diff options
Diffstat (limited to 'gcc/doc')
| -rw-r--r-- | gcc/doc/invoke.texi | 25 |
1 files changed, 19 insertions, 6 deletions
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi index 275986c..277574a 100644 --- a/gcc/doc/invoke.texi +++ b/gcc/doc/invoke.texi @@ -15709,12 +15709,25 @@ Use it to conform to a non-default application binary interface. @item -fno-common @opindex fno-common -In C, allocate even uninitialized global variables in the data section of the -object file, rather than generating them as common blocks. This has the -effect that if the same variable is declared (without @code{extern}) in -two different compilations, you will get an error when you link them. -The only reason this might be useful is if you wish to verify that the -program will work on other systems which always work this way. +In C code, controls the placement of uninitialized global variables. +Unix C compilers have traditionally permitted multiple definitions of +such variables in different compilation units by placing the variables +in a common block. +This is the behavior specified by @option{-fcommon}, and is the default +for GCC on most targets. +On the other hand, this behavior is not required by ISO C, and on some +targets may carry a speed or code size penalty on variable references. +The @option{-fno-common} option specifies that the compiler should place +uninitialized global variables in the data section of the object file, +rather than generating them as common blocks. +This has the effect that if the same variable is declared +(without @code{extern}) in two different compilations, +you will get a multiple-definition error when you link them. +In this case, you must compile with @option{-fcommon} instead. +Compiling with @option{-fno-common} is useful on targets for which +it provides better performance, or if you wish to verify that the +program will work on other systems which always treat uninitialized +variable declarations this way. @item -fno-ident @opindex fno-ident |