I am writing a code that reads information from a memory card (card.raw is the one we are provided but the code uses user input) and extracts the jpegs from it using the signatures that jpegs have of (0xff,0xd8,0xff,0x00 - 0xff). I am getting a segmentation fault because i am using malloc, but i dont see where i went wrong. I am pasting my code here any help would be appreciated.
#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <stdint.h>
typedef uint8_t BYTE;
int main(int argc, char *argv[])
{
//check terminal usage
if (argc != 2)
{
printf("Usage: ./recover image");
return 1;
}
//open inputted file and check for valid file
FILE *file = fopen(argv[1], "r");
if (!file)
{
printf("Invalid or missing file.");
return 1;
}
BYTE *buff = malloc(512 * sizeof(BYTE));
int counter = 0;
FILE *image = NULL;
char *name = malloc(8 * sizeof(char));
//loop till end of file reached and read a block of input
while(fread(buff, sizeof(BYTE), 512, file) == 1 && !feof(file))
{
bool foundJPEG = buff[0] == 0xff && buff[1] == 0xd8 && buff[2] == 0xff && ((buff[3] & 0xf0) == 0xe0);
//check if found jpeg, and open file for writing
if (foundJPEG)
{
sprintf(name, "%03i.jpg", counter);
image = fopen(name, "w");
}
//if image file open, write to it
if (image != NULL)
{
fwrite(buff, sizeof(BYTE), 512, image);
}
//if found a jpeg already, close it so new one can be written
if (foundJPEG && image != NULL)
{
fclose(image);
counter++;
}
}
free(name);
free(buff);
fclose(image);
fclose(file);
return 0;
}
There are three issues with the code above which are not mentioned in the comments:
The return value of fread is not 1 but 512, upon successful read. You exchanged the parameters for the blocksize and the blockcount -> fread definition. Therefore the while loop is not entered.
Don't try to save space with packing to much code into one statement. If would be more clever to separate the checks for the fread return value and the EOF and use a do ... while() loop, instead. Then you had the chance of seeing this issue in the debugger. This was exactly what i have done and how i found this out.
The second issue is that you close the image after rescuing the first 512 bytes, but you do not reset the file pointer image back to NULL along with the fclose statement.
As a consequence, the code would repeatedly write to an a file which is closed until a new block with a jpg header is found.
The third issue is that you only rescue the first 512 bytes of the jpg but not the whole jpg. You need to scan the input stream for the jpg end indicator FF D9 and copy bytes until it is found. ->jpg format
Related
Hello and thank you for taking a look.
I'm working through CS50x and am struggling with Recover. The aim is to open a .raw file, read its contents in 512-byte blocks, check the initial four bytes for .jpg headers, and then write each JPEG data to a new file.
I have a body of code written, and the file compiles. The debugger tells me that my buffer[512] variable remains empty/zeroed. This then means the program skips if/else conditions and the program exits.
While my logic within the While loop may be flawed, I haven't been able to step far enough into the program to consider this.
I looked up my issue before posting. Some sources like to use fread(buffer, 512, 1, input), but CS50 itself uses fread(buffer, 1, 512, input). Also, when initialising the filename, I have tried both char *filename = malloc(8 * sizeof(char)); and char filename[8];. For both lines I have tried each method and am still missing something.
My code is below. Thank you in advance for your time.
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
typedef uint8_t BYTE;
int main(int argc, char *argv[])
{
// First check the number of arguments is correct.
if (argc != 2)
{
printf("Correct Usage: ./recover.c [filename]\n");
return 1;
}
// Open the file.
FILE *inputFile = fopen(argv[1], "r");
if (inputFile == NULL)
{
printf("File not found.\n");
return 1;
}
// Create counter of number of files.
int counter = 0;
// Create filename variable
char *filename = malloc(8 * sizeof(char)); // 7 + 1 for \0
// Create a 512-size array buffer.
BYTE buffer[512];
// Initialise img file for scope access.
