Compiling pyd file on windows but "This program cannot be run in DOS mode."

Question

I was compiling a C file with gcc on Windows and got pyd file successfully. To my surprise, it shows "This program cannot be run in DOS mode" in hex. Although I still can call the function from it, the program crashed soon, caused by jpeg_read_header() from libjpeg library.

My question is what on earth made my program crashed.

Here are my guesses:

1. jpeg_read_header() : I tried both jpeg_mem_src() and jpeg_stdio_src() but it still crashed.

   int _read_dct_coefficients(FILE* input_file, int** all_dcts)
   {   
   JDIMENSION i, compnum, rownum, blocknum;
   JBLOCKARRAY row_ptrs[MAX_COMPONENTS];
   size_t block_row_size;
   int num_blocks = 0, cnt = 0;
   
   #ifdef LOG_DEBUG
   log_debug(__LINE__, "enter _read_dct_coefficients");
   #endif
   
   /* init decompression */
   srcinfo.err = jpeg_std_error(&jsrcerr);
   jpeg_create_decompress(&srcinfo);
   
   /* init compression */
   dstinfo.err = jpeg_std_error(&jdsterr);
   jpeg_create_compress(&dstinfo);
   
   jsrcerr.trace_level = jdsterr.trace_level;
   srcinfo.mem->max_memory_to_use = dstinfo.mem->max_memory_to_use;
   
   #ifdef LOG_DEBUG
   log_debug(__LINE__, "%%%%%%%MY TEST # 1%%%%%%%%");
   #endif  
   
   //***************************************************************
   
   unsigned int get_file_size(FILE *fp)  
   {  
       unsigned long filesize = -1;  
       if(fp == NULL)
           return filesize;
       fseek(fp, 0L, SEEK_END);
       filesize = ftell(fp);
       fclose(fp);
       return filesize;
   }
   
   int size = get_file_size(input_file);
   #ifdef LOG_DEBUG
   log_debug(__LINE__, "file size = %d", size);
   #endif
   char *tmp_buf = (unsigned char *)malloc(sizeof(char) * size);
   if (size != fread(tmp_buf, 1, size, input_file))
       log_debug(__LINE__, "cannot open.");
   
   jpeg_mem_src(&srcinfo, tmp_buf, size);
   /*
   jpeg_stdio_src(&srcinfo, input_file);
   */
   #ifdef LOG_DEBUG
   log_debug(__LINE__, "%%%%%%%MY TEST # 2%%%%%%%%");
   #endif  
   jpeg_read_header(&srcinfo, TRUE);
   
   ......
   
   }
   

2. Pyd file : It cannot be run in DOS mode?

Answer 1

A Python .pyd file is just a DLL, which is just a Windows PE file. Windows PE files by convention start with a stub that prints that message if you run them in DOS:

https://en.wikipedia.org/wiki/Portable_Executable#History

Pretty much every Windows EXE and DLL file contains this header; it doesn't imply anything special.