### 3.15 Specifying subprocesses and the switches to pass to them

`gcc` is a driver program. It performs its job by invoking a sequence of other programs to do the work of compiling, assembling and linking. GCC interprets its command-line parameters and uses these to deduce which programs it should invoke, and which command-line options it ought to place on their command lines. This behavior is controlled by spec strings. In most cases there is one spec string for each program that GCC can invoke, but a few programs have multiple spec strings to control their behavior. The spec strings built into GCC can be overridden by using the `-specs=` command-line switch to specify a spec file.

Spec files are plaintext files that are used to construct spec strings. They consist of a sequence of directives separated by blank lines. The type of directive is determined by the first non-whitespace character on the line and it can be one of the following:

`%`command
  Issues a command to the spec file processor. The commands that can appear here are:
  `%include <`file`>`
    Search for file and insert its text at the current point in the specs file.
  `%include_noerr <`file`>`
    Just like ‘`%include`’, but do not generate an error message if the include file cannot be found.
  `%rename` old_name new_name
    Rename the spec string old_name to new_name.
`*[`spec_name`]:`
  This tells the compiler to create, override or delete the named spec string. All lines after this directive up to the next directive or blank line are considered to be the text for the spec string. If this results in an empty string then the spec will be deleted. (Or, if the spec did not exist, then nothing will happened.) Otherwise, if the spec does not currently exist a new spec will be created. If the spec does exist then its contents will be overridden by the text of this directive, unless the first character of that text is the ‘`+`’ character, in which case the text will be appended to the spec.
`[`suffix`]:`
  Creates a new ‘`[suffix] spec`’ pair. All lines after this directive and up to the next directive or blank line are considered to make up the spec string for the indicated suffix. When the compiler encounters an input file with the named suffix, it will processes the spec string in order to work out how to compile that file. For example:

                .ZZ:
                z-compile -input %i

  This says that any input file whose name ends in ‘`.ZZ`’ should be passed to the program ‘`z-compile`’, which should be invoked with the command-line switch `-input` and with the result of performing the ‘`%i`’ substitution. (See below.)

  As an alternative to providing a spec string, the text that follows a suffix directive can be one of the following:

  `@`language
    This says that the suffix is an alias for a known language. This is similar to using the `-x` command-line switch to GCC to specify a language explicitly. For example:

                       .ZZ:
                       @c++

    Says that .ZZ files are, in fact, C++ source files.

  `#`name
    This causes an error messages saying:

                       name compiler not installed on this system.

  GCC already has an extensive list of suffixes built into it. This directive will add an entry to the end of the list of suffixes, but since the list is searched from the end backwards, it is effectively possible to override earlier entries using this technique.

GCC has the following spec strings built into it. Spec files can override these strings or create their own. Note that individual targets can also add their own spec strings to this list.

         asm          Options to pass to the assembler
         asm_final    Options to pass to the assembler post-processor
         cpp          Options to pass to the C preprocessor
         cc1          Options to pass to the C compiler
         cc1plus      Options to pass to the C++ compiler
         endfile      Object files to include at the end of the link
         link         Options to pass to the linker
         lib          Libraries to include on the command line to the linker
         libgcc       Decides which GCC support library to pass to the linker
         linker       Sets the name of the linker
         predefines   Defines to be passed to the C preprocessor
         signed_char  Defines to pass to CPP to say whether char is signed
                      by default
         startfile    Object files to include at the start of the link

Here is a small example of a spec file:

         %rename lib                 old_lib

         *lib:
         --start-group -lgcc -lc -leval1 --end-group %(old_lib)

This example renames the spec called ‘`lib`’ to ‘`old_lib`’ and then overrides the previous definition of ‘`lib`’ with a new one. The new definition adds in some extra command-line options before including the text of the old definition.

Spec strings are a list of command-line options to be passed to their corresponding program. In addition, the spec strings can contain ‘`%`’-prefixed sequences to substitute variable text or to conditionally insert text into the command line. Using these constructs it is possible to generate quite complex command lines.

