这是一个非常奇怪的问题，仅当使用-fPIC选项编译程序时才会发生。

使用gdb我可以打印线程局部变量，但是单步执行它们会导致崩溃。
thread.c

#include <pthread.h>
#include <stdlib.h>
#include <stdio.h>

#define MAX_NUMBER_OF_THREADS 2

struct mystruct {
    int   x;
    int   y;
};

__thread struct mystruct obj;

void* threadMain(void *args) {
    obj.x = 1;
    obj.y = 2;

    printf("obj.x = %d\n", obj.x);
    printf("obj.y = %d\n", obj.y);

    return NULL;
}

int main(int argc, char *arg[]) {
    pthread_t tid[MAX_NUMBER_OF_THREADS];
    int i = 0;

    for(i = 0; i < MAX_NUMBER_OF_THREADS; i++) {
        pthread_create(&tid[i], NULL, threadMain, NULL);
    }

    for(i = 0; i < MAX_NUMBER_OF_THREADS; i++) {
        pthread_join(tid[i], NULL);
    }

    return 0;
}

(gdb) b threadMain 
Breakpoint 1 at 0x4006a5: file thread.c, line 15.
(gdb) r
Starting program: /junk/test/thread 
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".
[New Thread 0x7ffff7fc7700 (LWP 31297)]
[Switching to Thread 0x7ffff7fc7700 (LWP 31297)]

Breakpoint 1, threadMain (args=0x0) at thread.c:15
15      obj.x = 1;
(gdb) p obj.x
$1 = 0
(gdb) n

Program received signal SIGSEGV, Segmentation fault.
threadMain (args=0x0) at thread.c:15
15      obj.x = 1;

Linux 3.13.0-66-generic #108-Ubuntu SMP Wed Oct 7 15:20:27 
GNU gdb (Ubuntu 7.7.1-0ubuntu5~14.04.2) 7.7.1
gcc (Ubuntu 4.8.4-2ubuntu1~14.04) 4.8.4

最佳答案

问题深深地存在于GAS，GNU汇编器的肠胃中，以及它如何生成DWARF调试信息。

编译器GCC负责为与位置无关的线程本地访问生成特定的指令序列，该序列在文档ELF Handling for Thread-Local Storage，第22页，第4.1.6节:x86-64通用动态TLS模型中记录。该顺序是:

0x00 .byte 0x66
0x01 leaq  x@tlsgd(%rip),%rdi
0x08 .word 0x6666
0x0a rex64
0x0b call __tls_get_addr@plt

threadMain:
.LFB2:
        .file 1 "thread.c"
        .loc 1 14 0
        .cfi_startproc
        pushq   %rbp
        .cfi_def_cfa_offset 16
        .cfi_offset 6, -16
        movq    %rsp, %rbp
        .cfi_def_cfa_register 6
        subq    $16, %rsp
        movq    %rdi, -8(%rbp)
        .loc 1 15 0
        .byte   0x66
        leaq    obj@tlsgd(%rip), %rdi
        .value  0x6666
        rex64
        call    __tls_get_addr@PLT
        movl    $1, (%rax)
        .loc 1 16 0
        ...

(gdb) disas/r threadMain                                                                                                                                                                                         
Dump of assembler code for function threadMain:                                                                                                                                                                  
   0x00000000004007f0 <+0>:     55      push   %rbp                                                                                                                                                              
   0x00000000004007f1 <+1>:     48 89 e5        mov    %rsp,%rbp                                                                                                                                                 
   0x00000000004007f4 <+4>:     48 83 ec 10     sub    $0x10,%rsp                                                                                                                                                
   0x00000000004007f8 <+8>:     48 89 7d f8     mov    %rdi,-0x8(%rbp)                                                                                                                                           
   0x00000000004007fc <+12>:    64 48 8b 04 25 00 00 00 00      mov    %fs:0x0,%rax
   0x0000000000400805 <+21>:    48 8d 80 f8 ff ff ff    lea    -0x8(%rax),%rax
   0x000000000040080c <+28>:    c7 00 01 00 00 00       movl   $0x1,(%rax)

64 48 8b 04 25 00 00 00 00      mov    %fs:0x0,%rax

cc                              int3
48 8b 04 25 00 00 00 00         mov    (0x0),%rax

64 cc                           fs:int3
8b 04 25 00 00 00 00            <garbage>

64                              fs:                # CPU DOESN'T SEE THIS!
48 8b 04 25 00 00 00 00         mov    (0x0),%rax  # <- CPU EXECUTES STARTING HERE!
# BOOM! SEGFAULT!

关于c - 使用-fPIC编译的程序在GDB中跳过线程局部变量时崩溃，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/33429912/