从暂停到理解：GDB 调试完整实践

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{
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  description = "GDB + C++20 debugging playground";
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  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
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  outputs = { nixpkgs, ... }:
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    let
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      systems = [ "x86_64-linux" "aarch64-linux" ];
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      forEachSystem = nixpkgs.lib.genAttrs systems;
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    in {
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      devShells = forEachSystem (system:
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        let pkgs = nixpkgs.legacyPackages.${system};
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        in {
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          default = pkgs.mkShell {
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            # mkShell 自带 stdenv 的 gcc/g++、objdump 和 nm，这里只需要补上 gdb 和 gdbserver。
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            packages = [ pkgs.gdb ];
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            # 关闭 cc-wrapper 默认注入的加固参数（栈保护、FORTIFY、调用后清零寄存器等），
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            # 让反汇编更接近普通发行版上 GCC 的输出。
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            hardeningDisable = [ "all" ];
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            # Nixpkgs 的 gdb 打过补丁，会从这个变量寻找分离的调试符号，这里给 glibc 用。
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            NIX_DEBUG_INFO_DIRS = "${pkgs.glibc.debug}/lib/debug";
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            shellHook = ''
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              g++ --version | head -n 1
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              gdb --version | head -n 1
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            '';
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          };
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        });
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    };
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}

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$ NIX_DEBUG=1 g++ -std=c++20 -g3 -O0 -c gdb_demo.cpp -o /dev/null 2>&1 | sed -n '/extra flags before/,/extra flags after/p'
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extra flags before to /nix/store/8sjgd7q3mdks1rb7rbcv0sallhvrhai5-gcc-15.3.0/bin/g++:
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  -fPIC
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  -fstack-clash-protection
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  -O2
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  -U_FORTIFY_SOURCE
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  -D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_FAST
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  -Wformat
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  -Wformat-security
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  -Werror=format-security
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  -fzero-call-used-regs=used-gpr
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  -fstrict-flex-arrays=1
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  -fstack-protector-strong
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  --param
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  ssp-buffer-size=4
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  -fno-strict-overflow
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  -fno-omit-frame-pointer
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  -mno-omit-leaf-frame-pointer
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  -mtls-dialect=gnu2
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original flags to /nix/store/8sjgd7q3mdks1rb7rbcv0sallhvrhai5-gcc-15.3.0/bin/g++:
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  -std=c++20
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  -g3
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  -O0
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  -c
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  gdb_demo.cpp
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  -o
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  /dev/null
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extra flags after to /nix/store/8sjgd7q3mdks1rb7rbcv0sallhvrhai5-gcc-15.3.0/bin/g++:

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(gdb) thread 1
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[Switching to thread 1 (Thread 0x7ffff7e90780 (LWP 165718))]
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#0  __syscall_cancel_arch () at ../sysdeps/unix/sysv/linux/x86_64/syscall_cancel.S:56
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warning: 56  ../sysdeps/unix/sysv/linux/x86_64/syscall_cancel.S: No such file or directory
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(gdb) bt
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#0  __syscall_cancel_arch () at ../sysdeps/unix/sysv/linux/x86_64/syscall_cancel.S:56
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#1  0x00007ffff789912c in __internal_syscall_cancel (a1=<optimized out>, a2=<optimized out>, a3=<optimized out>, a4=<optimized out>, a5=a5@entry=0,
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    a6=a6@entry=4294967295, nr=202) at cancellation.c:49
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#2  0x00007ffff78998ac in __futex_abstimed_wait_common64 (private=128, futex_word=0x7ffff77ff990, expected=<optimized out>, op=<optimized out>, abstime=0x0,
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    cancel=true) at futex-internal.c:57
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#3  __futex_abstimed_wait_common (futex_word=futex_word@entry=0x7ffff77ff990, expected=<optimized out>, clockid=clockid@entry=0, abstime=abstime@entry=0x0,
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    private=private@entry=128, cancel=cancel@entry=true) at futex-internal.c:87
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#4  0x00007ffff789993f in __GI___futex_abstimed_wait_cancelable64 (futex_word=futex_word@entry=0x7ffff77ff990, expected=<optimized out>,
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    clockid=clockid@entry=0, abstime=abstime@entry=0x0, private=private@entry=128) at futex-internal.c:139
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#5  0x00007ffff789ec8c in __pthread_clockjoin_ex (threadid=140737345746624, thread_return=0x0, clockid=0, abstime=0x0, block=<optimized out>)
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    at pthread_join_common.c:108
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#6  0x00007ffff7cf2f1b in std::thread::join() () from /nix/store/604gsr59rj7dzd0nrhp143rpvf7gyiaz-gcc-15.3.0-lib/lib/libstdc++.so.6
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#7  0x00005555555565af in main (argc=1, argv=0x7fffffffb7f8) at gdb_demo.cpp:36

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#include <sys/wait.h>
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#include <unistd.h>
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#include <iostream>
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#include <mutex>
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#include <string>
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#include <thread>
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#include <vector>
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int recursive_sum(const std::vector<int>& values, std::size_t index) {
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    if (index == values.size()) {
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        return 0;
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    }
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    return values[index] + recursive_sum(values, index + 1);
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}
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void worker(int& counter, std::mutex& mutex) {
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    for (int i = 0; i < 1000; ++i) {
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        std::lock_guard<std::mutex> lock(mutex);
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        ++counter;
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    }
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}
23

