- The C compiler actually
consists of
2 independent programs
- A syntax analyser and
code generator
program
- This program
first checks the
syntactic correctness and
variable/function type correctness
(definition and declaration conflicts) of
the C program
- When there are no syntax error,
the code generator will
translate the
C constructs into
assembler code
The code generator will also run the assembler on the assembler code to produce object (= machine) code
Note:
- When a C programmer says that
he/she
has compiled a program,
he/she could mean
one of the following:
- If "program" is
used to mean
one of the C program files
that make up the
multi-file program, then
he/she has
only performed
the above step on
that C program file
- If "program" is used to mean the entire project, he/she has compiled all source files and perform the linkage step (see the next step) on all object files to produce the executable program
- If "program" is
used to mean
one of the C program files
that make up the
multi-file program, then
he/she has
only performed
the above step on
that C program file
- This program
first checks the
syntactic correctness and
variable/function type correctness
(definition and declaration conflicts) of
the C program
- A "linking loader" program
- A "linking loader" program
combines the
machine code stored in
multiple files to
produce an
executable program
- The main task performed by
the "linking loader" is to
resolve the
variable and function reference
from one program file to
another program file
E.g.:
- When a global variable x is defined in program p1.c and the variable x is used (accessed) in another program p2.c, the "linking loader" must find the memory address used by the variable x and "link" this address to the instructions in program p2.c that refers to variable x.
- A "linking loader" program
combines the
machine code stored in
multiple files to
produce an
executable program
- A syntax analyser and
code generator
program
- How to
compile a
multi-files C program:
- Compile
each source file of the
multi-files C program
individually
- Link all the object files into an executable program
- Compile
each source file of the
multi-files C program
individually
- Important fact about the
C compiler:
- The C compiler
translates
each C program file
at a time -
independently from
each other
I.e.:
- All information on the variables (types) and functions (prototypes) gathered in the translation process of one C program file is discarded when the program translation process ends on one input C program file !!!
Schematically:
Note:
- The final step is to
link (= "merge") the
compiled code
(= program in machine instructions)
into one single
executable program
- The linkage step
checks for
missing items
(e.g.: some variable(s) or
function(s) were
not defined)
- The
linkage step
does
not perform
any checks on
data types !!!
(Data type checks are done in the compile (= translate) step)
- The linkage step
checks for
missing items
(e.g.: some variable(s) or
function(s) were
not defined)
- The C compiler
translates
each C program file
at a time -
independently from
each other
- Recall:
the
C compiler is
a one-pass compiler.
- When the C program consists
of multiple program files,
the compile command:
gcc -c programFile.cwill compile (= translate) the program file programFile.c individually (= without consulting any other program files !!!)
The C compiler will read each program file once (= one-pass !!!)
- When the C program consists
of multiple program files,
the compile command:
- Important recall:
implicit
function prototype assumption
- While translating each C program file (in a multi-files C program), the C compiler will make implicit function prototype assumptions for undefined and undeclared functions
It is therefore very important that:
- You declare all functions that you use inside every C program source file !!!