Linux programming: Compile code with dependencies

Question

I am new to linux programming and learning it from The Linux Programming Interface by Michael Kerrisk.

I have to compile my first program that has dependencies.

Directory structure:

--linux-programs
  |--seek_io.c
  |--lib
    |--tlpi_hdr.h
    |--error_functions.h
    |--error_functions.c
    |--get_num.h
    |--ename.c.inc

I want to compile seek_io.c program with dependencies in the lib directory, so that I can see how the program works.

I tried a few things, absolutely clueless on how they work following this stackoverflow answer. I get all sorts of errors as I am an absolute beginner to Linux programming, not to programming, linux OS and C.

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Trials:

gcc -I ./lib/ -c ./lib/error_functions.c and then gcc -o seek_io.c ./error_function.o gives error: /usr/lib/gcc/x86_64-linux-gnu/crt1.o: In function _start: (.text+0x20): undefined reference to main collect2: error: ld returned 1 exit status

After this run, on ls I find that my seek_io.c is not listed.

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Basically the author of the book says for tlpi_hdr.h file:

This header file includes various other header files used by many of the example programs, defines a Boolean data type, and defines macros for calculating the minimum and maximum of two numeric values. Using this header file allows us to make the example programs a bit shorter.

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Link to codes for files mentioned above:

tlpi_hdr.h

error_functions.h

error_functions.c

get_num.h

get_num.c

seek_io.c

Answer 1

The problem is with your second gcc command, where you're using the -o file to specify the output file where to store the resulting executable file, but passing it the name of the C source file seek_io.c instead...

gcc -o seek_io.c ./error_function.o

This means link file error_function.o and store the executable in seek_io.c. This fails because there is no main function, which is needed for a standalone executable, so your C source file is not overwritten by the failing link command.

You can fix this easily by passing the -o option a proper output file name, which in the case (of this link command) should be the name of the executable that you want to create, such as seek_io:

gcc -o seek_io seek_io.c ./error_function.o

(But this will fail without a -I ./lib/, since seek_io.c includes tlpi_hdr.h which is in that directory. If you add it to that command, it should work.)

You can also decide to split the compile and link steps in two separate steps (the command above will both compile seek_io.c into an object file and then link the two object files into an executable) with:

$ gcc -I ./lib/ -c ./lib/error_functions.c
$ gcc -I ./lib/ -c seek_io.c
$ gcc -o seek_io seek_io.o error_function.o

One final nitpick is that for the -I flag to specify the directories where to search for the include files, the more common usage has no space between the flag itself and the directory name, so you'll most commonly see -I./lib or even -Ilib.

$ gcc -Ilib -c ./lib/error_functions.c
$ gcc -Ilib -c seek_io.c
$ gcc -o seek_io seek_io.o error_function.o