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int main(int argc, char* argv[]) {
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    const std::string mode = argc > 1 ? argv[1] : "";
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    std::vector<int> values{3, 5, 7, 9};
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    const int total = recursive_sum(values, 0);
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    const double average = static_cast<double>(total) / (values.size() - 1);
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    std::cout << "total=" << total << ", average=" << average
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              << ", expected_average=6\n";
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    int counter = 0;
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    std::mutex mutex;
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    std::thread first(worker, std::ref(counter), std::ref(mutex));
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    std::thread second(worker, std::ref(counter), std::ref(mutex));
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    first.join();
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    second.join();
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    std::cout << "counter=" << counter << "\n";
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    if (mode == "--throw") {
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        std::cout << values.at(values.size()) << "\n";
42
    }
43

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    if (mode == "--fork") {
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        const pid_t pid = fork();
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        if (pid == 0) {
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            std::cout << "child: pid=" << getpid() << "\n";
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            return 0;
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        }
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        waitpid(pid, nullptr, 0);
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        std::cout << "parent: child " << pid << " exited\n";
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    }
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    if (mode == "--crash") {
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        int* pointer = nullptr;
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        std::cout << "about to dereference a null pointer\n";
57
        std::cout << *pointer << "\n";
58
    }
59
}

1
$ ./gdb_demo
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total=24, average=8, expected_average=6
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counter=2000
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$ ./gdb_demo --crash
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total=24, average=8, expected_average=6
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counter=2000
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about to dereference a null pointer
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Segmentation fault         (core dumped) ./gdb_demo --crash
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$ echo $?
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139
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$ ./gdb_demo --throw
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total=24, average=8, expected_average=6
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counter=2000
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terminate called after throwing an instance of 'std::out_of_range'
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  what():  vector::_M_range_check: __n (which is 4) >= this->size() (which is 4)
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Aborted                    (core dumped) ./gdb_demo --throw
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$ ./gdb_demo --fork
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total=24, average=8, expected_average=6
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counter=2000
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child: pid=170023
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parent: child 170023 exited
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$ ./gdb_demo --crash | cat
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$ nm gdb_demo_stripped
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nm: gdb_demo_stripped: no symbols

1
$ gdb -nx ./gdb_demo
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GNU gdb (GDB) 17.2
3
Copyright (C) 2025 Free Software Foundation, Inc.
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License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
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This is free software: you are free to change and redistribute it.
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There is NO WARRANTY, to the extent permitted by law.
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Type "show copying" and "show warranty" for details.
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This GDB was configured as "x86_64-unknown-linux-gnu".
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Type "show configuration" for configuration details.
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For bug reporting instructions, please see:
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<https://www.gnu.org/software/gdb/bugs/>.
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Find the GDB manual and other documentation resources online at:
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    <http://www.gnu.org/software/gdb/documentation/>.
14

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For help, type "help".
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Type "apropos word" to search for commands related to "word"...
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Reading symbols from ./gdb_demo...

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$ gdb -q -nx ./gdb_demo
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Reading symbols from ./gdb_demo...
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(gdb) break main
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Breakpoint 1 at 0x23ab: file gdb_demo.cpp, line 25.
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(gdb) break recursive_sum
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Breakpoint 2 at 0x22da: file gdb_demo.cpp, line 11.
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(gdb) break gdb_demo.cpp:14 if index == 2
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Breakpoint 3 at 0x22f8: file gdb_demo.cpp, line 14.
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(gdb) tbreak gdb_demo.cpp:38
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Temporary breakpoint 4 at 0x25be: file gdb_demo.cpp, line 38.
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(gdb) rbreak ^worker
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Breakpoint 5 at 0x233c: file gdb_demo.cpp, line 18.
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void worker(int&, std::mutex&);
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Successfully created breakpoint 5.
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(gdb) info breakpoints
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Num     Type           Disp Enb Address            What
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1       breakpoint     keep y   0x00000000000023ab in main(int, char**) at gdb_demo.cpp:25
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2       breakpoint     keep y   0x00000000000022da in recursive_sum(std::vector<int, std::allocator<int> > const&, unsigned long) at gdb_demo.cpp:11
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3       breakpoint     keep y   0x00000000000022f8 in recursive_sum(std::vector<int, std::allocator<int> > const&, unsigned long) at gdb_demo.cpp:14
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  stop only if index == 2
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4       breakpoint     del  y   0x00000000000025be in main(int, char**) at gdb_demo.cpp:38
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5       breakpoint     keep y   0x000000000000233c in worker(int&, std::mutex&) at gdb_demo.cpp:18
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(gdb) disable 2 4

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(gdb) run
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Starting program: /tmp/gdb-post/gdb_demo
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[Thread debugging using libthread_db enabled]
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Using host libthread_db library "/nix/store/lm3pknxi0ipypy3lxh1wmm8wvvavdwrn-glibc-2.42-84/lib/libthread_db.so.1".
5

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Breakpoint 1, main (argc=1, argv=0x7fffffffb7f8) at gdb_demo.cpp:25
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25      const std::string mode = argc > 1 ? argv[1] : "";
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(gdb) continue
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Continuing.
10

11
Breakpoint 3, recursive_sum (values=std::vector of length 4, capacity 4 = {...}, index=2) at gdb_demo.cpp:14
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14      return values[index] + recursive_sum(values, index + 1);
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(gdb) print index
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$1 = 2
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(gdb) info breakpoints
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Num     Type           Disp Enb Address            What
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1       breakpoint     keep y   0x00005555555563ab in main(int, char**) at gdb_demo.cpp:25
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  breakpoint already hit 1 time
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2       breakpoint     keep n   0x00005555555562da in recursive_sum(std::vector<int, std::allocator<int> > const&, unsigned long) at gdb_demo.cpp:11
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3       breakpoint     keep y   0x00005555555562f8 in recursive_sum(std::vector<int, std::allocator<int> > const&, unsigned long) at gdb_demo.cpp:14
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  stop only if index == 2
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  breakpoint already hit 1 time
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4       breakpoint     del  n   0x00005555555565be in main(int, char**) at gdb_demo.cpp:38
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5       breakpoint     keep y   0x000055555555633c in worker(int&, std::mutex&) at gdb_demo.cpp:18

