predefined macros

[cooper1200]

i have reached the chapter in the book im working though that discusses pre processing and i tried to play around with a few of the predefined macros such as _FUNC_ or _STDC_VERSION_. The first one said it was undeclared but asked if i men _unix when i tried that it said it was undeclared and asked me if i meant _unix again. the second one i called with a printf statment but the compiler just said it was undeclared.

do i need a header file for these or am i doing something wrong.
many thanks
coop

[Click_here]

Double underscores

 __FILE__

[laserlight]

i have reached the chapter in the book im working though that discusses pre processing and i tried to play around with a few of the predefined macros such as _FUNC_ or _STDC_VERSION_. The first one said it was undeclared but asked if i men _unix when i tried that it said it was undeclared and asked me if i meant _unix again. the second one i called with a printf statment but the compiler just said it was undeclared.

Neither of these predefined identifiers require a header inclusion. The first one is actually __func__ and the second one is actually __STDC_VERSION__ (note the consecutive underscores).

[cooper1200]

sorry guys im the double underscore wasnt clear in the book

[flp1969]

sorry guys im the double underscore wasnt clear in the book

A quick tip if you are using GCC. The option -dM will list all #defines, including those predefined; -E option will stop GCC after preprocessing, so, you can do:

$ gcc -dM -E - < /dev/null | less

To see all predefined macros GCC has available for you...

[cooper1200]

A quick tip if you are using GCC. The option -dM will list all #defines, including those predefined; -E option will stop GCC after preprocessing, so, you can do:

$ gcc -dM -E - < /dev/null | less

To see all predefined macros GCC has available for you...

i just tried that got an error message saying no file input and a load of weird characters i guess i dont have gcc installed directly as i use codeblocks ide although i have the man pages for gcc and codeblocks uses it as its compiler

[rstanley]

A quick tip if you are using GCC. The option -dM will list all #defines, including those predefined; -E option will stop GCC after preprocessing, so, you can do:

$ gcc -dM -E - < /dev/null | less

To see all predefined macros GCC has available for you...

That command does not show, __FILE__, __LINE__, and __func__.

And why __func__ is lowercase confuses me! ;^)

__FILE__ and __LINE__ are built into the preprocessor, and __func__ seems to be built into the compiler itself.

int main(void)
{
   char *file = __FILE__;
   int   line = __LINE__;
   const char *func = __func__;

   return 0;
}

/* compile with:
 gcc -E program-name.c
To see the output of the preprocessor
*/

[flp1969]

i just tried that got an error message saying no file input and a load of weird characters i guess i dont have gcc installed directly as i use codeblocks ide although i have the man pages for gcc and codeblocks uses it as its compiler

If you are using Windows and got GCC installed:

gcc -dM -E - < NUL | more

[cooper1200]

If you are using Windows and got GCC installed:

gcc -dM -E - < NUL | more

no i have mint although it is on a virtual machine

[flp1969]

That command does not show, __FILE__, __LINE__, and __func__.

These are created dynamically when a code is compiled... We are not compiling, but just preprocessing (-E option).

And why __func__ is lowercase confuses me! ;^)

Is defined this way by ISO 9989:1999. GCC defines also __FUNCTION__ if you are confused by the lowercase version.

/* compile with:
gcc -E program-name.c
To see the output of the preprocessor
*/

This will list everything done by the preprocessor, including #include lines, #if,#else,#endifs, #line, #error etc.... -dM option limits the listing to #define only.
If you want to preprocess only, you could do as below ("cpp" is C Pre-Processor, that's why, on Unixes, a C++ file extension is '.cc', '.c++', '.C' (uppercase) or '.cxx'):

cpp program-name.c

[rstanley]

@flp1969 please compile the sample program I provided in #7 above, with the command I provided, and you WILL see that __FILE__ and __LINE__ are resolved by the preprocessor, not the compiler, as I stated.

gcc -E program-name.c

WILL only run the preprocessor!

[flp1969]

@flp1969 please compile the sample program I provided in #7 above, with the command I provided, and you WILL see that __FILE__ and __LINE__ are resolved by the preprocessor, not the compiler, as I stated.

Yep... because __func__ isn't a predefined macro (as stated in ISO 9989:1999 6.4.2.2:1 and 6.10.8:1), it is an identifier defined by the compiler when used.

gcc -E program-name.c

WILL only run the preprocessor!

Yep, and I didn't say this wouldn't run only the preprocessor... I said, this will list all preprocessed info, while using -dM option will limit the listing to symbolic definitions only.

[flp1969]

I stand corrected: The -dM option will not "limit" the listing... it will list ALL defined symbols, and only defined symbols, except those dynamically created by the compiler (__FILE__ and __LINE__ -- this is strange, since gcc documentation includes "predefined" macros).

$ cpp -dM - < /dev/null | grep FILE
$ cat > test.c <<< 'int main(void){}'
$ cpp -dM test.c  | grep FILE
$

PS: It doen't make sense listing __LINE__, since will change for every line... But __FILE__ should be listed for a given filename (test.c above, for example)...

[Click_here]

It doen't make sense listing __LINE__, since will change for every line... But __FILE__ should be listed for a given filename (test.c above, for example)...

It's great for debugging - This is a quick example of how it can be used... [edit] I know that it isn't actually "thowing" an exception, it was just taken from some code I already had and simplified... [/edit]

#include <stdio.h>
#include <stdlib.h>


#define THROW_EXCEPTION(exType) my_throwing_exception( __FILE__ , \
                                                       __LINE__ , \
                                                       exType)

#define MALLOC_FAILED_EX 0


const char *getExceptionMessage(int exType)
{
    switch (exType)
    {
    case MALLOC_FAILED_EX:
        return "malloc call failed";

        /* ... */
    }
    return "Unknown error";
}

void my_throwing_exception( const char *fileName, int lineNumber, int exType)
{
    fprintf(stderr, "\nExeption found\nFile: %s\nLine: %d,\nMessage: %s", fileName, lineNumber, getExceptionMessage(exType));
}

int main(void)
{
    char *apple = NULL;
    int *banana = NULL;
    long int *grape = NULL;

    int result = EXIT_SUCCESS;

    if (NULL == (apple = malloc(sizeof(*apple))))
    {
        THROW_EXCEPTION(MALLOC_FAILED_EX);
        result =  EXIT_FAILURE;
    }

    free(apple);

    if (NULL == (banana = malloc(sizeof(*banana))))
    {
        THROW_EXCEPTION(MALLOC_FAILED_EX);
        result =  EXIT_FAILURE;
    }

    free(banana);

    if (NULL == (grape = malloc(sizeof(*grape))))
    {
        THROW_EXCEPTION(MALLOC_FAILED_EX);
        result =  EXIT_FAILURE;
    }

    free(grape);

    return result;
}

I first saw it used this way in a code tracer from R. Heathfield et. al., "C Unleashed", "Chapter 7: When Things Go Wrong: Code-Mending" (A great advanced book that was originally recommended by Salem)