ld - the GNU linker.
as - the GNU assembler.

addr2line - Converts addresses into filenames and line numbers.
ar - A utility for creating, modifying and extracting from archives.
c++filt - Filter to demangle encoded C++ symbols.
dlltool - Creates files for building and using DLLs.
elfedit - Allows alteration of ELF format files.
gprof - Displays profiling information.
gprofng - Collects and displays application performance data.
nlmconv - Converts object code into an NLM.
nm - Lists symbols from object files.
objcopy - Copies and translates object files.
objdump - Displays information from object files.
ranlib - Generates an index to the contents of an archive.
readelf - Displays information from any ELF format object file.
size - Lists the section sizes of an object or archive file.
strings - Lists printable strings from files.
strip - Discards symbols.

C99.6.7.8 #10 - 未初始化的全局/静态变量的初值为0

If an object that has automatic storage duration is not initialized explicitly, its
value is indeterminate. If an object that has static storage duration is not
initialized explicitly, then:
- if it has pointer type, it is initialized to a null pointer;
- if it has arithmetic type, it is initialized to (positive or unsigned) zero;
- if it is an aggregate, every member is initialized (recursively) according to these
rules;
- if it is a union, the first named member is initialized (recursively) according to
these rules.

// main.c
extern void f();
int a = 0;
int main() { f(); return 0; }

// f.c
extern int a;
int *pa = &a;
void f() { a = 1; *pa = 2; }

riscv64-linux-gnu-gcc -march=rv64g -fno-pic -c -O2 main.c
riscv64-linux-gnu-gcc -march=rv64g -fno-pic -c -O2 f.c

riscv64-linux-gnu-readelf -s a.o  # 查看ELF文件中的符号表

#include <stdio.h>
#include <math.h>

int main() {
  int a = 2;
  printf("%lf", sqrt(a));
  return 0;
}

gcc a.c       # undefined reference to 'sqrt'
gcc a.c -lm
gcc -lm a.c   # undefined reference to 'sqrt'

// a.c
void f() { }
extern void g();
int main() { g(); return 0; }

// b.cpp
extern void f();
void g() { f(); }
void g(int x) { f(); f(); }

gcc -c a.c && g++ -c b.cpp && gcc a.o b.o

// main.c
#include <stdio.h>
int a = 0, b = 0;
extern void f(), g();
int main() { 
  f(); printf("a = %d(0x%08x), b = %d(0x%08x)\n", a, a, b, b);
  g(); printf("a = %d(0x%08x), b = %d(0x%08x)\n", a, a, b, b);
  return 0;
}

// f.c
void f() { extern float  a; a = -1.0; }

// g.c
void g() { extern double a; a = -1.0; }

gcc main.c f.c g.c && ./a.out

// main.c
#include <stdio.h>
int a; // tentative definition
extern void f();
int main() { 
  a = 1; f(); printf("in main: a = %d\n", a);
  return 0;
}

// f.c
#include <stdio.h>
int a = 5;
void f() { printf("in f: a = %d\n", a); a ++; }

gcc main.c f.c # linking error: multiple definition of 'a'
gcc main.c f.c -fcommon && ./a.out