On 04/12/2015 11:39, Laszlo Ersek wrote: > (4) Linking those two files into a complete program is a violation of > "6.7 External definitions": > > [...] If an identifier declared with external linkage is used in an > expression (other than as part of the operand of a *sizeof* > operator), somewhere in the entire program there shall be exactly > one external definition for the identifier [...] > > Again, how does the argument go? > > - In each file we have exactly one tentative definition, and no > declaration that would be, in its own right, an external definition. > Based on (2), the files must behave exactly as-if there was one > external *definition* for "x" in each. > > - Argument (3) explains why "x" has external *linkage*. > > - Argument (4) applies to "x" because "x" has external linkage, and is > used in a non-sizeof expression. And the requirement set forth in (4) > is broken by the program because the program is required to behave > exactly as if "x" had two external definitions. > > In practical terms: > > - If you compile the first version of the program (without any special > options), it links together successfully: > > $ gcc -std=c89 -pedantic -Wall -Wextra -o f f1.c f2.c > > - If you compile the second (equivalent) version of the program, > linkage fails: > > $ gcc -std=c89 -pedantic -Wall -Wextra -o f-b f1-b.c f2-b.c > > /tmp/cc8Isbn8.o:(.bss+0x0): multiple definition of `x' > /tmp/cciQUlDz.o:(.bss+0x0): first defined here > collect2: error: ld returned 1 exit status > > - The gcc bug is that linking the first version *too* should fail, > without any particular options.
That's true, but it would break existing code that declares variables in headers without "extern". That's why Visual Studio does the same. Paolo _______________________________________________ edk2-devel mailing list [email protected] https://lists.01.org/mailman/listinfo/edk2-devel