Here is a table of all defined ‘`%`’-sequences for spec strings. Note that spaces are not generated automatically around the results of expanding these sequences. Therefore you can concatenate them together or combine them with constant text in a single argument.

`%%`
  Substitute one ‘`%`’ into the program name or argument.
`%i`
  Substitute the name of the input file being processed.
`%b`
  Substitute the basename of the input file being processed. This is the substring up to (and not including) the last period and not including the directory.
`%B`
  This is the same as ‘`%b`’, but include the file suffix (text after the last period).
`%d`
  Marks the argument containing or following the ‘`%d`’ as a temporary file name, so that that file will be deleted if GCC exits successfully. Unlike ‘`%g`’, this contributes no text to the argument.
`%g` suffix
  Substitute a file name that has suffix suffix and is chosen once per compilation, and mark the argument in the same way as ‘`%d`’. To reduce exposure to denial-of-service attacks, the file name is now chosen in a way that is hard to predict even when previously chosen file names are known. For example, ‘`%g.s ... %g.o ... %g.s`’ might turn into ‘`ccUVUUAU.s ccXYAXZ12.o ccUVUUAU.s`’. suffix matches the regexp ‘`[.A-Za-z]*`’ or the special string ‘`%O`’, which is treated exactly as if ‘`%O`’ had been preprocessed. Previously, ‘`%g`’ was simply substituted with a file name chosen once per compilation, without regard to any appended suffix (which was therefore treated just like ordinary text), making such attacks more likely to succeed.
`%u` suffix
  Like ‘`%g`’, but generates a new temporary file name even if ‘`%usuffix`’ was already seen.
`%U` suffix
  Substitutes the last file name generated with ‘`%usuffix`’, generating a new one if there is no such last file name. In the absence of any ‘`%usuffix`’, this is just like ‘`%gsuffix`’, except they don't share the same suffix space, so ‘`%g.s ... %U.s ... %g.s ... %U.s`’ would involve the generation of two distinct file names, one for each ‘`%g.s`’ and another for each ‘`%U.s`’. Previously, ‘`%U`’ was simply substituted with a file name chosen for the previous ‘`%u`’, without regard to any appended suffix.
`%j` suffix
  Substitutes the name of the `HOST_BIT_BUCKET`, if any, and if it is writable, and if save-temps is off; otherwise, substitute the name of a temporary file, just like ‘`%u`’. This temporary file is not meant for communication between processes, but rather as a junk disposal mechanism.
`%|`suffix
`%m` suffix
  Like ‘`%g`’, except if `-pipe` is in effect. In that case ‘`%|`’ substitutes a single dash and ‘`%m`’ substitutes nothing at all. These are the two most common ways to instruct a program that it should read from standard input or write to standard output. If you need something more elaborate you can use an ‘`%{pipe:`X`}`’ construct: see for example `f/lang-specs.h`.
`%.`SUFFIX
  Substitutes .SUFFIX for the suffixes of a matched switch's args when it is subsequently output with ‘`%*`’. SUFFIX is terminated by the next space or %.
`%w`
  Marks the argument containing or following the ‘`%w`’ as the designated output file of this compilation. This puts the argument into the sequence of arguments that ‘`%o`’ will substitute later.
`%o`
  Substitutes the names of all the output files, with spaces automatically placed around them. You should write spaces around the ‘`%o`’ as well or the results are undefined. ‘`%o`’ is for use in the specs for running the linker. Input files whose names have no recognized suffix are not compiled at all, but they are included among the output files, so they will be linked.
`%O`
  Substitutes the suffix for object files. Note that this is handled specially when it immediately follows ‘`%g, %u, or %U`’, because of the need for those to form complete file names. The handling is such that ‘`%O`’ is treated exactly as if it had already been substituted, except that ‘`%g, %u, and %U`’ do not currently support additional suffix characters following ‘`%O`’ as they would following, for example, ‘`.o`’.
`%p`
  Substitutes the standard macro predefinitions for the current target machine. Use this when running `cpp`.
`%P`
  Like ‘`%p`’, but puts ‘`__`’ before and after the name of each predefined macro, except for macros that start with ‘`__`’ or with ‘`_L`’, where L is an uppercase letter. This is for ISO C.
`%I`
  Substitute any of `-iprefix` (made from `GCC_EXEC_PREFIX`), `-isysroot` (made from `TARGET_SYSTEM_ROOT`), `-isystem` (made from `COMPILER_PATH` and `-B` options) and `-imultilib` as necessary.
`%s`
  Current argument is the name of a library or startup file of some sort. Search for that file in a standard list of directories and substitute the full name found.
`%e` str
  Print str as an error message. str is terminated by a newline. Use this when inconsistent options are detected.
`%(`name`)`
  Substitute the contents of spec string name at this point.
`%[`name`]`
  Like ‘`%(...)`’ but put ‘`__`’ around `-D` arguments.
`%x{`option`}`
  Accumulate an option for ‘`%X`’.
`%X`
  Output the accumulated linker options specified by `-Wl` or a ‘`%x`’ spec string.
`%Y`
  Output the accumulated assembler options specified by `-Wa`.
`%Z`
  Output the accumulated preprocessor options specified by `-Wp`.
`%a`
  Process the `asm` spec. This is used to compute the switches to be passed to the assembler.
`%A`
  Process the `asm_final` spec. This is a spec string for passing switches to an assembler post-processor, if such a program is needed.
`%l`
  Process the `link` spec. This is the spec for computing the command line passed to the linker. Typically it will make use of the ‘`%L %G %S %D and %E`’ sequences.
`%D`
  Dump out a `-L` option for each directory that GCC believes might contain startup files. If the target supports multilibs then the current multilib directory will be prepended to each of these paths.
`%L`
  Process the `lib` spec. This is a spec string for deciding which libraries should be included on the command line to the linker.
`%G`
  Process the `libgcc` spec. This is a spec string for deciding which GCC support library should be included on the command line to the linker.
`%S`
  Process the `startfile` spec. This is a spec for deciding which object files should be the first ones passed to the linker. Typically this might be a file named `crt0.o`.
`%E`
  Process the `endfile` spec. This is a spec string that specifies the last object files that will be passed to the linker.
`%C`
  Process the `cpp` spec. This is used to construct the arguments to be passed to the C preprocessor.
`%1`
  Process the `cc1` spec. This is used to construct the options to be passed to the actual C compiler (‘`cc1`’).
`%2`
  Process the `cc1plus` spec. This is used to construct the options to be passed to the actual C++ compiler (‘`cc1plus`’).
`%*`
  Substitute the variable part of a matched option. See below. Note that each comma in the substituted string is replaced by a single space.
`%<S`
  Remove all occurrences of `-S` from the command line. Note—this command is position dependent. ‘`%`’ commands in the spec string before this one will see `-S`, ‘`%`’ commands in the spec string after this one will not.
`%:`function`(`args`)`
  Call the named function function, passing it args. args is first processed as a nested spec string, then split into an argument vector in the usual fashion. The function returns a string which is processed as if it had appeared literally as part of the current spec.