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(gdb) delete 3
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(gdb) continue
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Continuing.
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total=24, average=8, expected_average=6
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[New Thread 0x7ffff77ff6c0 (LWP 154927)]
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[Switching to Thread 0x7ffff77ff6c0 (LWP 154927)]
7

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Thread 2 "gdb_demo" hit Breakpoint 5, worker (counter=@0x7fffffffb60c: 0, mutex=...) at gdb_demo.cpp:18
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18      for (int i = 0; i < 1000; ++i) {
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(gdb) continue
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Continuing.
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[New Thread 0x7ffff6ffe6c0 (LWP 154928)]
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[Switching to Thread 0x7ffff6ffe6c0 (LWP 154928)]
14

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Thread 3 "gdb_demo" hit Breakpoint 5, worker (counter=@0x7fffffffb60c: 0, mutex=...) at gdb_demo.cpp:18
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18      for (int i = 0; i < 1000; ++i) {
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(gdb) info breakpoints
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Num     Type           Disp Enb Address            What
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1       breakpoint     keep y   0x00005555555563ab in main(int, char**) at gdb_demo.cpp:25
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  breakpoint already hit 1 time
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2       breakpoint     keep n   0x00005555555562da in recursive_sum(std::vector<int, std::allocator<int> > const&, unsigned long) at gdb_demo.cpp:11
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4       breakpoint     del  n   0x00005555555565be in main(int, char**) at gdb_demo.cpp:38
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5       breakpoint     keep y   0x000055555555633c in worker(int&, std::mutex&) at gdb_demo.cpp:18
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  breakpoint already hit 2 times

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$ gdb -q -nx ./gdb_demo
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Reading symbols from ./gdb_demo...
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(gdb) start
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Temporary breakpoint 1 at 0x23ab: file gdb_demo.cpp, line 25.
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Starting program: /tmp/gdb-post/gdb_demo
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[Thread debugging using libthread_db enabled]
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Using host libthread_db library "/nix/store/lm3pknxi0ipypy3lxh1wmm8wvvavdwrn-glibc-2.42-84/lib/libthread_db.so.1".
8

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Temporary breakpoint 1, main (argc=1, argv=0x7fffffffb7f8) at gdb_demo.cpp:25
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25      const std::string mode = argc > 1 ? argv[1] : "";
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(gdb) advance 29
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main (argc=1, argv=0x7fffffffb7f8) at gdb_demo.cpp:29
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29      std::cout << "total=" << total << ", average=" << average
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(gdb) until 32
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total=24, average=8, expected_average=6
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main (argc=1, argv=0x7fffffffb7f8) at gdb_demo.cpp:32
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32      int counter = 0;

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$ gdb -q -nx ./gdb_demo
2
Reading symbols from ./gdb_demo...
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(gdb) break recursive_sum if index == 2
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Breakpoint 1 at 0x22da: file gdb_demo.cpp, line 11.
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(gdb) run
6
Starting program: /tmp/gdb-post/gdb_demo
7
[Thread debugging using libthread_db enabled]
8
Using host libthread_db library "/nix/store/lm3pknxi0ipypy3lxh1wmm8wvvavdwrn-glibc-2.42-84/lib/libthread_db.so.1".
9

10
Breakpoint 1, recursive_sum (values=std::vector of length 4, capacity 4 = {...}, index=2) at gdb_demo.cpp:11
11
11      if (index == values.size()) {
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(gdb) return 100
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Make recursive_sum(std::vector<int, std::allocator<int> > const&, unsigned long) return now? (y or n) y
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#0  0x0000555555556324 in recursive_sum (values=std::vector of length 4, capacity 4 = {...}, index=1) at gdb_demo.cpp:14
15
14      return values[index] + recursive_sum(values, index + 1);
16
(gdb) finish
17
Run till exit from #0  0x0000555555556324 in recursive_sum (values=std::vector of length 4, capacity 4 = {...}, index=1) at gdb_demo.cpp:14
18
0x0000555555556324 in recursive_sum (values=std::vector of length 4, capacity 4 = {...}, index=0) at gdb_demo.cpp:14
19
14      return values[index] + recursive_sum(values, index + 1);
20
Value returned is $1 = 105
21
(gdb) finish
22
Run till exit from #0  0x0000555555556324 in recursive_sum (values=std::vector of length 4, capacity 4 = {...}, index=0) at gdb_demo.cpp:14
23
0x000055555555642c in main (argc=1, argv=0x7fffffffb7f8) at gdb_demo.cpp:27
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27      const int total = recursive_sum(values, 0);
25
Value returned is $2 = 108
26
(gdb) next
27
28      const double average = static_cast<double>(total) / (values.size() - 1);
28
(gdb) print total
29
$3 = 108
30
(gdb) jump 32
31
Continuing at 0x5555555564f4.
32
[New Thread 0x7ffff77ff6c0 (LWP 156151)]
33
[New Thread 0x7ffff6ffe6c0 (LWP 156152)]
34
[Thread 0x7ffff77ff6c0 (LWP 156151) exited]
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counter=2000
36
[Thread 0x7ffff6ffe6c0 (LWP 156152) exited]
37
[Inferior 1 (process 156113) exited normally]

1
$ gdb -q -nx ./gdb_demo
2
Reading symbols from ./gdb_demo...
3
(gdb) break gdb_demo.cpp:12
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Breakpoint 1 at 0x22f1: file gdb_demo.cpp, line 12.
5
(gdb) run
6
Starting program: /tmp/gdb-post/gdb_demo
7
[Thread debugging using libthread_db enabled]
8
Using host libthread_db library "/nix/store/lm3pknxi0ipypy3lxh1wmm8wvvavdwrn-glibc-2.42-84/lib/libthread_db.so.1".
9

