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, which can be one of the following:
%
command
%include <
file>
%include_noerr <
file>
%include
’, but do not generate an error message if the include file cannot be found. %rename
old_name new_name
*[
spec_name]:
+
’ character, in which case the text is appended to the spec. [
suffix]:
.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 following a suffix directive can be one of the following:
@
language
-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
name compiler not installed on this system.
GCC already has an extensive list of suffixes built into it. This directive adds 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.
%%
%
’ into the program name or argument. %i
%b
%B
%b
’, but include the file suffix (text after the last period). %d
%d
’ as a temporary file name, so that that file is deleted if GCC exits successfully. Unlike ‘%g
’, this contributes no text to the argument. %g
suffix
%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
%g
’, but generates a new temporary file name each time it appears instead of once per compilation. %U
suffix
%g.s ... %U.s ... %g.s ... %U.s
’ involves 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
HOST_BIT_BUCKET
, if any, and if it is writable, and if -save-temps
is not used; 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
%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
%*
’. SUFFIX is terminated by the next space or %. %w
%w
’ as the designated output file of this compilation. This puts the argument into the sequence of arguments that ‘%o
’ substitutes. %o
%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 are linked. %O
%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 do following, for example, ‘.o
’. %p
cpp
. %P
%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
-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
%T
--script
option into the command line followed by the full path name found. If the file is not found then generate an error message. Note: the current working directory is not searched. %e
str
%(
name)
%x{
option}
%X
’. %X
-Wl
or a ‘%x
’ spec string. %Y
-Wa
. %Z
-Wp
. %a
asm
spec. This is used to compute the switches to be passed to the assembler. %A
asm_final
spec. This is a spec string for passing switches to an assembler post-processor, if such a program is needed. %l
link
spec. This is the spec for computing the command line passed to the linker. Typically it makes use of the ‘%L %G %S %D and %E
’ sequences. %D
-L
option for each directory that GCC believes might contain startup files. If the target supports multilibs then the current multilib directory is prepended to each of these paths. %L
lib
spec. This is a spec string for deciding which libraries are included on the command line to the linker. %G
libgcc
spec. This is a spec string for deciding which GCC support library is included on the command line to the linker. %S
startfile
spec. This is a spec for deciding which object files are the first ones passed to the linker. Typically this might be a file named crt0.o
. %E
endfile
spec. This is a spec string that specifies the last object files that are passed to the linker. %C
cpp
spec. This is used to construct the arguments to be passed to the C preprocessor. %1
cc1
spec. This is used to construct the options to be passed to the actual C compiler (cc1
). %2
cc1plus
spec. This is used to construct the options to be passed to the actual C++ compiler (cc1plus
). %*
%<S
-S
from the command line. Note—this command is position dependent. ‘%
’ commands in the spec string before this one see -S
, ‘%
’ commands in the spec string after this one do not. %:
function(
args)
The following built-in spec functions are provided:
getenv
getenv
spec function takes two arguments: an environment variable name and a string. If the environment variable is not defined, a fatal error is issued. Otherwise, the return value is the value of the environment variable concatenated with the string. For example, if TOPDIR
is defined as /path/to/top
, then: %:getenv(TOPDIR /include)
expands to /path/to/top/include
.
if-exists
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
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
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)}
remove-outfile
remove-outfile
spec function takes one argument. It looks for the first argument in the outfiles array and removes it. Here is a small example its usage: %:remove-outfile(-lm)
pass-through-libs
pass-through-libs
spec function takes any number of arguments. It finds any -l
options and any non-options ending in .a
(which it assumes are the names of linker input library archive files) and returns a result containing all the found arguments each prepended by -plugin-opt=-pass-through=
and joined by spaces. This list is intended to be passed to the LTO linker plugin. %:pass-through-libs(%G %L %G)
print-asm-header
print-asm-header
function takes no arguments and simply prints a banner like: Assembler 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}
-S
switch, if that switch is given to GCC. If that switch is 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}
’ matches the command-line option -foo
and outputs the command-line option -foo
. %W{S}
S
} but mark last argument supplied within as a file to be deleted on failure. %{S*}
-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 name starts with ‘o
’. %{o*} substitutes this text, including the space. Thus two arguments are generated. %{S*&T*}
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}
X
, if the -S
switch is given to GCC. %{!S:X}
X
, if the -S
switch is not given to GCC. %{S*:X}
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
is substituted once for each matching switch, with the %*
replaced by the part of that switch matching the *
. If %*
appears as the last part of a spec sequence then a space is added after the end of the last substitution. If there is more text in the sequence, however, then a space is not generated. This allows the %*
substitution to be used as part of a larger string. For example, a spec string like this:
%{mcu=*:--script=%*/memory.ld}
when matching an option like -mcu=newchip
produces:
--script=newchip/memory.ld
%{.S:X}
X
, if processing a file with suffix S
. %{!.S:X}
X
, if not processing a file with suffix S
. %{,S:X}
X
, if processing a file for language S
. %{!,S:X}
X
, if not processing a file for language S
. %{S|P:X}
X
if either -S
or -P
is 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}
outputs 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}
S
is given to GCC, substitutes X
; else if T
is 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.
© Free Software Foundation
Licensed under the GNU Free Documentation License, Version 1.3.
https://gcc.gnu.org/onlinedocs/gcc-5.4.0/gcc/Spec-Files.html