c - 头文件中const变量的多重定义

Question

我在flag.h 中定义了一些常量，所以link.c 和linkedlist.h 可以使用它。但是当我编译时:

clang -Wall main.c link.c linkedlist.c

我得到以下内容

/tmp/linkedlist-o2mcAI.o:(.rodata+0x0): multiple definition of `VARIABLE_NAME'

/tmp/link-oXhyfE.o:(.rodata+0x0): first defined here

对于 link.c 和 linkedlist.c 中使用的 flag.h 中的所有变量，以及最后:

clang: error: linker command failed with exit code 1 (use -v to see invocation)

---

ma​​in.c、flag.h、link.h、link.c示例代码, linkedlist.h 和 linkedlist.c:

main.c

#include <stdio.h>
#include "linkedlist.h"

int main(void) {
    LinkedList* list = LinkedList_new();
}

flag.h

#ifndef FLAG_H_
#define FLAG_H_

#include <limits.h>

#define FALSE 0
#define TRUE 1

const int OK = 1;
const int ERROR = -1;
const int FLAG = 0;

// other constants

#endif

link.h

#ifndef LINK_H_
#define LINK_H_

typedef struct Link {
    int value;
    struct Link* next;
} Link;

Link* Link_new(int value);

int useExample(int value);

// other methods

#endif

link.c

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

#include "link.h"
#include "flag.h"

Link* Link_new(int value)
{
    Link* link = malloc(sizeof(Link));
    link->value = value;
    link->next = NULL;
    return link;
}

useExample(int value)
{
    if (value == 0) {
        return OK; // defined in flag.h
    } else {
        return FLAG; // defined in flag.h
    }
}

// other methods

链表.h

#ifndef LINKEDLIST_H_
#define LINKEDLIST_H_

#include "link.h"

typedef struct LinkedList {
    Link* first;
    unsigned int size;
} LinkedList;

LinkedList* LinkedList_new();

int anotherUseExample(int value);

// other methods

#endif

链表.c

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

#include "linkedlist.h"
#include "flag.h"

LinkedList* LinkedList_new() {
    LinkedList* list = malloc(sizeof(LinkedList));
    list->first = NULL;
    list->size = 0;
    return list;
}

anotherUseExample(int value)
{
    if (value == 0) {
        return FLAG; // defined in flag.h
    } else {
        return ERROR; // defined in flag.h
    }
}

// other methods

---

那么如何在link.c和linkedlist.c中使用flag.h而不出现多重定义呢？

还有……我编码头文件和编译的方式是否正确？

---

-v 的完整输出:

