在头文件中找不到符号

Question

I have this c program:

#include    <sys/types.h>
#include    "ourhdr.h"

int     glob = 6;       /* external variable in initialized data */
char    buf[] = "a write to stdout\n";

int
main(void)
{
    int     var;        /* automatic variable on the stack */
    pid_t   pid;

    var = 88;
    if (write(STDOUT_FILENO, buf, sizeof(buf)-1) != sizeof(buf)-1)
        err_sys("write error");
    printf("before fork\n");    /* we don't flush stdout */

    if ( (pid = fork()) < 0)
        err_sys("fork error");
    else if (pid == 0) {        /* child */
        glob++;                 /* modify variables */
        var++;
    } else
        sleep(2);               /* parent */

    printf("pid = %d, glob = %d, var = %d\n", getpid(), glob, var);
    exit(0);
}

In the header file "ourhdr.h" (located in the same folder) i defined several functions, such as err_sys(). I get this error while compiling with gcc:

In function "main":
undefined reference to "err_sys"

How can i get this working? I can post the header file here if needed. Thank you.

** EDIT: ** This is the ourhdr.h file: http://pastebin.com/fMUiG4zU

Answer 1

You have included the header file with the declaration of err_sys . Ensure you have also an implementation that is passed to the linker. Background:

The compiler compiles a module to an object file: g++ -c modul1.c generates modul1.o g++ -c modul2.c generates modul2.o

Each of the modules can reference functions that are defined in a included header file. Up to here, no actual implementation of that function is needed.

In the next step, the linker links toghether all object files (and libraries): g++ modul1.o modul2.o generates ./a.out or similar

In the second step, an implementation for all used functions is needed. Be sure you provided it!

(PS: Some compilers allow compiling and linking with one command, if you have multiple modules you can perhaps add them to a single gcc-call. I'd recomment to use make though)

Answer 2

I think what's likely to happen is that you have a header, ourhdr.h , containing the definition of the function, but the implementation is in a different file: ourhdr.c . In that case, if you try to compile without including ourhdr.c you'll get a reference error:

$ gcc main.c
/bin/ld: /tmp/ccVzNF6w.o: in function `main':
main.c:(.text+0x38): undefined reference to `err_sys'

To fix it, you need to compile like this:

$ gcc main.c ourhdr.c

Another option is to define the body of the function in ourhdr.h :

// ourhdr.h

#include <stdio.h>

void err_sys(const char* str);

void err_sys(const char* str) {
    fprintf(stderr, "%s\n", str);
}

In that case, gcc main.c should work.

Answer 3

对我来说，如果你写的函数名写得正确，它编译和工作正常。