在 C 上读取二进制 PGM

Question

我正在制作一个用于读取 PGM 文件的库，但遇到了这个问题。

我的代码无法正确读取二进制 PGM 图像，看起来它读取了错误的值，因此生成的图像只有“噪声”

代码真的很简单：

void OpenPGM(PGMImage* pgm, const char* file){
    FILE *pgmfile = fopen (file, "rb");

    fscanf (pgmfile, "%s", pgm->magicNumber);
    fscanf (pgmfile, "%d %d", &(pgm->width),&(pgm->height));
    fscanf (pgmfile, "%d", &(pgm->maxValue));

    pgm->data = malloc(pgm->height * sizeof(unsigned char*));

    if (pgm->magicNumber[1] == '2')
    {
        for (int i = 0; i < pgm->height; ++i)
        {
            pgm->data[i] = (unsigned char*)malloc(pgm->width * sizeof(unsigned char*));
            for (int j = 0; j < pgm->width; ++j)            
                fscanf (pgmfile, "%d", &pgm->data[i][j]);           
        }
    } else {
        fgetc(pgmfile);// this should eat the last \n
        for (int i = 0; i < pgm->height; ++i)
        {
            pgm->data[i] = (unsigned char*)malloc(pgm->width * sizeof(unsigned char*));
            fread(pgm->data[i],sizeof(unsigned char*),pgm->width,pgmfile);//reading line by line
        }
    }
}

PGMImage 看起来像这样

typedef struct PGMImage {
    char magicNumber[2];
    unsigned char** data;
    unsigned int width;
    unsigned int height;
    unsigned int maxValue;
} PGMImage;

我做错了什么？

Answer 1

阅读图片时可能存在问题：

pgm->data[i] = (unsigned char*)malloc(pgm->width * sizeof(unsigned char*));
fread(pgm->data[i],sizeof(unsigned char*),pgm->width,pgmfile);//reading line by line

应该是：

pgm->data[i] = malloc(pgm->width * sizeof(unsigned char));
if(pgm->data[i]==NULL){fprintf(stderr,"malloc failed\n");exit(1);}
fread(pgm->data[i],sizeof(unsigned char),pgm->width,pgmfile);//reading line by line

确实，unsigned char* 是指向无符号字符的指针，sizeof(unsigned char*) 将是指针的大小（可能是 8 个字节）。因此，图像被读取，每次读取一行时读取 8 行。