$ cat /etc/os-release | head -2
PRETTY_NAME="Ubuntu 22.04.5 LTS"
NAME="Ubuntu"

$ uname -m
x86_64

$ gcc --version
gcc (Ubuntu 11.4.0-1ubuntu1~22.04.3) 11.4.0
Copyright (C) 2021 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ gcc -dumpversion
11
$ gcc -dumpmachine
x86_64-linux-gnu

$ ls -l /usr/bin/gcc /usr/bin/cc
lrwxrwxrwx 1 root root 20 Oct 17 02:00 /usr/bin/cc -> /etc/alternatives/cc
lrwxrwxrwx 1 root root  6 Aug  5  2021 /usr/bin/gcc -> gcc-11

$ dpkg -l | grep -E "^ii +(gcc|build-essential|binutils) "
ii  binutils                  2.38-4ubuntu2.12             amd64  GNU assembler, linker and binary utilities
ii  build-essential           12.9ubuntu3                  amd64  Informational list of build-essential packages
ii  gcc                       4:11.2.0-1ubuntu1            amd64  GNU C compiler

$ cat hello.c
#include <stdio.h>

int main(void)
{
    printf("hello, gcc!\n");
    return 0;
}

$ gcc hello.c          # 不带 -o，默认输出 a.out

$ ls -l a.out
-rwxr-xr-x 1 root root 15960 Oct 17 02:14 a.out

$ ./a.out
hello, gcc!

$ gcc hello.c -o hello
$ ./hello
hello, gcc!

$ gcc hello.c -o /tmp/gccdemo/basic/hello2    # -o 可写绝对路径
$ ls -l /tmp/gccdemo/basic/hello2
-rwxr-xr-x 1 root root 15960 Oct 17 02:14 /tmp/gccdemo/basic/hello2

$ gcc -v hello.c -o hello3
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/11/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 11.4.0-1ubuntu1~22.04.3' ...
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 11.4.0 (Ubuntu 11.4.0-1ubuntu1~22.04.3)
 /usr/lib/gcc/x86_64-linux-gnu/11/cc1 -quiet -v ... -o /tmp/ccEYmjhb.s
GNU C17 (Ubuntu 11.4.0-1ubuntu1~22.04.3) version 11.4.0 (x86_64-linux-gnu)
#include "..." search starts here:
#include <...> search starts here:
 /usr/lib/gcc/x86_64-linux-gnu/11/include
 /usr/local/include
 /usr/include/x86_64-linux-gnu
 /usr/include
End of search list.
 as -v --64 -o /tmp/cckViodD.o /tmp/ccEYmjhb.s
COMPILER_PATH=/usr/lib/gcc/x86_64-linux-gnu/11/:...
LIBRARY_PATH=/usr/lib/gcc/x86_64-linux-gnu/11/:...
COLLECT_GCC_OPTIONS='-v' '-o' 'hello3' '-mtune=generic' '-march=x86-64'

$ cat util.h
#ifndef UTIL_H
#define UTIL_H

int add(int a, int b);
long fact(int n);

#endif

$ cat util.c
#include "util.h"

int add(int a, int b)
{
    return a + b;
}

long fact(int n)
{
    long r = 1;
    while (n > 1)
        r *= n--;
    return r;
}

$ cat main.c
#include <stdio.h>
#include "util.h"

int main(void)
{
    printf("add(3,4)=%d, fact(5)=%ld\n", add(3, 4), fact(5));
    return 0;
}

$ gcc -c util.c          # 生成 util.o
$ gcc -c main.c          # 生成 main.o
$ gcc -c main.c -o main_obj.o   # -c 也能配合 -o 改名
$ ls -l *.o
-rw-r--r-- 1 root root 1360 Oct 17 02:14 main.o
-rw-r--r-- 1 root root 1296 Oct 17 02:14 main_obj.o
-rw-r--r-- 1 root root 1376 Oct 17 02:14 util.o

$ gcc main.o util.o -o app      # 链接 .o 成可执行
$ ./app
add(3,4)=5, fact(5)=120

$ gcc main.c util.c -o app_all  # 一步到位：源文件全部交给 gcc
$ ./app_all
add(3,4)=5, fact(5)=120

$ gcc -c util.c main.c          # -c 一次接多个源文件
$ ls -l *.o

int neg(int x)
{
    if (x < 0)
        return -x;      /* x>=0 时无返回值 */
}

static int helper(int flags, int mode)
{
    return flags;       /* mode 未使用 */
}

int main(void)
{
    int unused_var = 42;
    int overflow = 2147483647 + 1;
    printf("%d %d\n", neg(-5), helper(1, 2));
    return 0;
}

