許多情況下，標頭檔案和原始檔會單獨存放在不同的目錄中。

可以直接在.c檔案中利用#include“/path/file.h", 通過指定標頭檔案的路徑(可以是絕對路徑,也可以是相對路徑)來包含標頭檔案. 但這明顯降低了程式的可移植性. 在別的系統環境下編譯可能會出現問題.

  所以還是利用"-I"選項指定標頭檔案完整的包含路徑.

　 針對標頭檔案比較多的情況, 最好把它們統一放在一個目錄中,比如~/project/include. 這樣就不需為不同的標頭檔案指定不同的路徑. 如果你嫌每次輸入這麼多選項太麻煩,你可以通過設定環境變數來新增路徑:

　　$ C_INCLUDE_PATH=/opt/gdbm-1.8.3/include

　　$ export C_INCLUDE_PATH

　　$ LIBRART_PATH=/opt/gdbm-1.8.3/lib

　　$ export LIBRART_PATH

　　可一次指定多個搜尋路徑，":"用於分隔它們，"."表示當前路徑，如：

　　$ C_INCLUDE_PATH=.:/opt/gdbm-1.8.3/include:/net/include

　　$ LIBRARY_PATH=.:/opt/gdbm-1.8.3/lib:/net/lib

　　（可以新增多個路徑，路徑之間用:相隔，.代表當前目錄，若.在最前頭，也可省略）

　　當然，若想永久地新增這些路徑，可以在.bash_profile中新增上述語句.

   例如，假設存放原始檔的子目錄名為./src，而包含檔案則放在同層次的其他目錄下，如./inc。當我們在./src目錄下進行編譯工作時，如何告訴GCC到哪裡找標頭檔案呢？方法如下所示：
         $ gcc test.c  ,I../inc -o test
  上面的命令告訴GCC包含檔案存放在./inc目錄下，在當前目錄的上一級。如果在編譯時需要的包含檔案存放在多個目錄下，可以使用多個-I 來指定各個目錄：
       包含多個目錄： $ gcc test.c -I../inc -I../../inc2 -o test
    這裡指出了另一個包含子目錄inc2，較之前目錄它還要在再上兩級才能找到。

    C編譯器通過原始檔的字尾名來判斷是 C 程式還是 C++ 程式。在 Linux 中，C 原始檔的字尾名為 .c，而 C++原始檔的字尾名為 .C 或 .cpp。但是，gcc 命令只能編譯 C++ 原始檔，而不能自動和 C++程式使用的庫連線。因此，通常使用 g++ 命令來完成C++ 程式的編譯和連線，該程式會自動呼叫 gcc 實現編譯。

     -g生成除錯資訊。GNU 偵錯程式可利用該資訊。

    -w 不生成任何警告資訊。
    -Wall 生成所有警告資訊。

    -c只編譯並生成目標檔案

    the -c option says not  to run thelinker.  Then the output consists of object filesoutput by  the assembler.

 -c      Compile or assemble the source files, but do notlink.  The link-
          ing stage simply is not done.  The ultimate outputis in the form
          of an object file for each source file.

          By default, the object file name for a source file is made by
          replacing the suffix .c, .i, .s, etc., with .o.

          Unrecognized input files, not requiring compilation orassembly,
          are ignored.

  -S    Stop after the stage of compilation proper; do notassemble.  The
          output is in the form of an assembler code file for each non-
          assembler input file specified.

          By default, the assembler file name for a source file is madeby
          replacing the suffix .c, .i, etc., with .s.

          Input files that don\u2019t require compilation are ignored.

  -E    Stop after the preprocessing stage; do not run the compiler
          proper.  The output is in the form of preprocessedsource code,
          which is sent to the standard output.

          Input files which don't require preprocessing are ignored.

 -v      Print (on standard error output) the commandsexecuted to run the
          stages of compilation.  Also print the versionnumber of the com-
          piler driver program and of the preprocessor and the compiler
          proper.

 --version
          Display the version number and copyrights of the invoked GCC.

  -###
          Like -v except the commands are not executed and all commandargu-
          ments are quoted.  This is useful for shellscripts to capture the
          driver-generated command lines.