10
Breakpoint 1, recursive_sum (values=std::vector of length 4, capacity 4 = {...}, index=4) at gdb_demo.cpp:12
11
12          return 0;
12
(gdb) bt
13
#0  recursive_sum (values=std::vector of length 4, capacity 4 = {...}, index=4) at gdb_demo.cpp:12
14
#1  0x0000555555556324 in recursive_sum (values=std::vector of length 4, capacity 4 = {...}, index=3) at gdb_demo.cpp:14
15
#2  0x0000555555556324 in recursive_sum (values=std::vector of length 4, capacity 4 = {...}, index=2) at gdb_demo.cpp:14
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#3  0x0000555555556324 in recursive_sum (values=std::vector of length 4, capacity 4 = {...}, index=1) at gdb_demo.cpp:14
17
#4  0x0000555555556324 in recursive_sum (values=std::vector of length 4, capacity 4 = {...}, index=0) at gdb_demo.cpp:14
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#5  0x000055555555642c in main (argc=1, argv=0x7fffffffb7f8) at gdb_demo.cpp:27

1
(gdb) frame 2
2
#2  0x0000555555556324 in recursive_sum (values=std::vector of length 4, capacity 4 = {...}, index=2) at gdb_demo.cpp:14
3
14      return values[index] + recursive_sum(values, index + 1);
4
(gdb) print index
5
$1 = 2
6
(gdb) info args
7
values = std::vector of length 4, capacity 4 = {3, 5, 7, 9}
8
index = 2
9
(gdb) info locals
10
No locals.
11
(gdb) up
12
#3  0x0000555555556324 in recursive_sum (values=std::vector of length 4, capacity 4 = {...}, index=1) at gdb_demo.cpp:14
13
14      return values[index] + recursive_sum(values, index + 1);
14
(gdb) down
15
#2  0x0000555555556324 in recursive_sum (values=std::vector of length 4, capacity 4 = {...}, index=2) at gdb_demo.cpp:14
16
14      return values[index] + recursive_sum(values, index + 1);

1
(gdb) frame 0
2
#0  recursive_sum (values=std::vector of length 4, capacity 4 = {...}, index=4) at gdb_demo.cpp:12
3
12          return 0;
4
(gdb) finish
5
Run till exit from #0  recursive_sum (values=std::vector of length 4, capacity 4 = {...}, index=4) at gdb_demo.cpp:12
6
0x0000555555556324 in recursive_sum (values=std::vector of length 4, capacity 4 = {...}, index=3) at gdb_demo.cpp:14
7
14      return values[index] + recursive_sum(values, index + 1);
8
Value returned is $2 = 0
9
(gdb) finish
10
Run till exit from #0  0x0000555555556324 in recursive_sum (values=std::vector of length 4, capacity 4 = {...}, index=3) at gdb_demo.cpp:14
11
0x0000555555556324 in recursive_sum (values=std::vector of length 4, capacity 4 = {...}, index=2) at gdb_demo.cpp:14
12
14      return values[index] + recursive_sum(values, index + 1);
13
Value returned is $3 = 9
14
(gdb) finish
15
Run till exit from #0  0x0000555555556324 in recursive_sum (values=std::vector of length 4, capacity 4 = {...}, index=2) at gdb_demo.cpp:14
16
0x0000555555556324 in recursive_sum (values=std::vector of length 4, capacity 4 = {...}, index=1) at gdb_demo.cpp:14
17
14      return values[index] + recursive_sum(values, index + 1);
18
Value returned is $4 = 16

1
$ gdb -q -nx ./gdb_demo
2
Reading symbols from ./gdb_demo...
3
(gdb) break 29
4
Breakpoint 1 at 0x248d: file gdb_demo.cpp, line 29.
5
(gdb) run
6
Starting program: /tmp/gdb-post/gdb_demo
7
[Thread debugging using libthread_db enabled]
8
Using host libthread_db library "/nix/store/lm3pknxi0ipypy3lxh1wmm8wvvavdwrn-glibc-2.42-84/lib/libthread_db.so.1".
9

10
Breakpoint 1, main (argc=1, argv=0x7fffffffb7f8) at gdb_demo.cpp:29
11
29      std::cout << "total=" << total << ", average=" << average
12
(gdb) print total
13
$1 = 24
14
(gdb) print average
15
$2 = 8
16
(gdb) print $
17
$3 = 8
18
(gdb) print $1 + 1
19
$4 = 25
20
(gdb) print/x total
21
$5 = 0x18
22
(gdb) print/t total
23
$6 = 11000
24
(gdb) whatis values
25
type = std::vector<int>
26
(gdb) whatis values[0]
27
type = int
28
(gdb) ptype recursive_sum
29
type = int (const std::vector<int> &, std::size_t)
30
(gdb) print values[2]
31
$7 = 7
32
(gdb) print values.size()
33
$8 = 4
34
(gdb) print values.data()
35
Cannot evaluate function -- may be inlined
36
(gdb) print (double) total / values.size()
37
$9 = 6

1
(gdb) display total
2
1: total = 24
3
(gdb) display/x counter
4
2: /x counter = 0xffffffff
5
(gdb) next
6
30                << ", expected_average=6\n";
7
1: total = 24
8
2: /x counter = 0xffffffff
9
(gdb) info display
10
Auto-display expressions now in effect:
11
Num Enb Expression
12
1:   y  total
13
2:   y  /x counter
14
(gdb) undisplay 2
15
(gdb) next
16
total=24, average=8, expected_average=6
17
32      int counter = 0;
18
1: total = 24

