C语言 我不明白为什么读取不能正确填充我的缓冲区.CS50复苏

Question

我花了几个小时在gdb中浏览这段代码。我知道fread返回适当的字节数(512)。甚至检查了$eax打印来确认。有没有人能给我提示一下我的逻辑出了什么问题?

我认为头可能会从文件的开头偏移，所以我认为第一次读取逐字节查找十六进制匹配并设置fseek会做到这一点。没有这样的运气。打印匹配结果为0.

#include <stdio.h>
#include <getopt.h>
#include <string.h>
#include <stdlib.h>
#include <stdint.h>
typedef uint8_t       BYTE;
typedef enum { false, true } boolean;
int main(int argc, char *argv[])
{
// get filenames from cml input and open file
char *infile = argv[optind];
char *fileName = "image";
FILE *rawData = fopen(infile, "r");
FILE *imgJPG  = fopen(fileName, "w");
int match  = 0;
int imgCnt = 0;
// buffer to hold 512 bytes of file data - FAT file system
BYTE *FATbuffer = (BYTE *)malloc(sizeof(BYTE) * 512);
if (rawData == NULL)
{
printf("Error processing file. Exiting...");
return 1;
}
// begin reading raw data and writing it to buffer
while (fread(FATbuffer, sizeof(BYTE), 512, rawData) == 512)
{
if (imgCnt == 0)
{
for (int c = 0; c < 512; c++)
{
if (FATbuffer[c + 0] == 0xff &&
FATbuffer[c + 1] == 0x8d &&
FATbuffer[c + 2] == 0xff)
{
fseek(rawData, c, SEEK_SET);
imgCnt++;
match++;
}
}
}
else
{
if (FATbuffer[0] == 0xff &&
FATbuffer[1] == 0x8d &&
FATbuffer[2] == 0xff &&
imgCnt > 0)
{
sprintf(fileName, "%d.jpg", imgCnt);
fclose(imgJPG);
imgCnt++;
}
if (imgJPG == NULL)
{
printf("Error processing file. Exiting...");
return 3;
}
fwrite(FATbuffer, sizeof(BYTE), 512, imgJPG);
}
}
printf("%dn", match);
// file processed, free memory
free(FATbuffer);
return 0;
}

Answer 1

你的代码中有多个问题:

• 您应该测试命令行参数是否可用(在处理选项之后，您可能删除了用于发布的代码)。

• 文件必须以二进制模式打开，以避免可能的翻译结束。

• 你应该延迟打开imgJPG，直到你找到JPG标题。

• 不需要分配FATbuffer，定义一个512字节的自动存储数组就可以了。

• 每次扫描一个块的JPG签名，但如果它跨越512字节边界，并且当c大于509时，您访问FATbuffer数组末尾超过2字节，则可能会错过签名。

• fseek(rawData, c, SEEK_SET);设置文件位置在数组开始的偏移量处，而不是文件开始的偏移量。

• sprintf(fileName, "%d.jpg", imgCnt);尝试覆盖字符串常量。这具有未定义的行为。你的意思可能是:

  char fileName[64];
  snprintf(fileName, sizeof fileName, "image%d.jpg", imgCnt);
  FILE *imgJPG = fopen(fileName, "wb");
  

这是一个修改后的版本，可以提取嵌入在数据流中的任何地方的JPG文件:

#include <errno.h>
#include <getopt.h>
#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <string.h>
int main(int argc, char *argv[])
{
// get filenames from cml input and open file
if (optind >= argc)
return 1;
char *infile = argv[optind];
FILE *rawData = fopen(infile, "rb");  // open the disk image
char fileName[64];
FILE *imgJPG = NULL;
int imgCnt = 0;
// buffer to hold 512 bytes of file data - FAT file system
// add an extra 2 bytes to match signature across sector boundaries
uint8_t FATbuffer[514];
// begin reading raw data into buffer
int pos = 2;
while (fread(FATbuffer + 2, 1, 512, rawData) == 512) {
for (int c = pos; c < 512; c++) {
if (FATbuffer[c + 0] == 0xff &&
FATbuffer[c + 1] == 0x8d &&
FATbuffer[c + 2] == 0xff) {
// found signature: skip to a new file
if (imgJPG) {
// write the end of the current image
fwrite(FATbuffer + pos, c - pos, 1, imgJPG);
fclose(imgJPG);
}
pos = c;
imgCnt++;
snprintf(fileName, sizeof fileName, "image%d.jpg", imgCnt);
imgJPG = fopen(fileName, "wb");
if (imgJPG == NULL) {
fprintf(stderr, "Cannot create file %s: %sn",
fileName, strerror(errno));
return 3;
}
}
}
if (imgJPG) {
// write end of block to current image
fwrite(FATbuffer + pos, 512 - pos, 1, imgJPG);
}
// copy the last 2 bytes to test for signature overlapping blocks
FATbuffer[0] = FATbuffer[512];
FATbuffer[1] = FATbuffer[513];
// uncopied file data starts a 0 now.
pos = 0;
}
if (imgJPG) {
// write last 2 bytes to current image
fwrite(FATbuffer, 2, 1, imgJPG);
fclose(imgJPG);
}
printf("%dn", imgCnt != 0);
printf("%d images extractedn", imgCnt);
return 0;
}

如果可以假设签名位于扇区的开头，则可以简化代码:

#include <errno.h>
#include <getopt.h>
#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <string.h>
int main(int argc, char *argv[])
{
// get filenames from cml input and open file
if (optind >= argc)
return 1;
char *infile = argv[optind];
FILE *rawData = fopen(infile, "rb");  // open the disk image
char fileName[64];
FILE *imgJPG = NULL;
int imgCnt = 0;
// buffer to hold 512 bytes of file data - FAT file system
uint8_t FATbuffer[512];
// begin reading raw data into buffer
while (fread(FATbuffer, 1, 512, rawData) == 512) {
if (FATbuffer[c + 0] == 0xff &&
FATbuffer[c + 1] == 0x8d &&
FATbuffer[c + 2] == 0xff) {
// found signature: skip to a new file
if (imgJPG) {
fclose(imgJPG);
}
imgCnt++;
snprintf(fileName, sizeof fileName, "image%d.jpg", imgCnt);
imgJPG = fopen(fileName, "wb");
if (imgJPG == NULL) {
fprintf(stderr, "Cannot create file %s: %sn",
fileName, strerror(errno));
return 3;
}
}
if (imgJPG) {
// write end of block to current image
fwrite(FATbuffer, 512, 1, imgJPG);
}
}
if (imgJPG) {
fclose(imgJPG);
}
printf("%dn", imgCnt != 0);
printf("%d images extractedn", imgCnt);
return 0;
}