  -pipe
          Use pipes rather than temporary files for communicationbetween
          the various stages of compilation.  This fails towork on some
          systems where the assembler is unable to read from a pipe; butthe
          GNU assembler has no trouble.

優化選項：

    OptionsThat Control Optimization

      Theseoptions control various sorts ofoptimizations. Without any optimization option,the compiler's goal is to reduce the cost of compilation and tomake debugging produce the expectedresults.  Statements are independent: if you stopthe program with a  breakpoint betweenstatements(計算機程式指令或語句), you can then assign a new value to anyvariable or change the program  counter to any other statement in the function and get exactly theresults you would expect from the source code.

     Turningon optimization flags makes the compiler attempt to improve theperformance and/or code size at the expense ofcompilation time and possibly the ability to debug the program.

      The compiler performs optimization based on the knowledge it has ofthe program.  Usingthe -funit-at-a-time flag will allow the compilerto consider information gained from later functions in the filewhen compiling a function.  Compiling multiplefiles at once to a single output file (and using -funit-at-a-time)will allow the compiler to use information gained from all of thefiles when compiling each of them.

      Not all optimizations are controlled directly by aflag.  Only optimizations that have a flagare
      listed.

      -O
      -O1 Optimize.  Optimizing compilation takessomewhat more time, and a lot more memory for alarge  function.

     With -O, the compiler tries to reduce code size and execution time,without performingany   optimizations that take a great deal of compilation time.-O turnson the following optimization flags: -fdefer-pop -fmerge-constants-fthread-jumps -floop-optimize -fif-conversion-fif-conversion2 -fdelayed-branch-fguess-branch-probability-fcprop-registers  -Oalso turns on -fomit-frame-pointer on machines where doing so doesnot interfere with debugging.

          -O2 Optimize even more.  GCC performs nearly allsupported optimizations that do not involve a space-speedtradeoff.  The compiler does not perform loopunrolling or function inlining when you specify-O2.  As compared to -O, this option increasesboth compilation time and the performance of  thegenerated code.
     -O2 turns on all optimization flags specified by-O.  It also turns on the following optimizationflags: -fforce-mem -foptimize-sibling-calls -fstrength-reduce-fcse-follow-jumps
 -fcse-skip-blocks-frerun-cse-after-loop  -frerun-loop-opt-fgcse  -fgcse-lm -fgcse-sm  -fgcse-las -fdelete-null-pointer-checks-fexpensive-optimizations -fregmove -fschedule-insns
-fschedule-insns2 -fsched-interblock  -fsched-spec-fcaller-saves -fpeephole2 -freorder-blocks
-freorder-functions -fstrict-aliasing -funit-at-a-time-falign-functions -falign-jumps-falign-loops  -falign-labels-fcrossjumping

     Please note the warning under -fgcse about invoking -O2 on programsthat use computed gotos.

    -O3Optimize yet more.  -O3 turns on all optimizationsspecified by -O2 and also turns on the -fin-line-functions, -fweb,-frename-registers and -funswitch-loops options.

      -O0 Do not optimize.  This is the default.

      -Os Optimize for size.  -Os enables all -O2optimizations that do not typically increase code size.
       It also performs further optimizations designed to reduce codesize.

     -Os disables the following optimization flags:-falign-functions  -falign-jumps -falign-loops  -falign-labels -freorder-blocks  -fprefetch-loop-arrays

    Ifyou use multiple -O options, with or without level numbers, thelast such option is the one
that is effective.

      Optionsof the form -fflag specify machine-independentflags.  Most flags have both positive and negativeforms; the negative form of -ffoo would be-fno-foo.  In the table below, only one of theforms is listed---the one you typically will use. You can figure out the other form by either removing no or addingit.

     Thefollowing options control specific optimizations. They are either activated by -O options or are related to ones thatare.  You can use the following flags in the rarecases when "fine turning"of optimizations to be performed is desired.

