Compiling: where is function "_start"?

by Martin Monperrus

When linking programs, by convention, function "_start" is the entry point. Hence, it has to somewhere in the object files or libraries that are linked. Otherwise, the linker would say something like:

  tcc: error: _start not defined


  undefined reference to `_start'

Function start is provided by the implementation of the libc. However, there is no convention on the file name where the start function should be put.

GNU libc (glibc) puts it in crt1.o, newlib in crt0.o and dietlibc in start.o.

Once you know this, it makes things easier :)

Troubleshooting trace on a Debian system ("T" means the symbol is in the code (text) section, i.e. it is defined):

  $ nm /usr/lib/i386-linux-gnu/crt1.o | grep start
  00000000 D __data_start
  00000000 W data_start
          U __libc_start_main
  00000000 T _start
  $ nm /usr/lib/newlib/i486-linux-gnu/lib/crt0.o | grep start
  00000000 T _start
  $ nm /usr/lib/diet/lib/start.o | grep start
  00000000 T _start

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