1
(gdb) x/4dw &values[0]
2
0x55555556f320:  3  5  7  9
3
(gdb) x/16xb &values[0]
4
0x55555556f320:  0x03  0x00  0x00  0x00  0x05  0x00  0x00  0x00
5
0x55555556f328:  0x07  0x00  0x00  0x00  0x09  0x00  0x00  0x00
6
(gdb) x/2xg &values[0]
7
0x55555556f320:  0x0000000500000003  0x0000000900000007
8
(gdb) x/s argv[0]
9
0x7fffffffbf28:  "/tmp/gdb-post/gdb_demo"
10
(gdb) x/3i $pc
11
=> 0x555555556518 <main(int, char**)+404>:  lea    -0xf0(%rbp),%rax
12
   0x55555555651f <main(int, char**)+411>:  mov    %rax,%rdi
13
   0x555555556522 <main(int, char**)+414>:  call   0x555555557463 <_ZSt3refISt5mutexESt17reference_wrapperIT_ERS2_>
14
(gdb) print $pc
15
$6 = (void (*)(void)) 0x555555556518 <main(int, char**)+404>
16
(gdb) info registers rip rsp rbp
17
rip            0x555555556518      0x555555556518 <main(int, char**)+404>
18
rsp            0x7fffffffb5b0      0x7fffffffb5b0
19
rbp            0x7fffffffb6d0      0x7fffffffb6d0

1
$ gdb -q -nx ./gdb_demo
2
Reading symbols from ./gdb_demo...
3
(gdb) break recursive_sum
4
Breakpoint 1 at 0x22da: file gdb_demo.cpp, line 11.
5
(gdb) run
6
Starting program: /tmp/gdb-post/gdb_demo
7
[Thread debugging using libthread_db enabled]
8
Using host libthread_db library "/nix/store/lm3pknxi0ipypy3lxh1wmm8wvvavdwrn-glibc-2.42-84/lib/libthread_db.so.1".
9

10
Breakpoint 1, recursive_sum (values=std::vector of length 4, capacity 4 = {...}, index=0) at gdb_demo.cpp:11
11
11      if (index == values.size()) {
12
(gdb) disassemble /s recursive_sum
13
Dump of assembler code for function _Z13recursive_sumRKSt6vectorIiSaIiEEm:
14
gdb_demo.cpp:
15
10  int recursive_sum(const std::vector<int>& values, std::size_t index) {
16
   0x00005555555562c9 <+0>:  push   %rbp
17
   0x00005555555562ca <+1>:  mov    %rsp,%rbp
18
   0x00005555555562cd <+4>:  push   %rbx
19
   0x00005555555562ce <+5>:  sub    $0x18,%rsp
20
   0x00005555555562d2 <+9>:  mov    %rdi,-0x18(%rbp)
21
   0x00005555555562d6 <+13>:  mov    %rsi,-0x20(%rbp)
22

23
11      if (index == values.size()) {
24
=> 0x00005555555562da <+17>:  mov    -0x18(%rbp),%rax
25
   0x00005555555562de <+21>:  mov    %rax,%rdi
26
   0x00005555555562e1 <+24>:  call   0x555555556fae <_ZNKSt6vectorIiSaIiEE4sizeEv>
27
   0x00005555555562e6 <+29>:  cmp    %rax,-0x20(%rbp)
28
   0x00005555555562ea <+33>:  sete   %al
29
   0x00005555555562ed <+36>:  test   %al,%al
30
   0x00005555555562ef <+38>:  je     0x5555555562f8 <_Z13recursive_sumRKSt6vectorIiSaIiEEm+47>
31

32
12          return 0;
33
   0x00005555555562f1 <+40>:  mov    $0x0,%eax
34
   0x00005555555562f6 <+45>:  jmp    0x555555556326 <_Z13recursive_sumRKSt6vectorIiSaIiEEm+93>
35

36
13      }
37
14      return values[index] + recursive_sum(values, index + 1);
38
   0x00005555555562f8 <+47>:  mov    -0x20(%rbp),%rdx
39
   0x00005555555562fc <+51>:  mov    -0x18(%rbp),%rax
40
   0x0000555555556300 <+55>:  mov    %rdx,%rsi
41
   0x0000555555556303 <+58>:  mov    %rax,%rdi
42
   0x0000555555556306 <+61>:  call   0x555555557186 <_ZNKSt6vectorIiSaIiEEixEm>
43
   0x000055555555630b <+66>:  mov    (%rax),%ebx
44
   0x000055555555630d <+68>:  mov    -0x20(%rbp),%rax
45
   0x0000555555556311 <+72>:  lea    0x1(%rax),%rdx
46
   0x0000555555556315 <+76>:  mov    -0x18(%rbp),%rax
47
   0x0000555555556319 <+80>:  mov    %rdx,%rsi
48
   0x000055555555631c <+83>:  mov    %rax,%rdi
49
   0x000055555555631f <+86>:  call   0x5555555562c9 <_Z13recursive_sumRKSt6vectorIiSaIiEEm>
50
   0x0000555555556324 <+91>:  add    %ebx,%eax
51

52
15  }
53
   0x0000555555556326 <+93>:  mov    -0x8(%rbp),%rbx
54
   0x000055555555632a <+97>:  leave
55
   0x000055555555632b <+98>:  ret
56
End of assembler dump.

