fwrite与BMP文件返回0和分段错误

Question

我正在尝试读取一个简单的bmp文件然后反转颜色，然后使用fread和fwrite二进制模式将其保存回文件系统。

所以我做了

BMP_Image * img; 
BMP_Header * hdr;

fread(hdr,sizeof(BMP_Header),1, fptr); 

img->width = hdr->width;
img->height = hdr->height;
img->bytes_per_pixel = hdr->bits*BIT_TO_BYTE; 
img->header = *hdr;
img->data_size = hdr->size; 

获取BMP_Header是struct

的标题信息

然后读取图像数据并放入数组

fread(data, sizeof(char), img->data_size - 54, fptr);

然后我将图像数据中的每个元素减去255

int i = 0;
for(i = 0;i<img->data_size;i++){
   data[i] = 255 - data[i];
}

现在，当我想将文件写回文件系统时，我无法正常工作。我知道你必须做两次fwrite。首先写入标题，然后写入图像数据。

我到目前为止：

 fwrite(header, 1 ,dataSize, fptr_out); 

给出了sgmentation错误和0字节bmp文件。

以下是我的BMP_Header和BMP_image结构

的typedef

typedef unsigned short int uint16_t;
typedef unsigned int uint32_t;
typedef int int32_t;

typedef struct {
    uint16_t type;                      // Magic identifier
    uint32_t size;                      // File size in bytes
    uint16_t reserved1;     // Not used
    uint16_t reserved2;       // Not used
    uint32_t offset;                    // Offset to image data in bytes
    uint32_t header_size;               // Header size in bytes
    int32_t  width;                     // Width of the image
    int32_t  height;                    // Height of image
    uint16_t planes;                    // Number of color planes
    uint16_t bits;                      // Bits per pixel
    uint32_t compression;               // Compression type
    uint32_t imagesize;                 // Image size in bytes
    int32_t  xresolution;               // Pixels per meter
    int32_t  yresolution;               // Pixels per meter
    uint32_t ncolours;                  // Number of colors  
    uint32_t importantcolours;          // Important colors

} BMP_Header;

typedef struct {
    BMP_Header header;
    int data_size;
    int width;
    int height;
    int bytes_per_pixel; // This amount should be equals to number of bits/8
    char *data;
} BMP_Image;