clang version 3.3 (tags/RELEASE_33/rc3)
Target: i386-redhat-linux-gnu
Thread model: posix
 "/usr/bin/clang" -cc1 -triple i386-redhat-linux-gnu -emit-obj -mrelax-all -disable-free -disable-llvm-verifier -main-file-name main.c -mrelocation-model static -mdisable-fp-elim -fmath-errno -masm-verbose -mconstructor-aliases -fuse-init-array -target-cpu pentium4 -target-linker-version 2.23.52.0.1 -v -resource-dir /usr/bin/../lib/clang/3.3 -internal-isystem /usr/local/include -internal-isystem /usr/bin/../lib/clang/3.3/include -internal-externc-isystem /usr/include -internal-externc-isystem /usr/lib/gcc/i686-redhat-linux/4.8.1/include -Wall -fdebug-compilation-dir /home/jharvard/Desktop/Code/LinkedList -ferror-limit 19 -fmessage-length 80 -mstackrealign -fobjc-runtime=gcc -fobjc-default-synthesize-properties -fdiagnostics-show-option -fcolor-diagnostics -backend-option -vectorize-loops -o /tmp/main-JmZTmN.o -x c main.c
clang -cc1 version 3.3 based upon LLVM 3.3 default target i386-redhat-linux-gnu
#include "..." search starts here:
#include <...> search starts here:
 /usr/local/include
 /usr/bin/../lib/clang/3.3/include
 /usr/include
 /usr/lib/gcc/i686-redhat-linux/4.8.1/include
End of search list.
 "/usr/bin/clang" -cc1 -triple i386-redhat-linux-gnu -emit-obj -mrelax-all -disable-free -disable-llvm-verifier -main-file-name link.c -mrelocation-model static -mdisable-fp-elim -fmath-errno -masm-verbose -mconstructor-aliases -fuse-init-array -target-cpu pentium4 -target-linker-version 2.23.52.0.1 -v -resource-dir /usr/bin/../lib/clang/3.3 -internal-isystem /usr/local/include -internal-isystem /usr/bin/../lib/clang/3.3/include -internal-externc-isystem /usr/include -internal-externc-isystem /usr/lib/gcc/i686-redhat-linux/4.8.1/include -Wall -fdebug-compilation-dir /home/jharvard/Desktop/Code/LinkedList -ferror-limit 19 -fmessage-length 80 -mstackrealign -fobjc-runtime=gcc -fobjc-default-synthesize-properties -fdiagnostics-show-option -fcolor-diagnostics -backend-option -vectorize-loops -o /tmp/link-FtygcZ.o -x c link.c
clang -cc1 version 3.3 based upon LLVM 3.3 default target i386-redhat-linux-gnu
#include "..." search starts here:
#include <...> search starts here:
 /usr/local/include
 /usr/bin/../lib/clang/3.3/include
 /usr/include
 /usr/lib/gcc/i686-redhat-linux/4.8.1/include
End of search list.
 "/usr/bin/clang" -cc1 -triple i386-redhat-linux-gnu -emit-obj -mrelax-all -disable-free -disable-llvm-verifier -main-file-name linkedlist.c -mrelocation-model static -mdisable-fp-elim -fmath-errno -masm-verbose -mconstructor-aliases -fuse-init-array -target-cpu pentium4 -target-linker-version 2.23.52.0.1 -v -resource-dir /usr/bin/../lib/clang/3.3 -internal-isystem /usr/local/include -internal-isystem /usr/bin/../lib/clang/3.3/include -internal-externc-isystem /usr/include -internal-externc-isystem /usr/lib/gcc/i686-redhat-linux/4.8.1/include -Wall -fdebug-compilation-dir /home/jharvard/Desktop/Code/LinkedList -ferror-limit 19 -fmessage-length 80 -mstackrealign -fobjc-runtime=gcc -fobjc-default-synthesize-properties -fdiagnostics-show-option -fcolor-diagnostics -backend-option -vectorize-loops -o /tmp/linkedlist-n0zF1a.o -x c linkedlist.c
clang -cc1 version 3.3 based upon LLVM 3.3 default target i386-redhat-linux-gnu
#include "..." search starts here:
#include <...> search starts here:
 /usr/local/include
 /usr/bin/../lib/clang/3.3/include
 /usr/include
 /usr/lib/gcc/i686-redhat-linux/4.8.1/include
End of search list.
 "/usr/bin/ld" --eh-frame-hdr -m elf_i386 -dynamic-linker /lib/ld-linux.so.2 -o a.out /usr/lib/gcc/i686-redhat-linux/4.8.1/../../../crt1.o /usr/lib/gcc/i686-redhat-linux/4.8.1/../../../crti.o /usr/lib/gcc/i686-redhat-linux/4.8.1/crtbegin.o -L/usr/lib/gcc/i686-redhat-linux/4.8.1 -L/usr/lib/gcc/i686-redhat-linux/4.8.1/../../.. -L/lib -L/usr/lib /tmp/main-JmZTmN.o /tmp/link-FtygcZ.o /tmp/linkedlist-n0zF1a.o -lgcc --as-needed -lgcc_s --no-as-needed -lc -lgcc --as-needed -lgcc_s --no-as-needed /usr/lib/gcc/i686-redhat-linux/4.8.1/crtend.o /usr/lib/gcc/i686-redhat-linux/4.8.1/../../../crtn.o
/tmp/linkedlist-n0zF1a.o:(.rodata+0x4): multiple definition of `ERROR_indexOutOfBounds'
/tmp/link-FtygcZ.o:(.rodata+0x4): first defined here
/tmp/linkedlist-n0zF1a.o:(.rodata+0x8): multiple definition of `ERROR_invalidArgument'
/tmp/link-FtygcZ.o:(.rodata+0x8): first defined here
/tmp/linkedlist-n0zF1a.o:(.rodata+0x10): multiple definition of `FLAG_notFound'
/tmp/link-FtygcZ.o:(.rodata+0x10): first defined here
/tmp/linkedlist-n0zF1a.o:(.rodata+0xc): multiple definition of `FLAG_undefined'
/tmp/link-FtygcZ.o:(.rodata+0xc): first defined here
/tmp/linkedlist-n0zF1a.o:(.rodata+0x0): multiple definition of `OK'
/tmp/link-FtygcZ.o:(.rodata+0x0): first defined here
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Answer 1

C 中的#include 指令只是从头文件中复制文本。这意味着当你同时编译 link.c 和 linkedlist.c 时，flag.h 中的常量定义被复制到两者中，并且所有这些常量现在在 link.o 和 linkedlist.o` 中定义。当您链接程序时，您会得到一个名称，但会出现多重定义错误。

您需要将声明与定义分开，并创建一个 flag.c 文件，您在其中定义 const int OK = 1; 等，并在 flag.h 你将继续使用 const int OK; 等。这样，常量将被编译到一个目标文件中 - flag.o - 链接时不会出现多重定义错误。