  The following built-in spec functions are provided:

  `if-exists`
    The `if-exists` spec function takes one argument, an absolute pathname to a file. If the file exists, `if-exists` returns the pathname. Here is a small example of its usage:

                       *startfile:
                       crt0%O%s %:if-exists(crti%O%s) crtbegin%O%s

  `if-exists-else`
    The `if-exists-else` spec function is similar to the `if-exists` spec function, except that it takes two arguments. The first argument is an absolute pathname to a file. If the file exists, `if-exists-else` returns the pathname. If it does not exist, it returns the second argument. This way, `if-exists-else` can be used to select one file or another, based on the existence of the first. Here is a small example of its usage:

                       *startfile:
                       crt0%O%s %:if-exists(crti%O%s) \
                       %:if-exists-else(crtbeginT%O%s crtbegin%O%s)

  `replace-outfile`
    The `replace-outfile` spec function takes two arguments. It looks for the first argument in the outfiles array and replaces it with the second argument. Here is a small example of its usage:

                       %{fgnu-runtime:%:replace-outfile(-lobjc -lobjc-gnu)}

  `print-asm-header`
    The `print-asm-header` function takes no arguments and simply prints a banner like:

                       Assember options
                       ================

                       Use "-Wa,OPTION" to pass "OPTION" to the assembler.

