SDL 加载显示JPEG图片

在《 SDL点、线、面及图像加载 》一文中自己用SDL显示JPEG失败，现在有了解决方案。
关键函数：
SDL_rwops.h:

extern DECLSPEC SDL_RWops * SDLCALL SDL_RWFromFile(const char *file, const char *mode);

extern DECLSPEC void

SDL_image.h:

extern

查看IMG_jpg.c文件，发现需要预先声明LOAD_JPG才能使用函数​​IMG_LoadJPG_RW(SDL_RWops *src);​​

#include "SDL_image.h"

#ifdef LOAD_JPG

#include <jpeglib.h>

/* Define this for fast loading and not as good image quality */
/*#define FAST_JPEG*/

代码文件：

#include <stdio.h>
#include <stdlib.h>
#include <SDL.h>
#include <SDL_image.h>
int main(){
    char *file = "/home/edemon/Pictures/sun.jpg";
    SDL_Surface *screen;
    SDL_Surface *picture;
    if(SDL_Init(SDL_INIT_VIDEO) < 0){
        fprintf(stderr,"SDL_INIT_VIDEO error %s\n",SDL_GetError());
        exit(1);
    }
    //extern DECLSPEC SDL_RWops * SDLCALL SDL_RWFromFile(const char *file, const char *mode);
    SDL_RWops *rwops = SDL_RWFromFile(file,"rb");
    //extern DECLSPEC SDL_Surface * SDLCALL IMG_LoadJPG_RW(SDL_RWops *src);
    if(rwops == NULL){
        fprintf(stderr," SDL_RWFromFile, error is %s\n",SDL_GetError());
        exit(1);
    }
    picture = IMG_LoadJPG_RW(rwops);
    //picture =  IMG_Load(file);
    if(picture == NULL){
        fprintf(stderr,"IMG_Load error %s\n",SDL_GetError());
        exit(1);
    }
    screen = SDL_SetVideoMode(640,480,16,SDL_SWSURFACE);
    if(screen == NULL){
        fprintf(stderr,"couldn't set 640*480*16 bits color model, error is %s\n",SDL_GetError());
        exit(1);
    }
    /* int SDL_BlitSurface(SDL_Surface *src, SDL_Rect *srcrect, SDL_Surface *dst, SDL_Rect *dstrect); */
    if(SDL_BlitSurface(picture,NULL,screen,NULL) < 0){
        fprintf(stderr,"SDL_BlitSurface error %s\n",SDL_GetError());
        exit(1);
    }
    SDL_UpdateRect(screen, 0, 0, 0, 0);
    atexit(SDL_Quit);
    //extern DECLSPEC void SDLCALL SDL_FreeRW(SDL_RWops *area);
    SDL_FreeRW(rwops);
    SDL_FreeSurface(picture);
    SDL_Delay(5000);
    SDL_FreeSurface(screen);
    return 0;
}

在编译IMG_jpg.c时用D选项参数提前告诉编译器宏​​LOAD_JPG​​ —— 不是在代码文件中进行宏定义。

gcc -D LOAD_JPG ~/SDL_image-1.2.2/IMG_jpg.c -o IMG_jpg.o
gcc loadJPG.c IMG_jpg.o -o loadJPG -lSDL -lSDL_image

编译出现问题，缺少JPEG库。
​​​IMG_jpg.c:35:21: fatal error: jpeglib.h: No such file or directory​​

