is basically the same as GCC's, and many of the warning names are the same (though many aren't).
• Intel C++ Compiler (ICC): Use the MSVC syntax, but keep in mind that warning numbers are totally different. Example: #pragma warning(disable:1478 1786).
• PGI/Nvidia: There is a diag_suppress pragma: #pragma diag_suppress 1215,1444. Note that all warning numbers increased by one in 20.7 (the first Nvidia HPC release).
• TI (CCS): There is a diag_suppress pragma with the same syntax (but different warning numbers!) as PGI: pragma diag_suppress 1291,1718
• Oracle Developer Studio (ODS) (suncc): there is an error_messages pragma. Annoyingly, the warnings are different for the C and C++ compilers. Both of these disable basically the same warnings:
  • C: #pragma error_messages(off,E_DEPRECATED_ATT,E_DEPRECATED_ATT_MESS)
  • C++: #pragma error_messages(off,symdeprecated,symdeprecated2)
• IAR: also uses diag_suppress like PGI and TI, but the syntax is different. Some of the warning numbers are the same, but I others have diverged: #pragma diag_suppress=Pe1444,Pe1215
• Pelles C: similar to MSVC, though again the numbers are different #pragma warn(disable:2241)

For most compilers it is often a good idea to check the compiler version before trying to disable it, otherwise you'll just end up triggering another warning. For example, GCC 7 added support for the -Wimplicit-fallthrough warning, so if you care about GCC before 7 you should do something like

#if defined(__GNUC__) && (__GNUC__ >= 7)
#  pragma GCC diagnostic ignored "-Wimplicit-fallthrough"
#endif

For Clang and compilers based on Clang, such as newer versions of XL C/C++ and armclang, you can check to see if the compiler knows about a particular warning using the __has_warning() macro.

#if __has_warning("-Wimplicit-fallthrough")
#  pragma clang diagnostic ignored "-Wimplicit-fallthrough"
#endif

Of course you also have to check to see if the __has_warning() macro exists:

#if defined(__has_warning)
#  if __has_warning("-Wimplicit-fallthrough")
#    pragma clang diagnostic ignored "-Wimplicit-fallthrough"
#  endif
#endif

You may be tempted to do something like

#if !defined(__has_warning)
#  define __has_warning(warning)
#endif

So you can use __has_warning a bit more easily. Clang even suggests something similar for the __has_builtin() macro in their manual. Do not do this. Other code may check for __has_warning and fall back on checking compiler versions if it doesn't exist, and if you define __has_warning you'll break their code. The right way to do this is to create a macro in your namespace. For example:

#if defined(__has_warning)
#  define MY_HAS_WARNING(warning) __has_warning(warning)
#else
#  define MY_HAS_WARNING(warning) (0)
#endif

Then you can do stuff like

#if MY_HAS_WARNING(warning)
#  pragma clang diagnostic ignored "-Wimplicit-fallthrough"
#elif defined(__GNUC__) && (__GNUC__ >= 7)
#  pragma GCC diagnostic ignored "-Wimplicit-fallthrough"
#endif

Pushing and popping

Many compilers also support a way to push and pop warnings onto a stack. For example, this will disable a warning on GCC for one line of code, then return it to its previous state:

#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wdeprecated"
call_deprecated_function();
#pragma GCC diagnostic pop

Of course there isn't a lot of agreement across compilers about the syntax:

• GCC 4.6+: #pragma GCC diagnostic push / #pragma GCC diagnostic pop
• Clang: #pragma clang diagnostic push / #pragma clang diagnostic pop
• Intel 13+ (and probably earlier): #pragma warning(push) / #pragma warning(pop)
• MSVC 15+ (Visual Studio 9.0 / 2008): #pragma warning(push) / #pragma warning(pop)
• ARM 5.6+: #pragma push / #pragma pop
• TI 8.1+: #pragma diag_push / #pragma diag_pop
• Pelles C 2.90+ (and probably earlier): #pragma warning(push) / #pragma warning(pop)

If memory serves, for some very old versions of GCC (like 3.x, IIRC) the push/pop pragmas had to be outside of the function.

Hiding the gory details

For most compilers it's possible to hide the logic behind macros using _Pragma, which was introduced in C99. Even in non-C99 mode, most compilers support _Pragma; the big exception is MSVC, which has its own __pragma keyword with a different syntax. The standard _Pragma takes a string, Microsoft's version doesn't:

#if defined(_MSC_VER)
#  define PRAGMA_FOO __pragma(foo)
#else
#  define PRAGMA_FOO _Pragma("foo")
#endif
PRAGMA_FOO

Is roughly equivalent, once preprocessed, to

#pragma foo

This let’s us create macros so we can write code like

MY_DIAGNOSTIC_PUSH
MY_DIAGNOSTIC_DISABLE_DEPRECATED
call_deprecated_function();
MY_DIAGNOSTIC_POP

And hide away all the ugly version checks in the macro definitions.