FILE *img = NULL;
while (fread(buffer, sizeof(BYTE), 512, inputFile))
{
// If start of new JPEG:
if (buffer[0] == 0xff && buffer[1] == 0xd8 && buffer[2] == 0xff && (buffer[3] & 0xf0) == 0xe0)
{
if (counter == 0) // If the FIRST JPEG
{
// Make new file:
sprintf(filename, "%03i.jpg", counter);
img = fopen(filename, "w");
fwrite(buffer, sizeof(BYTE), 512, img);
}
else // If not the first JPEG
{
fclose(img); // Close previous file.
counter++;
// Make new file:
sprintf(filename, "%03i.jpg", counter); // Update filename.
img = fopen(filename, "w");
fwrite(buffer, sizeof(BYTE), 512, img);
}
}
else if (counter > 0) // buffer is continuation of previous.
{
fwrite(buffer, sizeof(BYTE), 512, img);
}
else
{
printf("I exited with no images.\n");
return 2;
}
}
free(filename);
fclose(img);
fclose(inputFile);
return 0;
}
The program exits (returns) after the first line in the raw file is read (assuming it's not a jpeg header, which is the case with the distro raw file). else if (counter > 0) evaluates to false, so the else branch executes.
Thank you everyone for your response. The issue is now fixed!
#DinoCoderSaurus (sorry, can't upvote yet) prompted me to realise that I had assumed (wrongly) that the data in the raw file would immediately begin with a .jpeg header (in fact it looks like the data begins with a hidden message, "surprise").
The Else condition was initially put there to avoid errors but of course it was prematurely exiting the While loop. The buffer was populated correctly after a couple of loops.
I then encountered the second problem (pointed out by #Some_programmer_dude) that counter++; was in the wrong place, which meant after the first new JPEG, no others could be written.
I'll also take your comments about best practice into consideration.
I have been trying to do this problem for at least a week now, and can't seem to understand where is the problem, I already checked everything in google, and dont know any programmer in real life to ask them personaly, so if anyone can help me it would be great.
None of the images generated load, and it doesnt recover 50 as it is suposed to, it recovers 986.
I get this results in check50:
:) recover.c exists.
:) recover.c compiles.
:) handles lack of forensic image
:( recovers 000.jpg correctly
recovered image does not match
:( recovers middle images correctly
recovered image does not match
:( recovers 049.jpg correctly
recovered image does not match
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <cs50.h>
typedef uint8_t BYTE;
#define BLOCK_SIZE 512
int main(int argc, char *argv[])
{
//it only accepts one comand argument in the name of an image
if (argc != 2)
{
printf("Usage: ./recover IMAGE");
return 1;
}
//check if it can open the image
FILE *file = fopen(argv[1], "r");
if (file == NULL)
{
printf("The image cannot be opened");
return 1;
}
bool jpg_before = false;
int counter = 0;
FILE *image = NULL;
char name[8];
unsigned char buffer[BLOCK_SIZE];
//while there is still jpegs in the file
while (fread(buffer, BLOCK_SIZE, 1, file) == 1)
{
if (buffer[0] == 0xff && buffer[1] == 0xd8 && buffer[2] == 0xff && (buffer[3] & 0xe0) == 0xe0)
{
jpg_before = true;
}
if(jpg_before == true)
{
sprintf(name, "%03i.jpg", counter);
counter++;
image = fopen(name, "a");
fwrite(buffer, BLOCK_SIZE, 1, image);
fclose(image);
}
}
fclose(file);
}
(also please keep in mind I'm new to programming, 16 years old and english is not my first lenguage)
When you detect a header in the input, you set jpg_before. But, you never clear it.
Once the flag is set, each block will be put into a different file.
Every output file should consist of a header, followed by the associated data blocks.
name[8] is a bit too small. The compiler will complain because the int could [in theory] be 10 or so digits, so the sprintf could overflow. Don't be stingy--use (e.g.): char name[20];
Output file should be opened with "w" instead of "a". If the program is run twice, the second time, the output file(s) will be incorrect.
