指向结构指针的指针答案

【问题标题】：Pointer to Pointer of structures指向结构指针的指针
【发布时间】：2020-09-07 17:28:08
【问题描述】：

我编写了这段代码，其中 readData 接受输入并将值存储在一个结构中，而 writeData 打印这些值。我需要将 struct Color* 指针值保留在堆内存中，以便我可以从 writeData 函数访问它。出于某种原因，它会在调用 writeData 时保留垃圾值。有人可以帮我弄清楚为什么会这样。谢谢

imageloader.c 文件：

typedef struct Color 
{
    uint8_t R;
    uint8_t G;
    uint8_t B;
} Color;
typedef struct Image
{
    Color **image;
    uint32_t rows;
    uint32_t cols;
} Image;

Image *readData(char *filename)
{
    FILE *fp = fopen(filename, "r");
    char format[3];
    int rows = 0, cols = 0, num = 0;
    uint8_t R_int = 0;
    uint8_t G_int = 0;
    uint8_t B_int = 0;
    fscanf(fp, "%s\n %d \n %d\n", format, &rows, &cols, &num);
    Color** p = (Color** )malloc(rows *cols * sizeof(Color*));
    for(int i =0; i< rows *cols; i++)
    {
        fscanf(fp, "%d %d %d", &R_int, &G_int, &G_int);
        Color* c = (Color *) malloc(sizeof(Color));
        c->R = R_int;
        c->G = G_int;
        c->B = B_int;
        p[i] = &c;
    }
    Image *img = (Image*)malloc(sizeof(Image));
    img->image = p;
    img->rows = rows;
    img->cols = cols;
    printf("%d", p[0]->R); //garbage value
    return img;
}
void writeData(Image *image)
{
}


}

主函数调用readData和writeData

int main(int argc, char **argv)
{
    Image *image;
    uint32_t rule;
    char *filename;
    processCLI(argc,argv,&filename);
    image = readData(filename);
    writeData(image);
    freeImage(image);
}

【问题讨论】：

p[i] = &c;
@Zindagi 这个调用有三个转换说明符和四个参数 fscanf(fp, "%s\n %d \n %d\n", format, &rows, &cols, &num);

标签： c pointers malloc

【解决方案1】：

打开编译器警告将帮助您找到此类错误。

$ clang -c -Wall -Wextra main.c
main.c:29:57: warning: data argument not used by format
      string [-Wformat-extra-args]
  ..."%s\n %d \n %d\n", format, &rows, &cols, &num);
     ~~~~~~~~~~~~~~~~~                        ^
main.c:33:32: warning: format specifies type 'int *' but
      the argument has type 'uint8_t *' (aka
      'unsigned char *') [-Wformat]
        fscanf(fp, "%d %d %d", &R_int, &G_int, &G_int);
                    ~~         ^~~~~~
                    %s
main.c:33:40: warning: format specifies type 'int *' but
      the argument has type 'uint8_t *' (aka
      'unsigned char *') [-Wformat]
        fscanf(fp, "%d %d %d", &R_int, &G_int, &G_int);
                       ~~              ^~~~~~
                       %s
main.c:33:48: warning: format specifies type 'int *' but
      the argument has type 'uint8_t *' (aka
      'unsigned char *') [-Wformat]
        fscanf(fp, "%d %d %d", &R_int, &G_int, &G_int);
                          ~~                   ^~~~~~
                          %s
main.c:38:14: warning: incompatible pointer types assigning
      to 'Color *' (aka 'struct Color *') from 'Color **'
      (aka 'struct Color **'); remove &
      [-Wincompatible-pointer-types]
        p[i] = &c;
             ^ ~~

【讨论】：