#include <stdlib.h>
#include <windows.h>
#include <stdio.h>

//#ifdef INT32
//#undef INT32
//#endif

#include <jpeglib.h>
#pragma comment(lib, "jpeglib.lib");

int jpegCapture(char* filename, int quality)
{
   HBITMAP hBMP;
   HWND desktopWnd;
   int width;
   int height;
   RECT rc;
   HDC hDC;
   HDC hDCmem;
   int posx;
   int posy;
   unsigned char scanline[1000000000];
   JSAMPROW row_pointer[1];	// pointer to JSAMPLE row[s]

   struct jpeg_compress_struct cinfo;
   struct jpeg_error_mgr jerr;
   FILE * outfile;
   COLORREF pixel;

   // Make capture bitmap
   desktopWnd = GetDesktopWindow();
   GetWindowRect(desktopWnd, &rc);
   width = rc.right - rc.left;
   height = rc.bottom - rc.top;
   hDC = GetDC(desktopWnd);
   hDCmem = CreateCompatibleDC(hDC);
   hBMP = CreateCompatibleBitmap(hDC, width, height);
   if (hBMP == NULL) return -2;
   SelectObject(hDCmem, hBMP);
   BitBlt(hDCmem, 0, 0, width, height, hDC, rc.left, rc.top, SRCCOPY);

   // Compress to JPEG using hDCmem
   cinfo.err = jpeg_std_error(&jerr);
   jpeg_create_compress(&cinfo);
   outfile = fopen(filename, "w+b");
   if (outfile == NULL) return -1;
   jpeg_stdio_dest(&cinfo, outfile);
   cinfo.image_width = width;
   cinfo.image_height = height;
   cinfo.input_components = 3;
   cinfo.in_color_space = JCS_RGB;
   jpeg_set_defaults(&cinfo);
   if (quality < 0) quality = 0;
   else if(quality > 100) quality = 100;
   jpeg_set_quality(&cinfo, quality, FALSE);
   jpeg_start_compress(&cinfo, TRUE);
   for (posy = 0; posy < height; posy++) {
      for (posx = 0; posx < width; posx++) {
         pixel = GetPixel(hDCmem, posx, posy);
         scanline[posx*3+0] = GetRValue(pixel);
         scanline[posx*3+1] = GetGValue(pixel);
         scanline[posx*3+2] = GetBValue(pixel);
      }
      row_pointer[0] = &scanline[0];
      (void) jpeg_write_scanlines(&cinfo, row_pointer, 1);
   }
   jpeg_finish_compress(&cinfo);

   jpeg_destroy_compress(&cinfo);
   fclose(outfile);
   DeleteDC(hDCmem);
   ReleaseDC(desktopWnd, hDC);
   return 0;
}

int WINAPI WinMain (HINSTANCE hThisInstance,
                    HINSTANCE hPrevInstance,
                    LPSTR lpszArgument,
                    int nFunsterStil)

{
    jpegCapture("hallo.jpg",75);
    return 0;
}

Es liegt an den Funktionen der jpeglib.h, aber was der Fehler ist, weiß ich nicht. Die Funktion hab ich von dieser Website "geklaut": http://members.aol.com/OlivThill/savejpg/savejpg1_c.htm

Bitte keine "Antworten" in Richtungen: "Warum nimmst du kein GDI+?"
Ich will in C und nicht in C++ programmieren.

http://www.c-plusplus.net/forum/viewtopic-var-t-is-95271-and-highlight-is-jpeglib.html
http://www.c-plusplus.net/forum/viewtopic-var-t-is-46819-and-highlight-is-jpeglib.html

Bitte verschieben, danke.

unsigned char scanline[1000000000];

Der Stack ist normalerweise nur 1 MB groß.