Why do you have to link the math library in C?

Question

If I include `<stdlib.h>` or `<stdio.h>` in a C program I don't have to link these when compiling but I do have to link to `<math.h>`, using `-lm` with gcc, for example: gcc test.c -o test -lmWhat is the reason for this? Why do I have to explicitly link the math library but not the other libraries?

Answer 1

Well, the math library (usually /usr/lib/libm.a) is not the only library that requires explicit linking at the cc/gcc command line ... for example, using anything defined in curses.h would require using -lcurses as part of your gcc command line.

All C/C++ compilers include certain library bindings by default - so we don't need to.

That's one reason why even the most simple C program that does absolutely nothing ....

int main( ) {

return 0;

}

... produces a ~150K executable

-rw-rw-r--+ 1 lji None 29 May 21 08:03 nothing.c

-rwxrwxr-x+ 1 lji None 148486 May 21 08:03 nothing.exe

But the compilers can't implicitly include everything ... only the libraries that just about all programs require.

This usually managed in a makefie anyway ... where a LIBS macro might be defined and used as follows:

LIBS = -lcurses -lm # ... other libs

$(CC) -o exec src1.c src2.c $(LIBS)