Here is the refactored code:
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
//#include <cs50.h>
typedef uint8_t BYTE;
#define BLOCK_SIZE 512
int
main(int argc, char *argv[])
{
// it only accepts one comand argument in the name of an image
if (argc != 2) {
printf("Usage: ./recover IMAGE");
return 1;
}
// check if it can open the image
FILE *file = fopen(argv[1], "r");
if (file == NULL) {
printf("The image cannot be opened");
return 1;
}
int counter = 0;
FILE *image = NULL;
char name[20];
unsigned char buffer[BLOCK_SIZE];
// while there is still jpegs in the file
while (fread(buffer, BLOCK_SIZE, 1, file) == 1) {
if (buffer[0] == 0xff && buffer[1] == 0xd8 && buffer[2] == 0xff && (buffer[3] & 0xe0) == 0xe0) {
if (image != NULL)
fclose(image);
sprintf(name, "%03i.jpg", counter);
counter++;
image = fopen(name, "w");
}
fwrite(buffer, BLOCK_SIZE, 1, image);
}
if (image != NULL)
fclose(image);
fclose(file);
}
UPDATE:
From the comments below:
Points 2 and 3 look to be handled by the file being opened for appending. Leaving the file open is probably a better idea, though. Faster and handles point 6. –
user4581301
If H is header and D is data, for an input of (e.g): H1,D1,D2,D3,D4,H2,D5,D6,D7:
Instead of two output files: F0:H1,D1,D2,D3,D4 and F1:H2,D5,D6,D7
We'd have: F0:H1, F1:D1, F2:D2, F3:D3, F4:D4, F5:H2, F6:D5, F7:D6, F8:D7
Although my refactored code was correct, the top section of my answer had an incorrect analysis of what OP's code was actually doing.
I've fixed that. But, to make user4581301's make sense, here is the original analysis:
When you detect a header in the input, you set jpg_before. But, you never clear it.
You only write to the output stream for the header block, so any data is not copied. So, each output file will only be 512 bytes
You immediately close the output stream after writing the header. It should be left open.
Every block must go to a given output file, not just the header.
name[8] is a bit too small. The compiler will complain because the int could [in theory] be 10 or so digits, so the sprintf could overflow. Don't be stingy--use (e.g.): char name[20];
Output file should be opened with "w" instead of "a". If the program is run twice, the second time, the output file(s) will be incorrect.
UPDATE #2:
First of all thanks! But it is giving me a segmentation fault, do you have any idea why? because everything seems correct –
Isa M
From code inspection, the only place that could segfault is the fwrite call (i.e. image is NULL).
I confirmed this by running the program under gdb [I have the cs50 recover input file]. When the program faults, just do tb to get a stack traceback.
image could be NULL for the following reasons:
The fopen for output file could fail (due to permissions, space, etc.) and return NULL. There was no check after the call as there was for opening the input file.
image starts out being NULL. If there is some sort of extra file data/file header before the first jpg header (e.g. before FF/D8/FF/E0) the if will not match on the first block read. The fwrite will be called even with a NULL in image.
Option (2) is what actually occurred because cs50's file has an extra header at the top of the file. You can see this by examining the file with a hex editor/dumper (e.g.) od or xxd:
00000000: 00000000 00000000 00000000 00000000 ................
*
00000200: 63733530 2E6C792F 73757270 72697365 cs50.ly/surprise
00000210: 00000000 00000000 00000000 00000000 ................
*
00000400: FFD8FFE0 00104A46 49460001 01000001 ......JFIF......
The code will not see a valid header (i.e. the if matches) until offset 400. So, there are two extraneous fread calls at the start until things sync up.
The fix is to change:
fwrite(buffer, BLOCK_SIZE, 1, image);
Into:
if (image != NULL)
fwrite(buffer, BLOCK_SIZE, 1, image);
I've written a few answers on this problem before. However, I forgot to include this. I just wrote the code but did not test it ;-)
To round things out, I've added more return code checking and added "rb" and "wb" to the fopen calls, just in case you're running on Windoze.
