How to disable GNU C extensions?

Question

As you can see in the following code, I've introduced a nested function within main():

#include <stdio.h>

int main(){
 int a=5;
 printf("%d\n",a);
 {
  int a=10;
  printf("%d\n",a);
 }
 printf("%d\n",a);

 //Nested function
 int main(int a){
 if(a>0)printf("%d\n",a--);
 return 0;
 }

 main(7);
 return 0;
}

As far as I could understand I used the -std=c99 flag in gcc to "disable" the unnecessary extensions, but I did not get any error.

gcc temp3.c -std=c99 -o temp3.out

Where have I made the mistake?

Answer 1

• Add -pedantic and -Werror to the command line.

Using GCC 6.1.0 on Mac OS X 10.11.6, with your original code in a file ped73.c and my default compilation options, I get:

$ gcc -O3 -g -std=c11 -Wall -Wextra -Werror -Wmissing-prototypes -Wstrict-prototypes \
>     -Wold-style-definition ped73.c -o ped73 
ped73.c:3:5: error: function declaration isn’t a prototype [-Werror=strict-prototypes]
 int main(){
     ^~~~
ped73.c: In function ‘main’:
ped73.c:3:5: error: old-style function definition [-Werror=old-style-definition]
ped73.c:13:6: error: ‘main’ takes only zero or two arguments [-Werror=main]
  int main(int a){
      ^~~~
ped73.c:13:6: error: ‘main’ is normally a non-static function [-Werror=main]
$

Renaming the nested function to nested and using int main(void), I get:

$ gcc -O3 -g-std=c11 -Wall -Wextra -Werror -Wmissing-prototypes -Wstrict-prototypes 
>     -Wold-style-definition -o ped73
$

Using the extra option -pedantic I get:

$ gcc -O3 -g -std=c11 -Wall -Wextra -Werror -Wmissing-prototypes -Wstrict-prototypes \
>     -Wold-style-definition -pedantic ped73.c -o ped73 
ped73.c: In function ‘main’:
ped73.c:13:2: error: ISO C forbids nested functions [-Werror=pedantic]
  int nested(int a){
  ^~~
cc1: all warnings being treated as errors
$

---

Then what's the point of -std=c99?

The -std=c99 flag disables the GNU extensions that GCC thinks should be disabled — such as POSIX versions, etc. See C Dialect Options for the meaning of -std=; see Warning Options for the meaning of -pedantic.

-Wpedantic
-pedantic

Issue all the warnings demanded by strict ISO C and ISO C++; reject all programs that use forbidden extensions, and some other programs that do not follow ISO C and ISO C++. For ISO C, follows the version of the ISO C standard specified by any -std option used.

Valid ISO C and ISO C++ programs should compile properly with or without this option (though a rare few require -ansi or a -std option specifying the required version of ISO C). However, without this option, certain GNU extensions and traditional C and C++ features are supported as well. With this option, they are rejected.

-Wpedantic does not cause warning messages for use of the alternate keywords whose names begin and end with __. Pedantic warnings are also disabled in the expression that follows __extension__. However, only system header files should use these escape routes; application programs should avoid them. See Alternate Keywords.

Some users try to use -Wpedantic to check programs for strict ISO C conformance. They soon find that it does not do quite what they want: it finds some non-ISO practices, but not all—only those for which ISO C requires a diagnostic, and some others for which diagnostics have been added.

A feature to report any failure to conform to ISO C might be useful in some instances, but would require considerable additional work and would be quite different from -Wpedantic. We don't have plans to support such a feature in the near future.

Where the standard specified with -std represents a GNU extended dialect of C, such as ‘gnu90’ or ‘gnu99’, there is a corresponding base standard, the version of ISO C on which the GNU extended dialect is based. Warnings from -Wpedantic are given where they are required by the base standard. (It does not make sense for such warnings to be given only for features not in the specified GNU C dialect, since by definition the GNU dialects of C include all features the compiler supports with the given option, and there would be nothing to warn about.)

And there's also a different option that gives pedantic errors:

-pedantic-errors

Give an error whenever the base standard (see -Wpedantic) requires a diagnostic, in some cases where there is undefined behavior at compile-time and in some other cases that do not prevent compilation of programs that are valid according to the standard. This is not equivalent to -Werror=pedantic, since there are errors enabled by this option and not enabled by the latter and vice versa.

---

There are multiple questions about which GCC compiler options to use, including:

• Recommended gcc warning options for C
• What is the purpos of using -pedantic in GCC/G++ compiler
• warning: implicit declaration of function when compiling C source

and no doubt there are many others that could be added to that list. Basically, the default options I use ensure that functions are declared before they are used (or, are defined as a static function before they are used), and that the function declarations have full prototypes — no empty parentheses () — and use the -Wall and -Wextra to spot a number of other routine problems, including mismatches between format strings and arguments to the printf() and scanf() families of functions.