    It is used to separate compiler options from assembler options in the `--target-help` output.

`%{S}`
  Substitutes the `-S` switch, if that switch was given to GCC. If that switch was not specified, this substitutes nothing. Note that the leading dash is omitted when specifying this option, and it is automatically inserted if the substitution is performed. Thus the spec string ‘`%{foo}`’ would match the command-line option `-foo` and would output the command line option `-foo`.
`%W{S}`
  Like %{`S`} but mark last argument supplied within as a file to be deleted on failure.
`%{S*}`
  Substitutes all the switches specified to GCC whose names start with `-S`, but which also take an argument. This is used for switches like `-o`, `-D`, `-I`, etc. GCC considers `-o foo` as being one switch whose names starts with ‘`o`’. %{o*} would substitute this text, including the space. Thus two arguments would be generated.
`%{S*&T*}`
  Like %{`S`*}, but preserve order of `S` and `T` options (the order of `S` and `T` in the spec is not significant). There can be any number of ampersand-separated variables; for each the wild card is optional. Useful for CPP as ‘`%{D*&U*&A*}`’.
`%{S:X}`
  Substitutes `X`, if the ‘`-S`’ switch was given to GCC.
`%{!S:X}`
  Substitutes `X`, if the ‘`-S`’ switch was not given to GCC.
`%{S*:X}`
  Substitutes `X` if one or more switches whose names start with `-S` are specified to GCC. Normally `X` is substituted only once, no matter how many such switches appeared. However, if `%*` appears somewhere in `X`, then `X` will be substituted once for each matching switch, with the `%*` replaced by the part of that switch that matched the `*`.
`%{.S:X}`
  Substitutes `X`, if processing a file with suffix `S`.
`%{!.S:X}`
  Substitutes `X`, if not processing a file with suffix `S`.
`%{S|P:X}`
  Substitutes `X` if either `-S` or `-P` was given to GCC. This may be combined with ‘`!`’, ‘`.`’, and `*` sequences as well, although they have a stronger binding than the ‘`|`’. If `%*` appears in `X`, all of the alternatives must be starred, and only the first matching alternative is substituted.

  For example, a spec string like this:

                %{.c:-foo} %{!.c:-bar} %{.c|d:-baz} %{!.c|d:-boggle}

  will output the following command-line options from the following input command-line options:

                fred.c        -foo -baz
                jim.d         -bar -boggle
                -d fred.c     -foo -baz -boggle
                -d jim.d      -bar -baz -boggle

`%{S:X; T:Y; :D}`
  If `S` was given to GCC, substitutes `X`; else if `T` was given to GCC, substitutes `Y`; else substitutes `D`. There can be as many clauses as you need. This may be combined with `.`, `!`, `|`, and `*` as needed.

The conditional text `X` in a %{`S`:`X`} or similar construct may contain other nested ‘`%`’ constructs or spaces, or even newlines. They are processed as usual, as described above. Trailing white space in `X` is ignored. White space may also appear anywhere on the left side of the colon in these constructs, except between `.` or `*` and the corresponding word.

The `-O`, `-f`, `-m`, and `-W` switches are handled specifically in these constructs. If another value of `-O` or the negated form of a `-f`, `-m`, or `-W` switch is found later in the command line, the earlier switch value is ignored, except with {`S`*} where `S` is just one letter, which passes all matching options.

The character ‘`|`’ at the beginning of the predicate text is used to indicate that a command should be piped to the following command, but only if `-pipe` is specified.

It is built into GCC which switches take arguments and which do not. (You might think it would be useful to generalize this to allow each compiler's spec to say which switches take arguments. But this cannot be done in a consistent fashion. GCC cannot even decide which input files have been specified without knowing which switches take arguments, and it must know which input files to compile in order to tell which compilers to run).

GCC also knows implicitly that arguments starting in `-l` are to be treated as compiler output files, and passed to the linker in their proper position among the other output files.