Here is the updated/fixed code (I've tested it this time ;-):
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <string.h>
#include <errno.h>
//#include <cs50.h>
typedef uint8_t BYTE;
#define BLOCK_SIZE 512
void
onerr(const char *action,const char *file)
{
printf("%s -- %s -- %s\n",action,file,strerror(errno));
exit(1);
}
int
main(int argc, char *argv[])
{
// it only accepts one comand argument in the name of an image
if (argc != 2) {
printf("Usage: ./recover IMAGE");
return 1;
}
// check if it can open the image
FILE *file = fopen(argv[1], "rb");
if (file == NULL)
onerr("The image cannot be opened",argv[1]);
int counter = 0;
FILE *image = NULL;
char name[20];
unsigned char buffer[BLOCK_SIZE];
// while there is still jpegs in the file
while (fread(buffer, BLOCK_SIZE, 1, file) == 1) {
if (buffer[0] == 0xff &&
buffer[1] == 0xd8 &&
buffer[2] == 0xff &&
(buffer[3] & 0xe0) == 0xe0) {
if (image != NULL)
fclose(image);
sprintf(name, "%03i.jpg", counter);
counter++;
image = fopen(name, "wb");
if (image == NULL)
onerr("unable to open output file",name);
}
#if 0
fwrite(buffer, BLOCK_SIZE, 1, image);
#else
if (image != NULL)
fwrite(buffer, BLOCK_SIZE, 1, image);
#endif
}
if (image != NULL)
fclose(image);
fclose(file);
return 0;
}
I am writing a program in C to recover images from a raw file for CS50 and I am having a strange problem. I have a variable int cnt that I was using for debug purposes and I got the program to work so I was removing leftover debug code. But when I remove the cnt declaration I start outputting corrupt files.
Before removing line 25 below I was outputing .jpg files that I could open and view, then I removed the line, recompiled, deleted the photos from the last run, and reran the program on the same .raw data and the new files I got were unrecognized. So I put the declaration back in, recompiled, deleted the old photos, and ran the program again and got good files. Does anyone know why removing an unused declaration is messing with my results? The offending declaration is on line 25.
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
int main(int argc, char *argv[])
{
if (argc != 2)
{
printf("Usage: ./recover image\n");
return 1;
}
int filesFound = 0;
FILE *inFile = fopen(argv[1], "r");
FILE *outFile = NULL;
if (inFile == NULL)
{
printf("Image file could not be opened\n");
return 1;
}
uint8_t buffer[512];
int cnt = 0;
while (!feof(inFile))
{
fread(buffer, 512, 1, inFile);
// check for start of jpg file
if (buffer[0] == 0xff && buffer[1] == 0xd8 && buffer[2] == 0xff && (buffer[3] & 0xf0) == 0xe0)
{
// start of jpg was found
if (outFile != NULL)
{
// close the current file and then open a new file to write to
fclose(outFile);
outFile = NULL;
}
// open a file to write to
char fName[4];
sprintf(fName, "%03i.jpg", filesFound);
outFile = fopen(fName, "w");
filesFound++;
}
if (outFile != NULL){
// we have found data to write and opened a file
fwrite(buffer, 512, 1, outFile);
}
}
//Be sure to close my files
fclose(inFile);
if (outFile != NULL)
{
fclose(outFile);
}
return 0;
}
char fName[4] does not have sufficient room for the name generated by "%03i.jpg", so you are overrunning the buffer. Make it larger and use snprintf, not sprintf, and test the return value to detect errors:
int result = snprintf(fName, sizeof fName, "%03i.jpg", filesFound);
if (sizeof fName <= result)
{
fprintf(stderr, "Internal error, buffer is too small for file name.\n");
exit(EXIT_FAILURE);
}
Instead of printing an error, you could instead use the return value of snprintf, which indicates the length needed, to allocate memory for a larger buffer and then redo the snprintf with that buffer.
(Note that snprintf may return a negative result if an error occurs. Normally, this will become a large number upon conversion to size_t for the comparison, so it will trigger this error message. However, in a robust program, you might want to insert a separate test for result < 0.)
I am learning how to code and I have no experience with that at all. I've successful got to PSET4 and stuck on recover. I've read everything online about this problem and i found out that many people have similar code as I do and it works. Does not work for me whatsoever. Please have a look and give me a hint what did I do wrong and how to correct it.