$ gcc warn.c -o warn_none
warn.c:17:31: warning: integer overflow in expression of type 'int' results in '-2147483648' [-Woverflow]

$ gcc -Wall warn.c -o warn_wall
warn.c:17:31: warning: integer overflow ... [-Woverflow]
warn.c:17:9:  warning: unused variable 'overflow' [-Wunused-variable]
warn.c:16:9:  warning: unused variable 'unused_var' [-Wunused-variable]
warn.c:7:1:   warning: control reaches end of non-void function [-Wreturn-type]

$ gcc -Wall -Wextra warn.c -o warn_both
warn.c:9:34: warning: unused parameter 'mode' [-Wunused-parameter]
   （-Wextra 补上了未使用的函数参数，共 5 条）

$ gcc -Wall -Werror warn.c -o warn_err
warn.c:17:31: error: integer overflow ... [-Werror=overflow]
...
cc1: all warnings being treated as errors

$ gcc -w warn.c -o warn_w     # -w 关闭一切警告，回显干净

$ gcc -O0 loop.c -o loop_O0    # 不优化（默认即 -O0）
$ gcc -O1 loop.c -o loop_O1
$ gcc -O2 loop.c -o loop_O2    # 发布常用档
$ gcc -O3 loop.c -o loop_O3    # 激进优化+向量化
$ gcc -Os loop.c -o loop_Os    # 按 -O2 收缩体积
$ gcc -Og loop.c -o loop_Og    # 优化但保持可调试
$ ls -l loop_O0 loop_O1 loop_O2 loop_O3 loop_Os loop_Og
-rwxr-xr-x 1 root root 15960 Oct 17 02:14 loop_O0
-rwxr-xr-x 1 root root 15968 Oct 17 02:14 loop_O1
-rwxr-xr-x 1 root root 15968 Oct 17 02:14 loop_O2
-rwxr-xr-x 1 root root 15968 Oct 17 02:14 loop_O3
-rwxr-xr-x 1 root root 15968 Oct 17 02:14 loop_Og
-rwxr-xr-x 1 root root 15968 Oct 17 02:14 loop_Os

$ time ./loop_O0 >/dev/null
real    0m0.274s

$ time ./loop_O1 >/dev/null
real    0m0.039s

$ time ./loop_O2 >/dev/null
real    0m0.001s

$ gcc -O0 -g loop.c -o loop_O0g
$ gcc -O2 -g loop.c -o loop_O2g
$ ls -l loop_O0 loop_O0g loop_O2 loop_O2g
-rwxr-xr-x 1 root root 15960 Oct 17 02:14 loop_O0
-rwxr-xr-x 1 root root 17232 Oct 17 02:14 loop_O0g   # -g 增加 DWARF 调试段
-rwxr-xr-x 1 root root 15968 Oct 17 02:14 loop_O2
-rwxr-xr-x 1 root root 17784 Oct 17 02:14 loop_O2g

$ cat modern.c
#include <stdio.h>

int main(void)
{
    int total = 0;
    for (int i = 1; i <= 10; i++)    /* C99 起允许在 for 里声明变量 */
        total += i;
    printf("total=%d\n", total);
    return 0;
}

$ gcc -std=c89 modern.c -o m89
modern.c:6:5: error: 'for' loop initial declarations are only allowed in C99 or C11 mode
    6 |     for (int i = 1; i <= 10; i++)
      |     ^~~
modern.c:6:5: note: use option '-std=c99', '-std=gnu99', '-std=c11' or '-std=gnu11' to compile your code

$ gcc -std=c99 modern.c -o m99 && ./m99
total=55

$ gcc -std=c11 modern.c -o m11
$ gcc -std=c17 modern.c -o m17

$ gcc -dM -E - </dev/null | grep -E "__STDC_VERSION__|__GNUC__ |__GNUC_MINOR__"
#define __GNUC__ 11
#define __STDC_VERSION__ 201710L
#define __GNUC_MINOR__ 4

$ gcc -std=c11 -dM -E - </dev/null | grep __STDC_VERSION__
#define __STDC_VERSION__ 201112L

$ tree -L2 . 2>/dev/null || find . -type f | sort
./include/mymath.h
./src/main.c
./mymath.c

$ cat src/main.c
#include <stdio.h>
#include <mymath.h>
...

$ gcc src/main.c -o app
src/main.c:2:10: fatal error: mymath.h: No such file or directory
    2 | #include <mymath.h>
      |          ^~~~~~~~~~
compilation terminated.

