3.16 Specifying Target Machine and Compiler Version

By default, GCC compiles code for the same type of machine that you are using. However, it can also be installed as a cross-compiler, to compile for some other type of machine. In fact, several different configurations of GCC, for different target machines, can be installed side by side. Then you specify which one to use with the `-b' option.

In addition, older and newer versions of GCC can be installed side by side. One of them (probably the newest) will be the default, but you may sometimes wish to use another.

-b machine

The argument machine specifies the target machine for compilation. This is useful when you have installed GCC as a cross-compiler.

The value to use for machine is the same as was specified as the machine type when configuring GCC as a cross-compiler. For example, if a cross-compiler was configured with `configure i386v', meaning to compile for an 80386 running System V, then you would specify `-b i386v' to run that cross compiler.

When you do not specify `-b', it normally means to compile for the same type of machine that you are using.

-V version

The argument version specifies which version of GCC to run. This is useful when multiple versions are installed. For example, version might be `2.0', meaning to run GCC version 2.0.

The default version, when you do not specify `-V', is the last version of GCC that you installed.

The `-b' and `-V' options actually work by controlling part of the file name used for the executable files and libraries used for compilation. A given version of GCC, for a given target machine, is normally kept in the directory `/usr/local/lib/gcc-lib/machine/version'.

Thus, sites can customize the effect of `-b' or `-V' either by changing the names of these directories or adding alternate names (or symbolic links). If in directory `/usr/local/lib/gcc-lib/' the file `80386' is a link to the file `i386v', then `-b 80386' becomes an alias for `-b i386v'.

In one respect, the `-b' or `-V' do not completely change to a different compiler: the top-level driver program gcc that you originally invoked continues to run and invoke the other executables (preprocessor, compiler per se, assembler and linker) that do the real work. However, since no real work is done in the driver program, it usually does not matter that the driver program in use is not the one for the specified target. It is common for the interface to the other executables to change incompatibly between compiler versions, so unless the version specified is very close to that of the driver (for example, `-V 3.0' with a driver program from GCC version 3.0.1), use of `-V' may not work; for example, using `-V 2.95.2' will not work with a driver program from GCC 3.0.

The only way that the driver program depends on the target machine is in the parsing and handling of special machine-specific options. However, this is controlled by a file which is found, along with the other executables, in the directory for the specified version and target machine. As a result, a single installed driver program adapts to any specified target machine, and sufficiently similar compiler versions.

The driver program executable does control one significant thing, however: the default version and target machine. Therefore, you can install different instances of the driver program, compiled for different targets or versions, under different names.

For example, if the driver for version 2.0 is installed as ogcc and that for version 2.1 is installed as gcc, then the command gcc will use version 2.1 by default, while ogcc will use 2.0 by default. However, you can choose either version with either command with the `-V' option.