C 不读取整个 BMP 文件 - fopen

Question

所以我正在尝试读取 C 中的 .bmp 文件。稍后我将使用 openssl 库加密该文件 - 但这只是背景信息。

我需要以二进制模式打开文件（显然），但无论出于何种原因，当我尝试打开文件时，它只读取 4 个字节。当我尝试输出我刚刚打开的这个确切文件（用于错误测试）时，它输出以下内容 - 88 24 AD FB.

在我的故障排除中，我决定在文本文件（54 字节）上尝试这个，我得到了完全相同的结果。

#include <openssl/conf.h>
#include <openssl/evp.h>
#include <openssl/err.h>

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

int main(){

    char * fileName="pic_original.bmp";

    //read the file from given filename in binary mode
    printf("Start to read the .bmp file \n");

    FILE *image;
    image = fopen(fileName,"rb");

    //print the size of the image (4 bytes every damn time)
    printf("Size of image: %d\n",sizeof(image));

    //output the exact file that was read (error testing)
    FILE *test;
    test = fopen("./test.bin", "w");
    fwrite(image, sizeof(image), 1, test);

    fclose(test);
    fclose(image);


    return 1;
}

这是图像（出于某种原因以 png 格式上传）

不太确定我哪里出错了，但我在 C 语言方面不是很熟练。

干杯，利亚姆

编辑 1：

//allocate memory for the header and image
char *headerBuf = (char *)malloc(54);
char *imageBuf = (char *)malloc(sizeof(image)-54); //this line is wrong - thanks to user EOF

//allocate memory for the final ciphertext
char *imagecipherCBC = (char *)malloc(sizeof(image)); //wrong also

//read first 54 bytes (header)
rewind(image);
fread(headerBuf,54,1,image);

//read the bitmap image until the end of the file
fread(imageBuf,sizeof(image),1,image); //also wrong

Answer 1

如果您的目标是加密文件，则将整个文件读入缓冲区，对其进行加密，然后将其保存为二进制文件。您可以通过将文件指针移动到末尾来找到文件大小。示例：

int main()
{
    FILE *fin;
    fin = fopen("pic_original.bmp", "rb");
    fseek(fin, 0, SEEK_END);
    int filesize = ftell(fin);
    rewind(fin);

    char *buf = malloc(filesize);
    fread(buf, 1, filesize, fin);
    fclose(fin);

    //encrypt the buffer...

    FILE *fout = fopen("output.bmp", "wb");
    fwrite(buf, 1, filesize, fout);
    fclose(fout);

    return 0;
}

这适用于任何文件。 OpenSSL已经有直接加密文件的功能

如果出于某种原因你想保持 header 不变，并且只更改后面的位，那么请单独阅读 header：

int main()
{
    FILE *fin = fopen("input.bmp", "rb");
    if(!fin) { printf("cannot open input\n"); return 0; }

    FILE *fout = fopen("output.bmp", "wb");
    if(!fout) { printf("cannot open output\n"); return 0; }

    fseek(fin, 0, SEEK_END);
    int filesize = ftell(fin);
    if(filesize <= 54)
    {
        printf("wrong filesize\n");
        return 0;
    }
    rewind(fin);

    char *header = malloc(54);
    char *buf = malloc(filesize - 54);
    //encrypt buf...
    fread(header, 1, 54, fin);
    fread(buf, 1, filesize - 54, fin);
    fclose(fin);

    fwrite(header, 1, 54, fout);
    fwrite(buf, 1, filesize - 54, fout);
    fclose(fout);

    free(header);
    free(buf);
    return 0;
}

我想这样做的好处是加密位图仍会被识别为位图。但是只有加密方法不会向输出添加额外的字节。

请注意，8 位、4 位和单色位图有一个调色板，它位于 54 字节标题之后，然后是图像位。

Answer 2

嗯，图像的大小当然是 4 个字节，这是 32 位机器上的文件指针。

我想你必须准备一些bmp文件的图像缓冲区作为一个简单的例子，然后如果你的文件不是太大，你可以对这个图像缓冲区的内容进行加密和解密。

static void read_from_image(char *imageBuf, int fileLength)
{
    const char * outFileName="c:/DEV/temp/test.bin";
    char headerBuf[54];
    char *imagecipherCBC;

    FILE *test;
    test = fopen(outFileName, "wb");

    //allocate memory for the final ciphertext
    imagecipherCBC = (char *)malloc(fileLength *sizeof(char));

    //read first 54 bytes (header)
    //fread(headerBuf,54,1,image);
    memcpy(headerBuf, imageBuf, 54 * sizeof(char));

    //read the bitmap image until the end of the file
    //fread(imageBuf,sizeof(image),1,image); //also wrong

    fwrite(imageBuf, fileLength * sizeof(char), 1, test);
    fflush(test);
    fclose(test);

    free(imagecipherCBC),imagecipherCBC = NULL;
    free(imageBuf),imageBuf = NULL;

    return;
}

您可以在主函数中设置文件长度和图像缓冲区。

    int main(int argc, char *argv[])
{
    const char * fileName="c:/DEV/temp/pic_original.bmp";

    int fileLength = 0;

    FILE *image;
    char *imageBuffer;

    imageBuffer = NULL;
    image = fopen(fileName,"rb");

    printf("read the file from given filename in binary mode \n");
    printf("Start to read the .bmp file \n");

    //try to get a file length;
    fseek(image, 0, SEEK_END);
    fileLength = ftell(image);
    fseek(image, 0, SEEK_SET);
    rewind(image);

    imageBuffer = (char*)malloc(fileLength * sizeof(char));

    //print the size of the image (4 bytes every damn time)
    printf("read the file from given filename in binary mode \n");
    printf("Size of image file pointer: %d\n",sizeof(image));
    printf("Size of image: %d\n",fileLength);

    //output the exact file that was read (error testing)
    fread(imageBuffer,sizeof(char),fileLength*sizeof(char), image);

    fclose(image);

    read_from_image(imageBuffer, fileLength);

    return 0;
}

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