1
$ gdb -q -nx ./gdb_demo
2
Reading symbols from ./gdb_demo...
3
(gdb) break 29
4
Breakpoint 1 at 0x248d: file gdb_demo.cpp, line 29.
5
(gdb) run
6
Starting program: /tmp/gdb-post/gdb_demo
7
[Thread debugging using libthread_db enabled]
8
Using host libthread_db library "/nix/store/lm3pknxi0ipypy3lxh1wmm8wvvavdwrn-glibc-2.42-84/lib/libthread_db.so.1".
9

10
Breakpoint 1, main (argc=1, argv=0x7fffffffb7f8) at gdb_demo.cpp:29
11
29      std::cout << "total=" << total << ", average=" << average
12
(gdb) print average
13
$1 = 8
14
(gdb) set variable average = 6
15
(gdb) print average
16
$2 = 6
17
(gdb) set $expected = (double) total / values.size()
18
(gdb) print $expected
19
$3 = 6
20
(gdb) print average == $expected
21
$4 = true
22
(gdb) next
23
30                << ", expected_average=6\n";
24
(gdb) next
25
total=24, average=6, expected_average=6
26
32      int counter = 0;
27
(gdb) continue
28
Continuing.
29
[New Thread 0x7ffff77ff6c0 (LWP 158570)]
30
[Thread 0x7ffff77ff6c0 (LWP 158570) exited]
31
[New Thread 0x7ffff6ffe6c0 (LWP 158571)]
32
counter=2000
33
[Thread 0x7ffff6ffe6c0 (LWP 158571) exited]
34
[Inferior 1 (process 158515) exited normally]
35
(gdb) print $_exitcode
36
$5 = 0

1
$ gdb -q -nx ./gdb_demo
2
Reading symbols from ./gdb_demo...
3
(gdb) break 34
4
Breakpoint 1 at 0x2518: file gdb_demo.cpp, line 34.
5
(gdb) run
6
Starting program: /tmp/gdb-post/gdb_demo
7
[Thread debugging using libthread_db enabled]
8
Using host libthread_db library "/nix/store/lm3pknxi0ipypy3lxh1wmm8wvvavdwrn-glibc-2.42-84/lib/libthread_db.so.1".
9
total=24, average=8, expected_average=6
10

11
Breakpoint 1, main (argc=1, argv=0x7fffffffb7f8) at gdb_demo.cpp:34
12
34      std::thread first(worker, std::ref(counter), std::ref(mutex));
13
(gdb) watch counter
14
Hardware watchpoint 2: counter
15
(gdb) continue
16
Continuing.
17
[New Thread 0x7ffff77ff6c0 (LWP 158939)]
18
[New Thread 0x7ffff6ffe6c0 (LWP 158940)]
19
[Thread 0x7ffff77ff6c0 (LWP 158939) exited]
20
counter=2000
21
[Thread 0x7ffff6ffe6c0 (LWP 158940) exited]
22

23
Watchpoint 2 deleted because the program has left the block in
24
which its expression is valid.
25
__libc_start_call_main (main=main@entry=0x555555556384 <main(int, char**)>, argc=argc@entry=1, argv=argv@entry=0x7fffffffb7f8)
26
    at ../sysdeps/nptl/libc_start_call_main.h:74
27
warning: 74  ../sysdeps/nptl/libc_start_call_main.h: No such file or directory

1
$ gdb -q -nx ./gdb_demo
2
Reading symbols from ./gdb_demo...
3
(gdb) break 34
4
Breakpoint 1 at 0x2518: file gdb_demo.cpp, line 34.
5
(gdb) run
6
Starting program: /tmp/gdb-post/gdb_demo
7
[Thread debugging using libthread_db enabled]
8
Using host libthread_db library "/nix/store/lm3pknxi0ipypy3lxh1wmm8wvvavdwrn-glibc-2.42-84/lib/libthread_db.so.1".
9
total=24, average=8, expected_average=6
10

11
Breakpoint 1, main (argc=1, argv=0x7fffffffb7f8) at gdb_demo.cpp:34
12
34      std::thread first(worker, std::ref(counter), std::ref(mutex));
13
(gdb) watch -l counter
14
Hardware watchpoint 2: -location counter
15
(gdb) continue
16
Continuing.
17
[New Thread 0x7ffff77ff6c0 (LWP 159309)]
18
[Switching to Thread 0x7ffff77ff6c0 (LWP 159309)]
19

20
Thread 2 "gdb_demo" hit Hardware watchpoint 2: -location counter
21

22
Old value = 0
23
New value = 1
24
worker (counter=@0x7fffffffb60c: 1, mutex=...) at gdb_demo.cpp:21
25
21      }
26
(gdb) info threads
27
  Id   Target Id                                     Frame
28
  1    Thread 0x7ffff7e90780 (LWP 159290) "gdb_demo" __GI___clone3 () at ../sysdeps/unix/sysv/linux/x86_64/clone3.S:62
29
* 2    Thread 0x7ffff77ff6c0 (LWP 159309) "gdb_demo" worker (counter=@0x7fffffffb60c: 1, mutex=...) at gdb_demo.cpp:21
30
(gdb) continue
31
Continuing.
32
[New Thread 0x7ffff6ffe6c0 (LWP 159310)]
33

34
Thread 2 "gdb_demo" hit Hardware watchpoint 2: -location counter
35

36
Old value = 1
37
New value = 2
38
worker (counter=@0x7fffffffb60c: 2, mutex=...) at gdb_demo.cpp:21
39
21      }
40
(gdb) info watchpoints
41
Num     Type           Disp Enb Address            What
42
2       hw watchpoint  keep y                      -location counter
43
  breakpoint already hit 2 times

1
(gdb) delete 2
2
(gdb) watch -l counter if counter == 1500
3
Hardware watchpoint 3: -location counter
4
(gdb) continue
5
Continuing.
6
[Thread 0x7ffff77ff6c0 (LWP 159309) exited]
7
[Switching to Thread 0x7ffff6ffe6c0 (LWP 159310)]
8