Here is everything about the pset4 recover i downloaded their card.raw from here card.raw
/** recovering JPEG files from a memory card
*
*/
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
typedef uint8_t BYTE;
int main(int argc, char* argv[])
{
// ensure proper usage
if (argc != 2)
{
fprintf(stderr,
"Usage: ./recover infile (the name of a forensic image from which to recover JPEGs)\n");
return 1;
}
// open input file (forensic image)
FILE* inptr = fopen(argv[1], "r");
if (inptr == NULL)
{
fprintf(stderr, "Could not open %s.\n", argv[1]);
return 2;
}
FILE* outptr = NULL;
// create a pointer array of 512 elements to store 512 bytes from the memory card
BYTE* buffer = malloc(sizeof(BYTE) * 512);
if (buffer == NULL)
{
return 3;
}
// count amount of jpeg files found
int jpeg = 0;
// string for a file name using sprintf
char filename[8] = { 0 };
// read memory card untill the end of file
while (fread(buffer, sizeof(BYTE) * 512, 1, inptr) != 0)
{
// check if jpeg is found
if (buffer[0] == 0xff && buffer[1] == 0xd8 && buffer[2] == 0xff
&& (buffer[3] >= 0xe0 || buffer[3] <= 0xef))
{
if (jpeg > 0)
{
fclose(outptr);
}
sprintf(filename, "%03d.JPEG", jpeg);
outptr = fopen(filename, "w");
jpeg++;
}
if (jpeg > 0)
{
fwrite(buffer, sizeof(BYTE) * 512, 1, outptr);
}
}
// free memory
free(buffer);
// close filename
fclose(outptr);
// close input file (forensic image)
fclose(inptr);
return 0;
}
The main problem is that you invoke undefined behavior because filename is not enough big. sprintf() need be 9 and 17 bytes with your code but you only has 8. So you have a buffer overflow.
Just change:
char filename[8] = { 0 };
to
char filename[17] = { 0 };
Because, you use an int, this value is implemented defined but in many system has an int with 32 bits. So the value possible are between -2^31 and 2^31 - 1 that make a maximum of 11 chars (-2147483648). We add the number of chars in ".JPEG", 5. We have 16 but you forget the null terminate byte of a c-string. So we are 17 maximum.
Modern compiler warning you: gcc version 7.1.1 20170516 (GCC):
In function ‘main’:
warning: ‘sprintf’ writing a terminating nul past the end of the destination [-Wformat-overflow ]
sprintf(filename, "%03d.JPEG", jpeg++);
^
note: ‘sprintf’ output between 9 and 17 bytes into a destination of size 8
sprintf(filename, "%03d.JPEG", jpeg++);
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Plus, your typedef is useless because a char world be always a byte in C. More than that you don't need a byte but an octet so like char, uint8_t would be always an octet in C. So you don't need typedef.
Again one thing, you allocate your buffer but it's useless because your buffer has a constant size. So just create an array is more simple.
#include <stdint.h>
#include <stdio.h>
int main(int argc, char *argv[]) {
if (argc != 2) {
fprintf(stderr, "Usage: ./recover infile (the name of a forensic image "
"from which to recover JPEGs)\n");
return 1;
}
FILE *inptr = fopen(argv[1], "r");
if (inptr == NULL) {
fprintf(stderr, "Could not open %s.\n", argv[1]);
return 2;
}
FILE *outptr = NULL;
uint8_t buffer[512];
size_t const buffer_size = sizeof buffer / sizeof *buffer;
size_t jpeg = 0;
while (fread(buffer, sizeof *buffer, buffer_size, inptr) == buffer_size) {
if (buffer[0] == 0xff && buffer[1] == 0xd8 && buffer[2] == 0xff &&
buffer[3] == 0xe0) {
if (outptr != NULL) {
fclose(outptr);
}
char filename[26];
sprintf(filename, "%03zu.JPEG", jpeg++);
outptr = fopen(filename, "w");
}
if (outptr != NULL) {
fwrite(buffer, sizeof *buffer, buffer_size, outptr);
}
}
if (outptr != NULL) {
fwrite(buffer, sizeof *buffer, buffer_size, outptr);
}
if (outptr != NULL) {
fclose(outptr);
}
fclose(inptr);
}
Note: This example is clearly not perfect, this will be better to make a true parser for jpeg file to have a better control flow. Here we suppose that all gonna be right.
how do you know that an instance of a JPEG image will always end with '\n'? Or better, how do you know that a JPEG image will be an exact multiple of 512?
You dont know.
So the posted code needs to calculate the actual value OR use some method to have the last call to fread() for any specific JPEG instance, to stop reading at the end of that image,
Then the check for the ID bytes of the next JPEG image will find the next image.
Otherwise, the start of the next image is already written to the prior output file and the check for a new image will fail.
In general this will result in the last created file containing more than one image.
This link: 'https://en.wikipedia.org/wiki/JPEG_File_Interchange_Format' is a web page that describes the format of a JPEG file.
On every digital camera that I have used, the SD card has a directory of all the files.
