SCons has built-in scanners for a number of languages. Sometimes these scanners fail to extract certain implicit dependencies due to limitations of the scanner implementation.

The following example illustrates a case where the built-in C scanner is unable to extract the implicit dependency on a header file.

#define FOO_HEADER <foo.h>
#include FOO_HEADER

int main() {
    return FOO;
}
      

% scons -Q
cc -o hello.o -c -I. hello.c
cc -o hello hello.o
%    [CHANGE CONTENTS OF foo.h]
% scons -Q
scons: `.' is up to date.

Apparently, the scanner does not know about the header dependency. Being not a full-fledged C preprocessor, the scanner does not expand the macro.

In these cases, you may also use the compiler to extract the implicit dependencies. ParseDepends can parse the contents of the compiler output in the style of Make, and explicitly establish all of the listed dependencies.

The following example uses ParseDepends to process a compiler generated dependency file which is generated as a side effect during compilation of the object file:

obj = Object('hello.c', CCFLAGS='-MD -MF hello.d', CPPPATH='.')
SideEffect('hello.d', obj)
ParseDepends('hello.d')
Program('hello', obj)
      

% scons -Q
cc -o hello.o -c -MD -MF hello.d -I. hello.c
cc -o hello hello.o
%    [CHANGE CONTENTS OF foo.h]
% scons -Q
cc -o hello.o -c -MD -MF hello.d -I. hello.c

Parsing dependencies from a compiler-generated .d file has a chicken-and-egg problem, that causes unnecessary rebuilds:

% scons -Q
cc -o hello.o -c -MD -MF hello.d -I. hello.c
cc -o hello hello.o
% scons -Q --debug=explain
scons: rebuilding `hello.o' because `foo.h' is a new dependency
cc -o hello.o -c -MD -MF hello.d -I. hello.c
% scons -Q
scons: `.' is up to date.
    

In the first pass, the dependency file is generated while the object file is compiled. At that time, SCons does not know about the dependency on foo.h. In the second pass, the object file is regenerated because foo.h is detected as a new dependency.

ParseDepends immediately reads the specified file at invocation time and just returns if the file does not exist. A dependency file generated during the build process is not automatically parsed again. Hence, the compiler-extracted dependencies are not stored in the signature database during the same build pass. This limitation of ParseDepends leads to unnecessary recompilations. Therefore, ParseDepends should only be used if scanners are not available for the employed language or not powerful enough for the specific task.