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Including files

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• Including files
  • The #include directive:

    #include <filename> - include file "filename" from the C's standard include directory (which is: /usr/include) #include "filename" - include file "filename" from a user specified diretory

    The default user specified include directory is:

    The current directory

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  • Specifying user specified include directories:

    gcc -Idir1 -Idir2 .... C-program

    The C compiler will look for include files included with #include "..." in directories dir1, dir2, etc.

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  • Common practice:

    Included file in C programs usually ends with the .h extention .h = header file extension. The include files are called header files because the #include command almost always is located at the top (head) of a C program file !!!

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  • Example:

    header1.h	include1.c
    #define square(x) ((x)*(x)) #define MAX 99999	#include "header1.h" int main( int argc, char* argv[] ) { int A[MAX]; int a, b; A[5] = square( a+b ); }

    Output of gcc -E include1.c:

    int main( ) { int A[99999]; int a, b; A[5] = ((a+b)*(a+b)); }

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  • Note: header file

    The #include command is often at the start (head) of a C program Hence, the included files are called header files in C

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• Nested includes
  • Fact:

    The #include directives can nest I.e.: an included file can contain #include directives to include other files

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  • Example: header1-1.h includes another header file header1-2.h:

    header2.h: #include "header2-2.h" #define MAX 99999 header2-2.h: #define square(x) ((x)*(x)) The main program: #include "header2.h" // Nested include int main( ) { int A[MAX]; int a, b; A[5] = square( a+b ); }

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/c/Cprep/include2.c

    How to run the program:

    To compile:   gcc -E include2.c          See output on terminal....

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• Recursive includes
  • Warning:

    You can create recursive file inclusion

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  • Example: recurse.h

    #include "recurse.h" // Include the header file itself... #define MAX 99999

    Result:

    recurse.h:2:21: error: #include nested too deeply

    You must scroll back a lot to see the error !!!

    Or, use this command (that will send the normal output to the null device so you can see the error message):

    gcc -E include-r.c > /dev/null

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/c/Cprep/include-r.c and /home/cs255001/demo/c/Cprep/recurse.h

    How to run the program:

    To compile:   gcc -E include-r.c > /dev/null          Watch the output onthe terminal