CMSC104

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Lecture 23: Separate Compilation

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Reading Assigned:  None

Homework Due:  None

Homework Assigned:  Homework 11 -- assigned last class

Classwork Assigned:  None

Topics Covered:

• Homework 11 questions?
• Quiz 4 Topics Questions?
• Note, we will start quiz 4 at 6:00, and you have 45 minutes
• Separate Compilation
  • So far all of our programs have been one file (eg. hw10.c)
  • This becomes unwieldy with large programs
  • C allows us to put pieces of programs into multiple .c files and compile them together
  • This is very similar to our current use of libraries, where the linker combines our compiled file with the library objects
  • When we provide multiple .c files to gcc, it will automatically combine them into one executable
  • Example get_values.c and sep_comp_ex.c
  • Example of the error when we compile just sep_comp_ex.c

    linux1[1]% gcc sep_comp_ex.c
    /tmp/ccnrem4d.o: In function `main':
    sep_comp_ex.c:(.text+0x17): undefined reference to `get_int'
    sep_comp_ex.c:(.text+0x2d): undefined reference to `get_double'
    sep_comp_ex.c:(.text+0x43): undefined reference to `get_char'
    collect2: ld returned 1 exit status
    

  • Example of the error when we compile just get_values.c

    linux1[1]% gcc get_values.c
    /usr/lib/gcc/i686-redhat-linux/4.4.5/../../../crt1.o: In function `_start':
    (.text+0x18): undefined reference to `main'
    collect2: ld returned 1 exit status
    

  • Why would get_values.c not compile on its own?
  • Why would sep_comp_ex.c not compile on its own?
• Referencing functions in separate files
  • In the example above I added a forward declaration to sep_comp_ex.c
  • That is one method to allow compilation when relying on external functions
  • The normal method is to create your own .h files
  • Example of my own get_values.h file containing the forward declarations
  • Example code using get_values.h sep_comp_ex2.c
  • This allows you to more easily reuse code you have written
  • Note that you still must ensure that the .h and .c files agree on the function signature (return type, name, and number and types of arguments)
  • Also note that the way #include works, it is just like you typed in all of the file that was #included into that exact point of your code