This what happens when you include #include <stdio.h> (adds input/ output functions like printf) ,for memory management #include <stdlib>, string manupulation #include <string.h> etc... you tell the compiler to copy the declarations for printf , scanf etc... those functions are in the header files such as stdio.h , stdlib etc... the code for these files are already complied they are part of GNU C library.
you try out verbose where these files located
gcc -v your_program.c -o your_program
the output would look like this
$ gcc -v test.c -o test.o
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-linux-gnu/13/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none:amdgcn-amdhsa
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 13.3.0-6ubuntu2~24.04' --with-bugurl=file:///usr/share/doc/gcc-13/README.Bugs --enable-languages=c,ada,c++,go,d,fortran,objc,obj-c++,m2 --prefix=/usr --with-gcc-major-version-only --program-suffix=-13 --program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id --libexecdir=/usr/libexec --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --enable-bootstrap --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-libstdcxx-backtrace --enable-gnu-unique-object --disable-vtable-verify --enable-plugin --enable-default-pie --with-system-zlib --enable-libphobos-checking=release --with-target-system-zlib=auto --enable-objc-gc=auto --enable-multiarch --disable-werror --enable-cet --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic --enable-offload-targets=nvptx-none=/build/gcc-13-fG75Ri/gcc-13-13.3.0/debian/tmp-nvptx/usr,amdgcn-amdhsa=/build/gcc-13-fG75Ri/gcc-13-13.3.0/debian/tmp-gcn/usr --enable-offload-defaulted --without-cuda-driver --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu --with-build-config=bootstrap-lto-lean --enable-link-serialization=2
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 13.3.0 (Ubuntu 13.3.0-6ubuntu2~24.04)
COLLECT_GCC_OPTIONS='-v' '-o' 'test.o' '-mtune=generic' '-march=x86-64' '-dumpdir' 'test.o-'
/usr/libexec/gcc/x86_64-linux-gnu/13/cc1 -quiet -v -imultiarch x86_64-linux-gnu test.c -quiet -dumpdir test.o- -dumpbase test.c -dumpbase-ext .c -mtune=generic -march=x86-64 -version -fasynchronous-unwind-tables -fstack-protector-strong -Wformat -Wformat-security -fstack-clash-protection -fcf-protection -o /tmp/ccRbksbn.s
GNU C17 (Ubuntu 13.3.0-6ubuntu2~24.04) version 13.3.0 (x86_64-linux-gnu)
compiled by GNU C version 13.3.0, GMP version 6.3.0, MPFR version 4.2.1, MPC version 1.3.1, isl version isl-0.26-GMP
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu"
ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/13/include-fixed/x86_64-linux-gnu"
ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/13/include-fixed"
ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/13/../../../../x86_64-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
/usr/lib/gcc/x86_64-linux-gnu/13/include
/usr/local/include
/usr/include/x86_64-linux-gnu
/usr/include
End of search list.
Compiler executable checksum: 38987c28e967c64056a6454abdef726e
COLLECT_GCC_OPTIONS='-v' '-o' 'test.o' '-mtune=generic' '-march=x86-64' '-dumpdir' 'test.o-'
as -v --64 -o /tmp/ccwriUWR.o /tmp/ccRbksbn.s
GNU assembler version 2.42 (x86_64-linux-gnu) using BFD version (GNU Binutils for Ubuntu) 2.42
COMPILER_PATH=/usr/libexec/gcc/x86_64-linux-gnu/13/:/usr/libexec/gcc/x86_64-linux-gnu/13/:/usr/libexec/gcc/x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/13/:/usr/lib/gcc/x86_64-linux-gnu/
LIBRARY_PATH=/usr/lib/gcc/x86_64-linux-gnu/13/:/usr/lib/gcc/x86_64-linux-gnu/13/../../../x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/13/../../../../lib/:/lib/x86_64-linux-gnu/:/lib/../lib/:/usr/lib/x86_64-linux-gnu/:/usr/lib/../lib/:/usr/lib/gcc/x86_64-linux-gnu/13/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-o' 'test.o' '-mtune=generic' '-march=x86-64' '-dumpdir' 'test.o.'
/usr/libexec/gcc/x86_64-linux-gnu/13/collect2 -plugin /usr/libexec/gcc/x86_64-linux-gnu/13/liblto_plugin.so -plugin-opt=/usr/libexec/gcc/x86_64-linux-gnu/13/lto-wrapper -plugin-opt=-fresolution=/tmp/ccO6Fe7I.res -plugin-opt=-pass-through=-lgcc -plugin-opt=-pass-through=-lgcc_s -plugin-opt=-pass-through=-lc -plugin-opt=-pass-through=-lgcc -plugin-opt=-pass-through=-lgcc_s --build-id --eh-frame-hdr -m elf_x86_64 --hash-style=gnu --as-needed -dynamic-linker /lib64/ld-linux-x86-64.so.2 -pie -z now -z relro -o test.o /usr/lib/gcc/x86_64-linux-gnu/13/../../../x86_64-linux-gnu/Scrt1.o /usr/lib/gcc/x86_64-linux-gnu/13/../../../x86_64-linux-gnu/crti.o /usr/lib/gcc/x86_64-linux-gnu/13/crtbeginS.o -L/usr/lib/gcc/x86_64-linux-gnu/13 -L/usr/lib/gcc/x86_64-linux-gnu/13/../../../x86_64-linux-gnu -L/usr/lib/gcc/x86_64-linux-gnu/13/../../../../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/13/../../.. /tmp/ccwriUWR.o -lgcc --push-state --as-needed -lgcc_s --pop-state -lc -lgcc --push-state --as-needed -lgcc_s --pop-state /usr/lib/gcc/x86_64-linux-gnu/13/crtendS.o /usr/lib/gcc/x86_64-linux-gnu/13/../../../x86_64-linux-gnu/crtn.o
COLLECT_GCC_OPTIONS='-v' '-o' 'test.o' '-mtune=generic' '-march=x86-64' '-dumpdir' 'test.o.'
here you can see three phases:
Compilation of your .c files into .o object files, via calls to cc1 and as.
Linking of those object files together plus the startup files (Scrt1.o, crti.o, crtn.o) and the pre‐built C runtime libraries (GCC’s support libraries and the C standard library).
Result is your final executable.
In your verbose dump the key line is buried in the collect2/ld invocation:
… -plugin-opt=-pass-through=-lc … -lc …
That -lc is the linker flag that tells it:
“Pull in the C standard library (libc), which already contains the compiled code for
printf,fopen, etc.”
You do not compile stdio.c (or any of the .c sources of glibc) yourself. The C library ships as pre-compiled archives (libc.a) and shared objects (libc.so), and GCC drivers automatically pass -lc at link time so that all your <stdio.h> declarations get resolved to real code in libc.
a good read would be https://www.gnu.org/software/libc/manual/html_node/Header-Files.html and How does the compilation/linking process work?