The easy way: Hedley

Now that you understand the mechanics of how to do stuff like this portably while keeping your code clean, you understand what one of my projects, Hedley does. Instead of digging through tons of documentation and/or installing as many versions of as many compilers as you can to test with, you can just include Hedley (it is a single public domain C/C++ header) and be done with it. For example:

#include "hedley.h"

HEDLEY_DIAGNOSTIC_PUSH
HEDLEY_DIAGNOSTIC_DISABLE_DEPRECATED
call_deprecated();
HEDLEY_DIAGNOSTIC_POP

Will disable the warning about calling a deprecated function on GCC, Clang, ICC, PGI, MSVC, TI, IAR, ODS, Pelles C, and possibly others (I probably won't bother updating this answer as I update Hedley). And, on compilers which aren't known to work, the macros will be preprocessed away to nothing, so your code will continue to work with any compiler. Of course HEDLEY_DIAGNOSTIC_DISABLE_DEPRECATED isn't the only warning Hedley knows about, nor is disabling warnings all Hedley can do, but hopefully you get the idea.

Answer 5

Use:

#define DIAG_STR(s) #s
#define DIAG_JOINSTR(x,y) DIAG_STR(x ## y)
#ifdef _MSC_VER
#define DIAG_DO_PRAGMA(x) __pragma (#x)
#define DIAG_PRAGMA(compiler,x) DIAG_DO_PRAGMA(warning(x))
#else
#define DIAG_DO_PRAGMA(x) _Pragma (#x)
#define DIAG_PRAGMA(compiler,x) DIAG_DO_PRAGMA(compiler diagnostic x)
#endif
#if defined(__clang__)
# define DISABLE_WARNING(gcc_unused,clang_option,msvc_unused) DIAG_PRAGMA(clang,push) DIAG_PRAGMA(clang,ignored DIAG_JOINSTR(-W,clang_option))
# define ENABLE_WARNING(gcc_unused,clang_option,msvc_unused) DIAG_PRAGMA(clang,pop)
#elif defined(_MSC_VER)
# define DISABLE_WARNING(gcc_unused,clang_unused,msvc_errorcode) DIAG_PRAGMA(msvc,push) DIAG_DO_PRAGMA(warning(disable:##msvc_errorcode))
# define ENABLE_WARNING(gcc_unused,clang_unused,msvc_errorcode) DIAG_PRAGMA(msvc,pop)
#elif defined(__GNUC__)
#if ((__GNUC__ * 100) + __GNUC_MINOR__) >= 406
# define DISABLE_WARNING(gcc_option,clang_unused,msvc_unused) DIAG_PRAGMA(GCC,push) DIAG_PRAGMA(GCC,ignored DIAG_JOINSTR(-W,gcc_option))
# define ENABLE_WARNING(gcc_option,clang_unused,msvc_unused) DIAG_PRAGMA(GCC,pop)
#else
# define DISABLE_WARNING(gcc_option,clang_unused,msvc_unused) DIAG_PRAGMA(GCC,ignored DIAG_JOINSTR(-W,gcc_option))
# define ENABLE_WARNING(gcc_option,clang_option,msvc_unused) DIAG_PRAGMA(GCC,warning DIAG_JOINSTR(-W,gcc_option))
#endif
#endif

This should do the trick for GCC, Clang and MSVC.

It can be called with e.g.:

DISABLE_WARNING(unused-variable,unused-variable,42)
[.... some code with warnings in here ....]
ENABLE_WARNING(unused-variable,unused-variable,42)

See 7 Pragmas, Controlling Diagnostics via Pragmas and Pragma directives and the __pragma and _Pragma keywords for more details.

You need at least version 4.02 to use these kind of pragmas for GCC, and I am not sure about MSVC and Clang about the versions.

It looks like the push pop pragma handling for GCC is a little bit broken. If you enable the warning again, you still get the warning for the block that was inside the DISABLE_WARNING/ENABLE_WARNING block. For some versions of GCC it works, and for some it doesn't.

Answer 6

#pragma GCC diagnostic ignored "-Wformat"

Replace "-Wformat" with the name of your warning flag.

AFAIK there is no way to use push/pop semantics for this option.

Answer 7

I had same issue with external libraries like ROS headers. I like to use following options in CMakeLists.txt for stricter compilation:

set(CMAKE_CXX_FLAGS "-std=c++0x -Wall -Wextra -Wstrict-aliasing -pedantic -Werror -Wunreachable-code ${CMAKE_CXX_FLAGS}")

However, doing this causes all kind of pedantic errors in externally included libraries as well. The solution is to disable all pedantic warnings before you include external libraries and re-enable them like this:

// Save compiler switches
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wpedantic"

// Bad headers with a problem goes here
#include <ros/ros.h>
#include <sensor_msgs/LaserScan.h>

// Restore compiler switches
#pragma GCC diagnostic pop

Answer 8

Here is a way to do this in IAR. Try this:

#pragma diag_suppress=Pe177
void foo1(void)
{
   /* The following line of code would normally provoke diagnostic 
      message #177-D: variable "x" was declared but never referenced.
      Instead, we have suppressed this warning throughout the entire 
      scope of foo1(). 
   */
   int x;
}
#pragma diag_default=Pe177

See official documentation for reference.

Answer 9

Rather than silencing the warnings, GCC style is usually to use either standard C constructs or the __attribute__ extension to tell the compiler more about your intention.

For instance, the warning about assignment used as a condition is suppressed by putting the assignment in parentheses, i.e. if ((p=malloc(cnt))) instead of if (p=malloc(cnt)).

Warnings about unused function arguments can be suppressed by some odd __attribute__ I can never remember, or by self-assignment, etc.

But generally I prefer just globally disabling any warning option that generates warnings for things that will occur in correct code.