Developing Large C Programs

1
Developing Large C Programs

• Multi-File Program
• Compilation Process
• Building Large Programs
• Building Libraries
• Debugging
• Profiling

2
Multi-File Program
/ prog1.c / int x1 5 char x2 messge
1,message 2,message 3 float x3
4.500 static float localvar 0.0 static void
fn2(char) float fn1 ( int a int b ) /
body of fn1 appears here / static void fn2 (
char a ) / body of fn2 appears here /
Can be used by every function but only in this
file
3
Multi-File Program
/ prog2.c / extern int x1 extern char
x2 extern float x3 extern float fn1( int,
int ) static int localvar, anothervar int main
( int argc, char argv ) / body of main
program appears here /
defined somewhere else
4
Separate Compilation

• gcc -c prog1.c
• gcc -c prog2.c
• gcc -o prog prog1.o prog2.o
• prog1.o and prog2.o is relocatable object files
• If prog1.c is changed. Repeat only step 1 and 3.

5
Header file (or .h file, include file)

• / header.h /
• extern int x
• / prog1.c /
• include header.h
• fn1()
• / proc2.c /
• include header.h
• int x
• main()
• Only one change is needed to change xs
  definition.

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No multiple definitions

• Multiple definitions of the same name cause
  errors at compile time.
• Only one definition for each name can exist
• Compiler allocates memory space for it
• Other definitions must use extern
• Compiler does not allocate memory space but knows
  how much space it needs
• Every name must be defined
• compiler needs to know the type of every name.
  So, provide type information for every name in
  the file.

7
No multiple definitions (2)

• If the same .h file is included in several places
  in a program (e.g. a.c includes b.h and c.h,
  which both include d.h), it will cause multiple
  definition error at compile time.
• To prevent this problem, enclose include file
  with ifndef define and endif. For example,
• ifndef _MY_HEADER_H
• define _MY_HEADER_H
• / body of header.h /
• endif

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Make

• After a change to source file or header file,
  recompilation is needed
• Make program keeps track which files need
  recompiling
• Make looks at the dependencies among files and
  file update times.

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• main.c
• include treepack.h
• include listpack.h
• treepack.c
• include treepack.h
• include graphpack.h
• listpack.c
• include listpack.h
• include graphpack.h
• graphpack.c
• include graphpack.h
• graphpack.h
• include tables.i

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Makefile

• Dependencies is defined by programmer in Makefile
• Object file is dependent to source and header
  file, executable file is dependent to object file
• comment
• target prerequisites
• command(s)

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tables.i
graphpack.h treepack.h treepack.c listpack.h l
istpack.c main.c
treepack.o listpack.o main.o
diplayprog
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• Makefile to create displayprog
• displayprog main.o treepack.o listpack.o
• gcc main.o treepack.o listpack.o -o displayprog
• main.o main.c treepack.h listpack.h
• gcc -c main.c
• treepack.o treepack.c treepack.h graphpack.h
• gcc -c treepack.c
• listpack.o listpack.c listpack.h graphpack.h
• gcc -c listpack.c
• graphpack.h table.i
• touch graphpack.h
• change file update time to current time

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Make

• make
• make the first target in Makefile
• make lttargetgt
• make the specified target
• e.g. make treepack.o

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Macros in Makefile
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Compilation Process

• Preprocessing
• Expand include files, macros
• Compilation
• Translate C code to assembly code
• Assemble
• Translate assembly code to machine code
  (relocatable object code)
• Linking
• Combine object files and resolve symbol
  references to produce executable code

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Compilation Process
myprog.c header.h expanded macros
Assembly code myprog.s
Object file myprog.o
library object files
Executable file myprog
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Preprocessing

• Process commands
• include ltstdio.hgt
• define K 1024
• define MAX(X,Y) ((X)gt(Y)? (X) (Y))
• Insert include files
• Replace macros with text strings

