A while ago I started reading Expert C Programming, a book on the intricacies of C, written almost 20 years ago (in 1994). I can very greatly recommend this book, even for seasoned C developers. Even although I know about most of the subjects addressed in the book, I picked up a few new things as well.

Anyway, the book made me look at my default compiler flags again. I tended not to use -Wall at all, but I've since then expanded my CFLAGS to -Wall -Wextra -Wdeclaration-after-statement -Wmissing-field-initializers -Wshadow -Wno-unused-parameter. When running with the new flags, the MongoDB driver for PHP started throwing (among others) the following warning during compilation:

/home/derick/dev/php/derickr-mongo-php-driver/bson.c:1069:12: warning: declaration of ‘buffer’ shadows a global declaration [-Wshadow]
  char *buffer;
        ^

At first I found this odd, as I could not find the declaration at all, until I looked closer at the location where the compiler though I had the error:

In file included from /home/derick/dev/php/derickr-mongo-php-driver/bson.c:26:0:
/home/derick/dev/php/derickr-mongo-php-driver/php_mongo.h:301:3: warning: shadowed declaration is here [-Wshadow]
} buffer;
  ^

And this pointed me to the buffer struct:

typedef struct {
    char *start;
    char *pos;
    char *end;
} buffer;

This typedef declares buffer into the global scope and therefore the compiler rightfully warns us about this when trying to compile char *buffer; in a function. The fix was to add the prefix mongo_ so that the new name becomes mongo_buffer. The mongo_ prefix is what we use for all the other structs we define as well.