多个定义collect2：错误：ld在C中返回了1个退出状态

Question

Process_struct.h //header file

#define MAX_PROCS 5
#define EXIT 1
#define TRUE 1
/*******************************/
FILE *file=NULL;
/*******************************/
static FILE *outLog=NULL;
pthread_t producer;//Producer Thread ID
pthread_t consumer[MAX_PROCS];//consumer thread ID

This is the error I get when I go to run my Makefile:

    /tmp/ccvDJUQI.o:(.bss+0x8): multiple definition of `file'
    /tmp/cc4RWdZ4.o:(.bss+0x8): first defined here
    collect2: error: ld returned 1 exit status
    make: *** [Multiprocessor] Error 1

The build tells me that I have multiple definition of file in my program. Well the only section that I'm calling my file is in my header file. With my header file code above, there is no multiple definition of file in the file. The file is used throughout both of my .c program files but is only appears in my header file as a variable. I'm not sure why this error appears. Can anyone help me with fixing this error?

Answer 1

If you have more than one .c file and include your header into them then each one of them will have that file defined inside (as the preprocessor just takes your header code and stuffs it into it before compilation). If you then compile them together (as in a library) you will get that particular linker error.

The solution is to move these definitions: FILE *file=NULL; static FILE *outLog=NULL; pthread_t producer;//Producer Thread ID pthread_t consumer[MAX_PROCS];//consumer thread ID FILE *file=NULL; static FILE *outLog=NULL; pthread_t producer;//Producer Thread ID pthread_t consumer[MAX_PROCS];//consumer thread ID

into just one of your .c files. Eventually you could leave them into the header marked with the extern keyword to let the linker know they're defined elsewhere. Having definitions inside header files is usually not such a good practice. The header files should also be guarded against multiple inclusions conflicts as in:

#ifndef MY_HEADER_H__
#define MY_HEADER_H__

... your code here

#endif

Another solution would be to make all of them static into your header. However this would mean that each .c file will access its own data.