If you need to compile a large number of C++ files by using GCC with the same options, for example, typing the options for each file is tedious. You can avoid this repetitive task by defining a variable or macro in make as follows:
# Define macros for name of compiler CXX= g++ # Define a macro for the GCC flags CXXFLAGS= -O2 -g -mcpu=i686 # A rule for building an object file form.o: form.C form.h $(CXX) -c $(CXXFLAGS) form.CIn this example, CXX and CXXFLAGS are make variables. (GNU make prefers to call them variables, but most Unix make utilities call them macros.)
To use a variable anywhere in the makefile, start with a dollar sign ($) followed by the variable within parentheses. GNU make replaces all occurrences of a variable with its definition; thus, it replaces all occurrences of $(CXXFLAGS) with the string -O2 -g -mcpu=i686.
GNU make has several predefined variables that have special meanings. This table lists these variables. In addition to the variables listed here, GNU make considers all environment variables (such as PATH and HOME) to be predefined variables as well.
| Variable | Meaning |
|---|---|
| $% | Member name for targets that are archives. If the target is libDisp.a(image.o), for example, $% is image.o. |
| $* | Name of the target file without the extension. |
| $+ | Names of all dependent files with duplicate dependencies, listed in their order of occurrence. |
| $< | The name of the first dependent file. |
| $? | Names of all dependent files (with spaces between the names) that are newer than the target. |
| $@ | Complete name of the target. If the target is libDisp.a image.o), for example, $@ is libDisp.a. |
| $^ | Names of all dependent files, with spaces between the names. Duplicates are removed from the dependent filenames. |
| AR | Name of the archive-maintaining program (default value: ar). |
| ARFLAGS | Flags for the archive-maintaining program (default value: rv). |
| AS | Name of the assembler program that converts the assembly language to object code (default value: as). |
| ASFLAGS | Flags for the assembler. |
| CC | Name of the C compiler (default value: cc). |
| CFLAGS | Flags that are passed to the C compiler. |
| CO | Name of the program that extracts a file from RCS (default value: co). |
| COFLAGS | Flags for the RCS co program. |
| CPP | Name of the C preprocessor (default value: $(CC) -E). |
| CPPFLAGS | Flags for the C preprocessor. |
| CXX | Name of the C++ compiler (default value: g++). |
| CXXFLAGS | Flags that are passed to the C++ compiler. |
| FC | Name of the FORTRAN compiler (default value: f77). |
| FFLAGS | Flags for the FORTRAN compiler. |
| LDFLAGS | Flags for the compiler when it’s supposed to invoke the linker ld. |
| RM | Name of the command to delete a file (Default value: rm -f). |