What does GCC's "-wrapper" flag do?

Question

From the GCC manual, there is the following overall option:

-wrapper
Invoke all subcommands under a wrapper program.
The name of the wrapper program and its parameters
are passed as a comma separated list.

gcc -c t.c -wrapper gdb,--args

This will invoke all subprograms of gcc under gdb --args', thus the invocation of cc1 will begdb --args cc1 ...'.

I'm having trouble understanding the example and the purpose of the flag.

gcc -c t.c will create a t.o.
and then what? the object file is sent to gdb?
or is gdb given the responsibility of creating the object file (asummingly adding debugging information)?

Answer 1

Yes, for debugging the compiler itself. Or otherwise "trace" what is going on in the compiler - you could for example print the arguments passed to cc1 itself by adding a program that does that and then runs cc1.

gdb is not in charge of generating anything, it is just wrapping around cc1 whihc is the "compiler proper" - when you run gcc -c t.c the compiler first runs cpp -o t.i t.c to preprocess the t.c file. Then it runs cc1 -o t.s t.i and finally as -o t.o t.s (or something along those lines. With the wrapper, it would run those commands as, for example, gdb --args cc1 -o t.s t.i.

Edit: This is of course much simplified compared to a "real" compile - there's a whole bunch of arguments passed to cc1, etc.

Answer 2

During compilation gcc invokes some other programs (actual assembler, linker etc), and with -wrapper flag they are invoked within said wrapper. In your example, all subcommands are executed within gdb, which is useful for debugging gcc.

Another example: to get list of all invoked subcommands one can wrap them within echo (of course, they are not executed this way):

$ gcc 1.c  -wrapper echo
/usr/lib/gcc/x86_64-linux-gnu/4.6/cc1 -quiet -imultilib . -imultiarch x86_64-linux-gnu 1.c -quiet -dumpbase 1.c -mtune=generic -march=x86-64 -auxbase 1 -fstack-protector -o /tmp/cc7cQrsT.s
as --64 -o /tmp/ccaLYkv9.o /tmp/cc7cQrsT.s
/usr/lib/gcc/x86_64-linux-gnu/4.6/collect2 --sysroot=/ --build-id --no-add-needed --as-needed --eh-frame-hdr -m elf_x86_64 --hash-style=gnu -dynamic-linker /lib64/ld-linux-x86-64.so.2 -z relro /usr/lib/gcc/x86_64-linux-gnu/4.6/../../../x86_64-linux-gnu/crt1.o /usr/lib/gcc/x86_64-linux-gnu/4.6/../../../x86_64-linux-gnu/crti.o /usr/lib/gcc/x86_64-linux-gnu/4.6/crtbegin.o -L/usr/lib/gcc/x86_64-linux-gnu/4.6 -L/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../x86_64-linux-gnu -L/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../../lib -L/lib/x86_64-linux-gnu -L/lib/../lib -L/usr/lib/x86_64-linux-gnu -L/usr/lib/../lib -L/usr/lib/gcc/x86_64-linux-gnu/4.6/../../.. /tmp/ccaLYkv9.o -lgcc --as-needed -lgcc_s --no-as-needed -lc -lgcc --as-needed -lgcc_s --no-as-needed /usr/lib/gcc/x86_64-linux-gnu/4.6/crtend.o /usr/lib/gcc/x86_64-linux-gnu/4.6/../../../x86_64-linux-gnu/crtn.o

Answer 3

You could have tried it on a simple hello world.

gcc will call different subcommands. Each of these subcommands will be prefixed with the wrapper. Giving gdb as a wrapper means that you want to debug the compiler.