9
Thread 3 "gdb_demo" hit Hardware watchpoint 3: -location counter
10

11
Old value = 1499
12
New value = 1500
13
worker (counter=@0x7fffffffb60c: 1500, mutex=...) at gdb_demo.cpp:21
14
21      }
15
(gdb) print i
16
$1 = 499
17
(gdb) info threads
18
  Id   Target Id                                     Frame
19
  1    Thread 0x7ffff7e90780 (LWP 159290) "gdb_demo" __syscall_cancel_arch () at ../sysdeps/unix/sysv/linux/x86_64/syscall_cancel.S:56
20
* 3    Thread 0x7ffff6ffe6c0 (LWP 159310) "gdb_demo" worker (counter=@0x7fffffffb60c: 1500, mutex=...) at gdb_demo.cpp:21

1
(gdb) thread 1
2
[Switching to thread 1 (Thread 0x7ffff7e90780 (LWP 159290))]
3
#0  __syscall_cancel_arch () at ../sysdeps/unix/sysv/linux/x86_64/syscall_cancel.S:56
4
warning: 56  ../sysdeps/unix/sysv/linux/x86_64/syscall_cancel.S: No such file or directory
5
(gdb) bt
6
#0  __syscall_cancel_arch () at ../sysdeps/unix/sysv/linux/x86_64/syscall_cancel.S:56
7
#1  0x00007ffff789912c in __internal_syscall_cancel (a1=<optimized out>, a2=<optimized out>, a3=<optimized out>, a4=<optimized out>, a5=a5@entry=0,
8
    a6=a6@entry=4294967295, nr=202) at cancellation.c:49
9
#2  0x00007ffff78998ac in __futex_abstimed_wait_common64 (private=128, futex_word=0x7ffff6ffe990, expected=<optimized out>, op=<optimized out>, abstime=0x0,
10
    cancel=true) at futex-internal.c:57
11
#3  __futex_abstimed_wait_common (futex_word=futex_word@entry=0x7ffff6ffe990, expected=<optimized out>, clockid=clockid@entry=0, abstime=abstime@entry=0x0,
12
    private=private@entry=128, cancel=cancel@entry=true) at futex-internal.c:87
13
#4  0x00007ffff789993f in __GI___futex_abstimed_wait_cancelable64 (futex_word=futex_word@entry=0x7ffff6ffe990, expected=<optimized out>,
14
    clockid=clockid@entry=0, abstime=abstime@entry=0x0, private=private@entry=128) at futex-internal.c:139
15
#5  0x00007ffff789ec8c in __pthread_clockjoin_ex (threadid=140737337353920, thread_return=0x0, clockid=0, abstime=0x0, block=<optimized out>)
16
    at pthread_join_common.c:108
17
#6  0x00007ffff7cf2f1b in std::thread::join() () from /nix/store/604gsr59rj7dzd0nrhp143rpvf7gyiaz-gcc-15.3.0-lib/lib/libstdc++.so.6
18
#7  0x00005555555565be in main (argc=1, argv=0x7fffffffb7f8) at gdb_demo.cpp:37
19
(gdb) thread apply all bt
20

21
Thread 3 (Thread 0x7ffff6ffe6c0 (LWP 159310) "gdb_demo"):
22
#0  worker (counter=@0x7fffffffb60c: 1500, mutex=...) at gdb_demo.cpp:21
23
#1  0x000055555555889a in std::__invoke_impl<void, void (*)(int&, std::mutex&), std::reference_wrapper<int>, std::reference_wrapper<std::mutex> > (__f=@0x55555556f8d8: 0x55555555632c <worker(int&, std::mutex&)>) at /nix/store/8sjgd7q3mdks1rb7rbcv0sallhvrhai5-gcc-15.3.0/include/c++/15.3.0/bits/invoke.h:63
24
#2  0x00005555555587f0 in std::__invoke<void (*)(int&, std::mutex&), std::reference_wrapper<int>, std::reference_wrapper<std::mutex> > (__fn=@0x55555556f8d8: 0x55555555632c <worker(int&, std::mutex&)>) at /nix/store/8sjgd7q3mdks1rb7rbcv0sallhvrhai5-gcc-15.3.0/include/c++/15.3.0/bits/invoke.h:98
25
#3  0x000055555555873d in std::thread::_Invoker<std::tuple<void (*)(int&, std::mutex&), std::reference_wrapper<int>, std::reference_wrapper<std::mutex> > >::_M_invoke<0ul, 1ul, 2ul> (this=0x55555556f8c8) at /nix/store/8sjgd7q3mdks1rb7rbcv0sallhvrhai5-gcc-15.3.0/include/c++/15.3.0/bits/std_thread.h:303
26
#4  0x00005555555586da in std::thread::_Invoker<std::tuple<void (*)(int&, std::mutex&), std::reference_wrapper<int>, std::reference_wrapper<std::mutex> > >::operator() (this=0x55555556f8c8) at /nix/store/8sjgd7q3mdks1rb7rbcv0sallhvrhai5-gcc-15.3.0/include/c++/15.3.0/bits/std_thread.h:310
27
#5  0x00005555555586be in std::thread::_State_impl<std::thread::_Invoker<std::tuple<void (*)(int&, std::mutex&), std::reference_wrapper<int>, std::reference_wrapper<std::mutex> > > >::_M_run (this=0x55555556f8c0) at /nix/store/8sjgd7q3mdks1rb7rbcv0sallhvrhai5-gcc-15.3.0/include/c++/15.3.0/bits/std_thread.h:255
28
#6  0x00007ffff7cf2ea4 in execute_native_thread_routine () from /nix/store/604gsr59rj7dzd0nrhp143rpvf7gyiaz-gcc-15.3.0-lib/lib/libstdc++.so.6
29
#7  0x00007ffff789ce73 in start_thread (arg=<optimized out>) at pthread_create.c:448
30
#8  0x00007ffff79242bc in __GI___clone3 () at ../sysdeps/unix/sysv/linux/x86_64/clone3.S:78
31