$ gcc -Iinclude src/main.c -o app
/usr/bin/ld: /tmp/ccKoEegX.o: in function `main':
main.c:(.text+0x15): undefined reference to `circle_area'
collect2: error: ld returned 1 exit status

$ gcc -Iinclude src/main.c mymath.c -o app_full
$ ./app_full
area=12.5664

$ gcc -Iinclude -v src/main.c -o app 2>&1 | sed -n "/#include <...>/,/End of search/p"
#include <...> search starts here:
 include
 /usr/lib/gcc/x86_64-linux-gnu/11/include
 /usr/local/include
 /usr/include/x86_64-linux-gnu
 /usr/include
End of search list.

$ cat mymath.h
double circle_area(double r);

$ cat mymath.c
#include "mymath.h"

double circle_area(double r)
{
    return 3.141592653589793 * r * r;
}

$ gcc -c mymath.c
$ ar rcs libmylib.a mymath.o      # r 替换/插入、c 创建、s 建索引
$ ls -l libmylib.a mymath.o
-rw-r--r-- 1 root root 1800 Oct 17 02:14 libmylib.a
-rw-r--r--r-- 1 root root 1344 Oct 17 02:14 mymath.o

$ cat main.c
#include <stdio.h>
#include "mymath.h"

int main(void)
{
    printf("area=%.4f\n", circle_area(2.0));
    return 0;
}

$ gcc main.c -lmylib -o app_noflag        # 不给 -L，默认路径找不到 libmylib.a
/usr/bin/ld: cannot find -lmylib
collect2: error: ld returned 1 exit status

$ gcc main.c -L. -lmylib -o app           # -L. 把当前目录加入库搜索路径
$ ./app
area=12.5664

$ gcc -L. -lmylib main.c -o app_bad       # 库在源文件之前
/usr/bin/ld: main.c:(.text+0x15): undefined reference to `circle_area'

$ gcc -fPIC -shared mymath.c -o libmymath.so   # 位置无关代码 + 共享对象
$ ls -l libmymath.so
-rwxr-xr-x 1 root root 15216 Oct 17 02:14 libmymath.so

$ gcc main.c -L. -lmymath -o app
$ ./app
./app: error while loading shared libraries: libmymath.so: cannot open shared object file: No such file or directory

$ ldd app
        linux-vdso.so.1 (0x00007ffcae49d000)
        libmymath.so => not found
        libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007d927af9f000)

# 解法一：LD_LIBRARY_PATH 环境变量（临时）
$ LD_LIBRARY_PATH=/tmp/gccdemo/shlib ./app
area=12.5664

# 解法二：编译期烧写 rpath（永久）
$ gcc main.c -L. -lmymath -Wl,-rpath=/tmp/gccdemo/shlib -o app_rpath
$ ./app_rpath
area=12.5664

$ ldd app_rpath
        libmymath.so => /tmp/gccdemo/shlib/libmymath.so (0x000071e6e45c7000)

$ readelf -d app | grep -E "RPATH|RUNPATH|NEEDED"
 0x0000000000000001 (NEEDED)             Shared library: [libmymath.so]

$ gcc hello.c -o hello_dyn
$ gcc -static hello.c -o hello_static

$ ls -l hello_dyn hello_static
-rwxr-xr-x 1 root root  15960 Oct 17 02:14 hello_dyn
-rwxr-xr-x 1 root root 900368 Oct 17 02:14 hello_static    # 体积 ×57

$ ./hello_static
hello, static link!

$ ldd hello_dyn
        libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x0000761d75fc5000)

$ ldd hello_static
        not a dynamic executable

$ cat thread.c
#include <pthread.h>
#include <stdio.h>

static void *work(void *arg)
{
    printf("thread %d running\n", *(int *)arg);
    return NULL;
}

int main(void)
{
    pthread_t t1, t2;
    int a = 1, b = 2;
    pthread_create(&t1, NULL, work, &a);
    pthread_create(&t2, NULL, work, &b);
    pthread_join(t1, NULL);
    pthread_join(t2, NULL);
    puts("all threads done");
    return 0;
}

$ gcc thread.c -pthread -o t_pthread
$ ./t_pthread
thread 1 running
thread 2 running
all threads done

$ ls -l /usr/lib/x86_64-linux-gnu/libpthread*
-rw-r--r-- 1 root root     8 Sep  3 14:20 /usr/lib/x86_64-linux-gnu/libpthread.a
-rw-r--r-- 1 root root 21448 Sep  3 14:20 /usr/lib/x86_64-linux-gnu/libpthread.so.0

