Possible Duplicate:
What is a “translation unit” in C++
Possible duplicate: What is a "translation unit" in C++
x.h :
void f();
x.c :
void f(){}
main.c :
#include"x.h"
int main(){
f();
}
then I use: gcc -o a.out main.c why it say f is a undefined symbol?
That's a linker error. You need to compile main.c and x.c at the same time, or compile them separately without linking and link the resulting object files.
For example:
gcc -o x.o -c x.c
gcc -o main.o -c main.c
gcc -o myexecutable main.o x.o
This is because x.h merely contains a declaration, which does not define a symbol. x.c contains the definition.
Try
gcc -o a.out main.c x.c
You also have to include the x.h in the x.c file, and make sure that the definition is not used twice by the following code:
#ifndef X_H
#define X_H
f();
#endif
And then use gcc -o a.out main.c x.c
Because f isn't defined in x.h, it's only declared. You need to link x.o into your executable. If you are not using an IDE, you might want to set up a Makefile to handle this, and call make to build your project.
It should say this during compiling, but probably during linking. You haven't compiled x.c yet (gcc -c x.c)?