32
Thread 1 (Thread 0x7ffff7e90780 (LWP 159290) "gdb_demo"):
33
#0  __syscall_cancel_arch () at ../sysdeps/unix/sysv/linux/x86_64/syscall_cancel.S:56
34
#1  0x00007ffff789912c in __internal_syscall_cancel (a1=<optimized out>, a2=<optimized out>, a3=<optimized out>, a4=<optimized out>, a5=a5@entry=0, a6=a6@entry=4294967295, nr=202) at cancellation.c:49
35
#2  0x00007ffff78998ac in __futex_abstimed_wait_common64 (private=128, futex_word=0x7ffff6ffe990, expected=<optimized out>, op=<optimized out>, abstime=0x0, cancel=true) at futex-internal.c:57
36
#3  __futex_abstimed_wait_common (futex_word=futex_word@entry=0x7ffff6ffe990, expected=<optimized out>, clockid=clockid@entry=0, abstime=abstime@entry=0x0, private=private@entry=128, cancel=cancel@entry=true) at futex-internal.c:87
37
#4  0x00007ffff789993f in __GI___futex_abstimed_wait_cancelable64 (futex_word=futex_word@entry=0x7ffff6ffe990, expected=<optimized out>, clockid=clockid@entry=0, abstime=abstime@entry=0x0, private=private@entry=128) at futex-internal.c:139
38
#5  0x00007ffff789ec8c in __pthread_clockjoin_ex (threadid=140737337353920, thread_return=0x0, clockid=0, abstime=0x0, block=<optimized out>) at pthread_join_common.c:108
39
#6  0x00007ffff7cf2f1b in std::thread::join() () from /nix/store/604gsr59rj7dzd0nrhp143rpvf7gyiaz-gcc-15.3.0-lib/lib/libstdc++.so.6
40
#7  0x00005555555565be in main (argc=1, argv=0x7fffffffb7f8) at gdb_demo.cpp:37

1
$ gdb -q -nx --args ./gdb_demo --fork
2
Reading symbols from ./gdb_demo...
3
(gdb) show follow-fork-mode
4
Debugger response to a program call of fork or vfork is "parent".
5
(gdb) set follow-fork-mode child
6
(gdb) set detach-on-fork off
7
(gdb) break 47
8
Breakpoint 1 at 0x2697: file gdb_demo.cpp, line 47.
9
(gdb) run
10
Starting program: /tmp/gdb-post/gdb_demo --fork
11
[Thread debugging using libthread_db enabled]
12
Using host libthread_db library "/nix/store/lm3pknxi0ipypy3lxh1wmm8wvvavdwrn-glibc-2.42-84/lib/libthread_db.so.1".
13
total=24, average=8, expected_average=6
14
[New Thread 0x7ffff77ff6c0 (LWP 162029)]
15
[Thread 0x7ffff77ff6c0 (LWP 162029) exited]
16
[New Thread 0x7ffff6ffe6c0 (LWP 162030)]
17
counter=2000
18
[Thread 0x7ffff6ffe6c0 (LWP 162030) exited]
19
[Attaching after Thread 0x7ffff7e90780 (LWP 162026) fork to child process 162031]
20
[New inferior 2 (process 162031)]
21
[Thread debugging using libthread_db enabled]
22
Using host libthread_db library "/nix/store/lm3pknxi0ipypy3lxh1wmm8wvvavdwrn-glibc-2.42-84/lib/libthread_db.so.1".
23
[Switching to Thread 0x7ffff7e90780 (LWP 162031)]
24

25
Thread 2.1 "gdb_demo" hit Breakpoint 1.2, main (argc=2, argv=0x7fffffffb7f8) at gdb_demo.cpp:47
26
47              std::cout << "child: pid=" << getpid() << "\n";
27
(gdb) info inferiors
28
  Num  Description       Connection           Executable
29
  1    process 162026    1 (native)           /tmp/gdb-post/gdb_demo
30
* 2    process 162031    1 (native)           /tmp/gdb-post/gdb_demo
31
(gdb) print pid
32
$1 = 0
33
(gdb) inferior 1
34
[Switching to inferior 1 [process 162026] (/tmp/gdb-post/gdb_demo)]
35
[Switching to thread 1.1 (Thread 0x7ffff7e90780 (LWP 162026))]
36
#0  arch_fork (ctid=0x7ffff7e90a50) at ../sysdeps/unix/sysv/linux/arch-fork.h:50
37
warning: 50  ../sysdeps/unix/sysv/linux/arch-fork.h: No such file or directory
38
(gdb) bt 2
39
#0  arch_fork (ctid=0x7ffff7e90a50) at ../sysdeps/unix/sysv/linux/arch-fork.h:50
40
#1  __GI__Fork () at ../sysdeps/nptl/_Fork.c:33
41
(More stack frames follow...)
42
(gdb) inferior 2
43
[Switching to inferior 2 [process 162031] (/tmp/gdb-post/gdb_demo)]
44
[Switching to thread 2.1 (Thread 0x7ffff7e90780 (LWP 162031))]
45
#0  main (argc=2, argv=0x7fffffffb7f8) at gdb_demo.cpp:47
46
47              std::cout << "child: pid=" << getpid() << "\n";
47
(gdb) continue
48
Continuing.
49
child: pid=162031
50
[Inferior 2 (process 162031) exited normally]
51
(gdb) info inferiors
52
  Num  Description       Connection           Executable
53
  1    process 162026    1 (native)           /tmp/gdb-post/gdb_demo
54
* 2    <null>                                 /tmp/gdb-post/gdb_demo