$ ldd t_pthread
        libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x0000765710fb4000)

$ cat version.c
#include <stdio.h>

int main(void)
{
#if VER >= 3
    puts("new path");
#else
    puts("legacy path");
#endif
#ifdef DEBUG
    puts("[debug] build with DEBUG");
#endif
    return 0;
}

$ gcc -DVER=1 version.c -o v1 && ./v1
ver=1 (legacy path)

$ gcc -DVER=1 -DDEBUG version.c -o v2 && ./v2
[debug] build with DEBUG

$ gcc -DVER=3 -DDEBUG version.c -o v3 && ./v3
[debug] build with DEBUG
ver=3 (new path)

$ gcc -DAPP_NAME=\"build-server\" msg.c -o msg && ./msg    # 字符串宏：内嵌引号
app=build-server

$ gcc -dM -E - </dev/null | grep -E "__linux|__x86_64|__SIZEOF_LONG__"
#define __linux 1
#define __unix 1
#define __SIZEOF_LONG__ 8
#define __x86_64 1

$ gcc -dM -E - </dev/null | grep -c "#define"
367

$ cat sum.c
#include <stdio.h>
#define N 5

int sum_to(int n)
{
    int s = 0;
    for (int i = 1; i <= n; i++)
        s += i;
    return s;
}

int main(void)
{
    printf("sum1..%d=%d\n", N, sum_to(N));
    return 0;
}

# 阶段一：-E 只做预处理，输出展开后的纯 C
$ gcc -E sum.c -o sum.i
$ ls -l sum.i
-rw-r--r-- 1 root root 18076 Oct 17 02:14 sum.i

$ tail -14 sum.i
# 5 "sum.c"
int sum_to(int n)
{
    int s = 0;
    for (int i = 1; i <= n; i++)
        s += i;
    return s;
}

int main(void)
{
    printf("sum1..%d=%d\n", 5, sum_to(5));   /* N 已替换为 5 */
    return 0;
}

$ grep -c "extern" sum.i
93    /* 头文件 stdio.h 全量展开，可见 -E 产物之大 */

# 阶段二：-S 生成汇编
$ gcc -S sum.c -o sum.s
$ head -20 sum.s
        .file   "sum.c"
        .text
        .globl  sum_to
        .type   sum_to, @function
sum_to:
.LFB0:
        .cfi_startproc
        endbr64
        pushq   %rbp
        movq    %rsp, %rbp
        movl    %edi, -20(%rbp)
        movl    $0, -8(%rbp)
        movl    $1, -4(%rbp)
        ...

$ gcc -S -O2 sum.c -o sum_O2.s    # -O2 后循环被改写，main 直接内联常量 15
$ grep -A2 "movl.*\$15" sum_O2.s | head -3
        movl    $15, %ecx

# 阶段三：-c 生成目标文件
$ gcc -c sum.c -o sum.o && file sum.o
sum.o: ELF 64-bit LSB relocatable, x86-64, version 1 (SYSV), not stripped

$ gcc -save-temps sum.c -o sum
$ ls sum.*
sum  sum.i  sum.o  sum.s

$ cat release.rsp
-Wall
-Wextra
-O2
-o app_release

$ gcc @release.rsp main.c util.c
$ ./app_release
add(2,3)=5

$ gcc @release.rsp -o app_single main.c    # rsp 里 -o 与命令行可混用
/usr/bin/ld: /tmp/ccgYLZUL.o: in function `main':
main.c:(.text.startup+0x13): undefined reference to `add'
collect2: error: ld returned 1 exit status

$ cat syntax.c
#include <stdio.h>

int main(void)
{
    printf("oops\n")          /* 漏分号 */
    return 0;
}

$ gcc syntax.c -o syntax
syntax.c:5:21: error: expected ';' before 'return'
    5 |     printf("oops\n")
      |                     ^
      |                     ;

$ cat undecl.c
int main(void)
{
    x = 3;
    return 0;
}

$ gcc undecl.c -o undecl
undecl.c:3:5: error: 'x' undeclared (first use in this function)
    3 |     x = 3;
      |     ^

$ cat linkerr.c
extern void missing_fn(int);

int main(void)
{
    missing_fn(1);
    return 0;
}

$ gcc linkerr.c -o linkerr
/usr/bin/ld: /tmp/ccswl83u.o: in function `main':
linkerr.c:(.text+0x9): undefined reference to `missing_fn'
collect2: error: ld returned 1 exit status

$ gcc nofile.c -o nofile
cc1: fatal error: nofile.c: No such file or directory

$ gcc
gcc: fatal error: no input files
compilation terminated.