C Runtime

Mixing content from:

• Understanding the C Runtime: crt0, crt1, crti, and crtn
  https://www.inferara.com/en/blog/c-runtime/
• Documentation from ulibc
  https://github.com/kraj/uClibc/blob/master/docs/crt.txt
• gcc docs
  http://gcc.gnu.org/onlinedocs/gccint/Initialization.html

crt0.o / crt*.o the startup objects

Historically crt0.o (C Runtime Zero). Modern toolchains split crt0 and use crt1.o (the primary startup object).

• crt1.o : entry point; provides _start symbol
• crti.o : initialization; prologues of .init and .finit sections
• crtn.o : termination; epilogue of .init and .finit sections

crt1 Variants

• Scrt1.o : used in place of crt1.o to generate PIEs
• gcrt1.o : used in place of crt1.o to generate profiling info. (see gprof)
• Mcrt1.o : like gcrt1.o; for prof; useless on Linux
• grcrt1.o, rcrt1.o : wtf

C++ constructor stuffs, I guess

• crtbegin{,S,T}.o to find start of constructors (gcc); S for shared objects/PIEs. T for static executables.
• crtend{,S}.o : find start of destructors (gcc);

Instead of function calls, the startup objects are concatenated at link time.

General linking order:

crt1.o crti.o crtbegin.o [-L paths] [user objects] [gcc libs] [C libs] [gcc libs] crtend.o crtn.o

typical file locations

• /usr/lib/{Scrt1, crt1, crti, crtn, gcrt1}.o
• /lib64/gcc/<m>/<v>/ : crtbegin, crtend , etc.

notes

• The crt0 / crt1 provides the entry point _start for the binary loader.

initialization + crti.o

• stack initialization, if not done by the kernel
• set up memory (data, BSS)
• prepare argc, argv for main
• invoking constructors for global/static objects (C++)
• library initialization
• then calls main(argc, argv, envp)

cleanup + crtn.o

• when main returns, calls OS exit routines (e.g. _exit)

$ readelf -s /usr/lib/crt1.o 
Num: Value  Size  Type   Bind   Vis      Ndx Name
 0: [...]    0   NOTYPE  LOCAL  DEFAULT  UND 
 1: [...]    0   SECTION LOCAL  DEFAULT    1 .text
 2: [...]    5   FUNC    GLOBAL HIDDEN     1 _dl_relocate_sta[...]
 3: [...]   38   FUNC    GLOBAL DEFAULT    1 _start
 4: [...]    0   NOTYPE  GLOBAL DEFAULT  UND main
 5: [...]    0   NOTYPE  WEAK   DEFAULT   11 data_start
 6: [...]    4   OBJECT  GLOBAL DEFAULT    3 _IO_stdin_used
 7: [...]    0   NOTYPE  GLOBAL DEFAULT  UND __libc_start_main
 8: [...]    0   NOTYPE  GLOBAL DEFAULT   11 __data_start

$ readelf -s /usr/lib/crti.o 
Symbol table '.symtab' contains 4 entries:
Num: Value  Size  Type   Bind   Vis      Ndx Name
 0: [...]    0   NOTYPE  LOCAL  DEFAULT  UND 
 1: [...]    0   NOTYPE  WEAK   DEFAULT  UND __gmon_start__
 2: [...]    0   FUNC    GLOBAL HIDDEN     5 _init
 3: [...]    0   FUNC    GLOBAL HIDDEN     7 _fini