我写了这段代码从cs50 pset4恢复50个图像。代码只能检索到4张图片，所有的图片都不是正确的。它编译得很好。当我使用printf进行调试时，似乎if(found)代码段在name_count == 0的次数超过预期时运行了多次。
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <stdbool.h>
typedef uint8_t BYTE;
const int BLOCK_SIZE = 152;
bool is_a_jpeg(BYTE *buffer);
int main(int argc, char *argv[])
{
//Check if there are exactly two commandline arguments
if (argc != 2)
{
printf("usage: ./IMAGEn");
return 1;
}
//Open a storage device and chack if it has data
FILE *input = fopen(argv[1], "r");
if (input == NULL)
{
fclose(input);
printf("Could not open file.n");
return 2;
}
//Create a file to write into and clear it
FILE *img = NULL;
//Declar an interger of counting images
int name_count = 0;
// create buffer
BYTE buffer[BLOCK_SIZE];
//Declare space for saving the filename
char filename[8];
//Declare a bolean variable used to check for already found images
bool found = false;
//A function for reading through the device looking for images
while (fread(buffer, BLOCK_SIZE, 1, input))
{
//If a jpeg image is found notify the program(set found = true)
//and start writing the data to a new file
if (is_a_jpeg(buffer))
{
found = true;
//If we are not writing the first image, close the previous one
if (name_count > 0)
{
fclose(img);
}
//Create incrementing filenames for each new picture i.e 000.jpg, 001.jpg etc.
sprintf(filename, "%03d.jpg", name_count);
//Open an initially created empty file and start writing to it
img = fopen (filename, "w");
name_count++;
fwrite(buffer, BLOCK_SIZE, 1, img);
}
//Continue writing to a file as soon as it is found until another JPEG image is found
if(found)
{
fwrite(buffer, BLOCK_SIZE, 1, img);
}
}
//Close all the files
fclose(input);
fclose(img);
return 0;
}
//Function to check for a JPEG Image
bool is_a_jpeg(BYTE *buffer)
{
return buffer[0] == 0xff &&
buffer[1] == 0xd8 &&
buffer[2] == 0xff &&
(buffer[3] & 0xf0) == 0xe0;
}
cs50查询结果
每次if(found)代码运行时打印点。在关闭第一个图像之前，代码不必要地在第一个图像上花费大量时间
张贴的代码有一些问题:
-
块大小应该是512字节，而不是152。
-
如果
fopen失败，则不应该使用fclose(file)。这有未定义的行为。 -
你在最后无条件关闭
img，如果没有找到JPG文件或如果最后一个JPG文件无法打开，这可能有未定义的行为。 -
磁盘映像文件和jpg目标文件都必须以二进制模式打开。如果您在Windows上工作，这可能会解释意外行为。
下面是修改后的版本:
#include <errno.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <string.h>
typedef uint8_t BYTE;
const int BLOCK_SIZE = 512;
bool is_a_jpeg(const BYTE *buffer);
int main(int argc, char *argv[]) {
//Check if there are exactly two commandline arguments
if (argc != 2) {
printf("usage: ./IMAGEn");
return 1;
}
//Open a storage device and check if it has data
FILE *input = fopen(argv[1], "rb");
if (input == NULL) {
fprintf(stderr, "Could not open file %s: %s.n", argv[1], strerror(errno));
return 2;
}
//Create a file to write into and clear it
FILE *img = NULL;
//Declare an integer of counting images
int name_count = 0;
// create buffer
BYTE buffer[BLOCK_SIZE];
//Declare space for saving the filename
char filename[16];
//A function for reading through the device looking for images
while (fread(buffer, BLOCK_SIZE, 1, input)) {
//If a jpeg image is found notify the program(set found = true)
//and start writing the data to a new file
if (is_a_jpeg(buffer)) {
//close the current image file if any
if (img) {
fclose(img);
img = NULL;
}
//Create incrementing filenames for each new picture i.e 000.jpg, 001.jpg etc.
sprintf(filename, "%03d.jpg", name_count);
name_count++;
//Create an empty file and start writing to it
img = fopen(filename, "wb");
if (img == NULL) {
fprintf(stderr, "Could not open output file %s: %s.n", filename, strerror(errno));
}
}
//Continue writing to a file as soon as it is found until another JPEG image is found
if (img) {
if (!fwrite(buffer, BLOCK_SIZE, 1, img)) {
fprintf(stderr, "Error writing to %s: %s.n", filename, strerror(errno));
fclose(img);
img = NULL;
}
}
}
//Close all the files
fclose(input);
if (img)
fclose(img);
return 0;
}
//Function to check for a JPEG Image
bool is_a_jpeg(const BYTE *buffer) {
return buffer[0] == 0xff &&
buffer[1] == 0xd8 &&
buffer[2] == 0xff &&
(buffer[3] & 0xf0) == 0xe0;
}