需要安装JPEG库。
资源：
​​​git clone https://github.com/LuaDist/libjpeg.git​​

安装时的输出信息：

make[1]: Entering directory '/home/edemon/Downloads/libjpeg'
 /bin/mkdir -p '/usr/local/lib'
 /bin/bash ./libtool   --mode=install /usr/bin/install -c   libjpeg.la '/usr/local/lib'
libtool: install: /usr/bin/install -c .libs/libjpeg.so.9.1.0 /usr/local/lib/libjpeg.so.9.1.0
libtool: install: (cd /usr/local/lib && { ln -s -f libjpeg.so.9.1.0 libjpeg.so.9 || { rm -f libjpeg.so.9 && ln -s libjpeg.so.9.1.0 libjpeg.so.9; }; })
libtool: install: (cd /usr/local/lib && { ln -s -f libjpeg.so.9.1.0 libjpeg.so || { rm -f libjpeg.so && ln -s libjpeg.so.9.1.0 libjpeg.so; }; })
libtool: install: /usr/bin/install -c .libs/libjpeg.lai /usr/local/lib/libjpeg.la
libtool: install: /usr/bin/install -c .libs/libjpeg.a /usr/local/lib/libjpeg.a
libtool: install: chmod 644 /usr/local/lib/libjpeg.a
libtool: install: ranlib /usr/local/lib/libjpeg.a
libtool: finish: PATH="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/sbin" ldconfig -n /usr/local/lib
----------------------------------------------------------------------
Libraries have been installed in:
   /usr/local/lib

If you ever happen to want to link against installed libraries
in a given directory, LIBDIR, you must either use libtool, and
specify the full pathname of the library, or use the `-LLIBDIR'
flag during linking and do at least one of the following:
   - add LIBDIR to the `LD_LIBRARY_PATH' environment variable
     during execution
   - add LIBDIR to the `LD_RUN_PATH' environment variable
     during linking
   - use the `-Wl,-rpath -Wl,LIBDIR' linker flag
   - have your system administrator add LIBDIR to `/etc/ld.so.conf'

See any operating system documentation about shared libraries for
more information, such as the ld(1) and ld.so(8) manual pages.
----------------------------------------------------------------------
 /bin/mkdir -p '/usr/local/bin'
  /bin/bash ./libtool   --mode=install /usr/bin/install -c cjpeg djpeg jpegtran rdjpgcom wrjpgcom '/usr/local/bin'
libtool: install: /usr/bin/install -c .libs/cjpeg /usr/local/bin/cjpeg
libtool: install: /usr/bin/install -c .libs/djpeg /usr/local/bin/djpeg
libtool: install: /usr/bin/install -c .libs/jpegtran /usr/local/bin/jpegtran
libtool: install: /usr/bin/install -c rdjpgcom /usr/local/bin/rdjpgcom
libtool: install: /usr/bin/install -c wrjpgcom /usr/local/bin/wrjpgcom
/bin/bash /home/edemon/Downloads/libjpeg/install-sh -d /usr/local/include
/usr/bin/install -c -m 644 jconfig.h /usr/local/include/jconfig.h
 /bin/mkdir -p '/usr/local/include'
 /usr/bin/install -c -m 644 jerror.h jmorecfg.h jpeglib.h '/usr/local/include'
 /bin/mkdir -p '/usr/local/share/man/man1'
 /usr/bin/install -c -m 644 cjpeg.1 djpeg.1 jpegtran.1 rdjpgcom.1 wrjpgcom.1 '/usr/local/share/man/man1'
make[1]: Leaving directory '/home/edemon/Downloads/libjpeg'

查看共享库的信息：

edemon@ubuntu1:/usr/local/lib$ file libjpeg.so
libjpeg.so: symbolic link to `libjpeg.so.9.1.0'

然后，我们编译、运行：

gcc -D LOAD_JPG -c ~/SDL_image-1.2.2/IMG_jpg.c -o IMG_jpg.o
gcc loadJPG.c IMG_jpg.o -o loadJPG -lSDL -lSDL_image -ljpeg

在运行的时候发现一件怪事，gcc怎么也找不到共享库文件。网上查阅：

ldconfig命令的用途, 主要是在默认搜寻目录(/lib和/usr/lib)以及动态库配置文件/etc/ld.so.conf内所列的目录下, 搜索出可共享的动态链接库(格式如lib*.so*), 进而创建出动态装入程序(ld.so)所需的连接和缓存文件. 缓存文件默认为​​/etc/ld.so.cache​​​, 此文件保存已排好序的动态链接库名字列表. ​

原来如此，我确定了路径​​/usr/local/lib​​​包含在lib搜索目录中，然后ldconfig一下。
再次运行：