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Include Files

• All code in include file is inserted where
  include appears
• include ltstdio.hgt tells preprocessor to look for
  stdio.h in standard directories, e.g.
  /usr/local/include /usr/include
• gcc -Iltdirgt preprocessor looks in directory
  dir first, e.g. gcc -I/include
• include header.h look in the source files
  directory first. Can put relative pathname, e.g.
  ../include/header

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Library

• Library is a collection of object codes used by
  many programs

strcopy.c strsearch.c strcmp.c
myprog.c yourprog.c
string.h
strcopy.o strsearch.o strcmp.o
myprog.o yourprog.o
libstring.a libstring.so
myprog yourprog
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Types of Libraries

• Archive Library
• statically linked
• create by
• ar -r libmylib.a myprog1.o myprog2.o
• Shared Object
• dynamically linked
• create by
• ld -G -o libmylib.so myprog1.o myprog2.o
• or
• gcc -shared -o libmylib.so myprog1.o myprog2.o

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C Library

• The C library contains most standard functions.
• It is always automatically linked.
• /usr/lib/libc.a /usr/lib/libc.so

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Header Files

• Header files for standard library functions, type
  definitions, constants
• /usr/include/.h /usr/local/include/.h
• ctype.h type checking functions
• error.h error numbers
• math.h mathematic
• stddef.h some types macros
• stdio.h input-output
• stdlib.h general purpose
• string.h string
• time.h time

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Linking with Library

• Include needed header files in source code. (man
  page for the function tells what to include) For
  example, to use functions in math library
• include ltmath.hgt
• Lots of functions (e.g. printf) are in libc,
  which is automatically linked. No linker option
  needed.
• Functions outside libc must be explicitly linked.
  For example, math library libm.so
• gcc myprog.c -o myprog -lm

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Linking with Libraries

• Libraries are in /usr/lib /usr/local/lib
• gcc -o myprog myprog.c -L. -R. -lmylib
• -Lltdirgt tells where to find library to be linked
  (or set LIBRARY_PATHltdirgt)
• -Rltdirgt tells where to find library at run-time
  (or set LD_LIBRARY_PATHltdirgt)
• -static statically link

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Debugging

• Compiler insert some code to help debugger get
  information about running program
• gdb
• gcc -g -o myprog myprog.c
• gdb myprog

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Gdb Commands

• help read manual
• break ltlinefunctiongt set breakpoint
• run start program
• step continue to next line,
• step over subroutine
• stepi step one instruction
• next same as step but go
• through subroutine too
• continue continue to next breakpoint
• display ltexprgt print value of expression
• quit quit

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• gdb myprog
• (gdb) break func1
• (gdb) run
• Starting program /home/abc/myprog
• Breakpoint 1, func1 () at myprog.c5
• 5 printf(s\n, hello)
• (gdb) step
• hello
• 6 printf(s\n, world)
• 7
• (gdb) continue
• Continuing.
• Program exited with code 01.
• (gdb)

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Profiling

• Profiler counts how many times a function is
  called and the time the function spends.
• Compiler inserts some code to produce profiling
  information while the program runs.
• gcc -pg -o myprog myprog.c
• myprog
• gprof -b myprog

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• gprof -ab myprog
• index time self children called name
• 0.06 0.11 1/1
  _start 2
• 1 100.0 0.06 0.11 1 main
  1
• 0.08 0.00 1000000/1000000
  sqrt 3
• 0.00 0.03 1000/1000
  printf 4
• -----------------------------------------------
• 
  ltspontaneousgt
• 2 100.0 0.00 0.17
  _start 2
• 0.06 0.11 1/1
  main 1
• 0.00 0.00 2/3
  atexit 18
• 0.00 0.00 1/1
  exit 19
• -----------------------------------------------
• 0.08 0.00 1000000/1000000
  main 1
• 3 47.1 0.08 0.00 1000000 sqrt
  3
• -----------------------------------------------
• 0.00 0.03 1000/1000
  main 1
• 4 17.6 0.00 0.03 1000
  printf 4
• 0.01 0.02 1000/1000
  _doprnt 5