   -fno-default-inline
    Donot make member functions inline by default merely because they aredefined inside the class scope (C++only).  Otherwise, when you specify -O, memberfunctions defined inside class scopeare compiled inline by default;i.e., you don\u2019t need to add inline in front of the memberfunction name.
 

   關於編譯選項的次序： Order does matter when you useseveral options of the same kind; for example, ifyou specify -L more than once, the directories are searched in theorder specified.

  Overall Options
          -c  -S  -E  -ofile  -pipe  -pass-exit-codes -xlanguage  -v  -###
          --help  --target-help --version

 C Language Options
          -ansi  -std=standard  -aux-infofilename -fno-asm  -fno-builtin
          -fno-builtin-function -fhosted -ffreestanding  -fms-extensions
          -trigraphs  -no-integrated-cpp -traditional  -traditional-cpp
          -fallow-single-precision  -fcond-mismatch-fsigned-bitfields
          -fsigned-char -funsigned-bitfields -funsigned-char
          -fwritable-strings

 C++ Language Options
          -fabi-version=n -fno-access-control  -fcheck-new-fconserve-space
          -fno-const-strings -fno-elide-constructors-fno-enforce-eh-specs
          -ffor-scope  -fno-for-scope -fno-gnu-keywords -fno-implicit-tem-
          plates -fno-implicit-inline-templates -fno-implement-inlines
          -fms-extensions -fno-nonansi-builtins -fno-operator-names
          -fno-optional-diags  -fpermissive-frepo  -fno-rtti  -fstats
          -ftemplate-depth-n -fno-threadsafe-statics -fuse-cxa-atexit
          -fno-weak  -nostdinc++-fno-default-inline  -fvisibil-
          ity-inlines-hidden -Wabi  -Wctor-dtor-privacy-Wnon-virtual-dtor
          -Wreorder -Weffc++  -Wno-deprecated-Wno-non-template-friend
          -Wold-style-cast -Woverloaded-virtual -Wno-pmf-conversions
          -Wsign-promo
 

下面的是一個呼叫數學庫 libm.a 中 sin函式的的例子，建立檔案calc.c：

    #include<math.h>

    #include<stdio.h>

    int main(void)

   {

    double x= 2.0;

    double y= sin (x);

    printf("The value of sin(2.0) is %f\n", y);

    return0;

  }

嘗試單獨從該檔案生成一個可執行檔案將導致一個連結階段的錯誤：函式sin，未在本程式中定義也不在預設庫‘libc.a’中；

    gcc -Wallcalc.c /usr/lib/libm.a -o calc

為避免在命令列中指定長長的路徑，編譯器為連結函式庫提供了快捷的選項‘-l’。例如，下面的命令      $gcc -Wall calc.c -lm -o calc

與我們上面指定庫全路徑‘/usr/lib/libm.a’的命令等價。

程式通常要使用很多 -l 選項來指定要連結的數學庫，圖形庫，網路庫等。

1）在實現共享庫時，要將原始檔編譯為相對地址編碼的格式。
2）Gcc選項 -fpic是實現1）中要求的選項。pic是position independent code的縮寫。如：gcc -c-fpic component1.c component2.c

3）在連結時定位共享庫：用-L指定絕對路徑，也可以是用-l指定相對路徑。

   預設搜尋路徑是/lib和/usr/lib

4)在執行時，應用程式的搜尋路徑包括：環境變數LD_LIBRARY_PATH指定的路徑、/etc/ld.so.cache中的共享庫（由ldconfig生成）、/lib和/usr/lib。另外還有一個環境變數LD_PRELOAD,在這裡定義的共享庫會比任何前述路徑優先搜尋。

   gdb除錯時：檢視資料

       print variable       檢視變數

       print *[email protected]     檢視陣列（array是陣列指標，len是需要資料長度）

       可以通過新增引數來設定輸出格式：

           /x 按十六進位制格式顯示變數。

           /d 按十進位制格式顯示變數。

           /u 按十六進位制格式顯示無符號整型。

           /o 按八進位制格式顯示變數。

           /t 按二進位制格式顯示變數。

           /a 按十六進位制格式顯示變數。

           /c 按字元格式顯示變數。

           /f 按浮數格式顯示變數。