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crt1

https://en.wikipedia.org/wiki/Crt0

Stands for C runtime.

Major functions:

  • contains the _start symbol, which is the conventional symbol entry point (can be customized on ELF header)
  • prepares the values of argc, argv and environ
  • calls main
  • passes the return value of main back to an exit system call

Present in:

  • the source at sysdeps/x86_64/start.S
  • the build under csu/, which seems to start for C Start Up
  • the install under lib/

In Ubuntu 14.04, we can confirm this with:

nm /usr/lib/x86_64-linux-gnu/crt1.o

In Ubuntu 14.04 it is part of the libc6-dev package, which also contains libc.so. It is provided by glibc as well.

GCC makes the linker link with it by default. This can be seen from:

gcc -v main.c

which shows that the linker command is of type:

 /usr/lib/gcc/x86_64-linux-gnu/4.8/collect2 /usr/lib/gcc/x86_64-linux-gnu/4.8/../../../x86_64-linux-gnu/crt1.o /tmp/ccaVhizy.o [...]

A raw ld call however does not do this: you need to pass those files to the linker as well.

you can tell GCC not to add those files with -nostartfiles.

_start does arch specific initializations and then the common __libc_start_main in csu/libc-start.c.

This then calls:

result = main (argc, argv, __environ MAIN_AUXVEC_PARAM);

and:

exit(result);

crt0

http://stackoverflow.com/questions/2709998/crt0-o-and-crt1-o-whats-the-difference

crti

crtn

Also linked by default by GCC.

crtbegin

crtend

Part of GCC, so not documented here.

Just to let you know that not all crt are glibc libraries.

Come from libgcc/crtstuff.c.

Mcrt1

Scrt1

http://stackoverflow.com/questions/16436035/whats-the-usage-of-mcrt1-o-and-scrt1-o

init-first

TODO. This actually seems to be the actual initial code that is run, not _start, even though _start is set at the ELF header as the entry point. What is going on?

Just try to do b _init on gdb.

Bibliography