Suggest using that directory and the info in the linked web page to find each JPEG image and to determine when the end of that image has been encountered. (I.E. the 0xFF 0xD9)
I'm driving myself crazy trying to figure out what is happening with me code.
I'm currently in CS50's pset4. Recover Challenge.
For those who don't know what is it about:
We're given a file called card.raw in which there are some deleted photos. Our task is to implement a program that can do a bit of forensics (idyllically) and recover the lost photos.
Hereby I attach my code:
#include <stdio.h>
#include <stdint.h>
int main(int argc, char *argv[])
{
if (argc != 2)
{
fprintf(stderr, "Usage: ./recover file\n");
return 1;
}
//declaring pointer infile and giving the address of argv[1];
char *infile = argv[1];
//Opening file
FILE *raw_data;
raw_data = fopen(infile, "r");
//Checking for NULL error
if(raw_data == NULL)
{
fprintf(stderr, "Could not open file.\n");
return 2;
}
uint8_t buffer[512]; //Delcaring unsigned int variable type. Array of 512 bytes.
int counter = 0; //Declaring counter for counting jpegs files
FILE *outfile; //Setting pointer named outfile for printing here
char filename[8]; //declaring 'filename' variable for storing the file's name
//While we can reads blocks of memory of 512 bytes from raw_data (aka the address from the infile) into buffer:
while (fread(buffer, 512, 1, raw_data))
{
//Condition for tracking the first bytes that form a JPEG file
if(buffer[0] == 0xff &&
buffer[1] == 0xd8 &&
buffer[2] == 0xff &&
(buffer[3] & 0xf0) == 0xe0)
{
if(counter == 0) //If this is the 1st file, then name the file with
//counter value with 3 digits (%03d)
{
sprintf(filename, "%03d.jpg", counter); // And 3 digits (%i3)
outfile = fopen(filename, "w"); //Open file named outfile in write mode
counter++;
}
else //If this is not the first JPG opened, firstly close the
{ // current open file, and then open a new one with the
fclose(outfile); // current counter value and 3 digits for its name
sprintf(filename, "%03d.jpg", counter);
outfile = fopen(filename, "w"); //Open file named 'outfile' in write mode
counter++;
}
}
fwrite(buffer, 1, sizeof(buffer), outfile); /* Write function that takes buffer data (aka the
pointer to the array of elements to be written,
writes 1 byte of elements of the syze buffer (512)
and it writes it to the output, aka 'outfile' */
}
fclose(outfile); //Remember to close the last file once we get out of the while-loop
}
Here's the tricky part:
I've successfully recovered all the problem images.
But, if I run the code several times, let's say for example, 5 times, I end up having a Segmentation Fault.
When I run check50, I get the following message (I will attach an image with both the segmentation fault after some successful runs and the check50 veredict). Click here to see the image
I just can't get it. I supose there might be some trouble with memory, but I just don't know what is it.
Thank you very much for your time and your help guys. StackOVerFlow is always such a nice place to seek for guidance.
EDIT
If I run echo $? once the Segmentation Fault has prompted, I get a value of 139.
Here's the terminal prompt screenshot
EDIT
Just as #Thomas Dickey has pointed out, the program was writing on a file regardless of having an open file yet or not.
I've updated and fixed a bit my code in order to keep it cleaner, and added an if condition in order to fix it.
Here's the solution:
#include <stdio.h>
#include <stdint.h>
int main(int argc, char *argv[])
{
if (argc != 2)
{
fprintf(stderr, "Usage: ./recover file\n");
return 1;
}
//declaring pointer infile and giving the address of argv[1];
char *infile = argv[1];
//Opening file
FILE *raw_data;
raw_data = fopen(infile, "r");
//Checking for NULL error
if(raw_data == NULL)
{
fprintf(stderr, "Could not open file.\n");
return 2;
}
uint8_t buffer[512]; //Delcaring unsigned int variable type. Array of 512 bytes.
int counter = 0; //Declaring counter for counting jpegs files
FILE *outfile; //Setting pointer named outfile for printing here
char filename[8]; //declaring 'filename' variable for storing the file's name
//While we can reads blocks of memory of 512 bytes from raw_data (aka the address from the infile) into buffer:
while (fread(buffer, 512, 1, raw_data))
{
//Condition for tracking the first bytes that form a JPEG file
if(buffer[0] == 0xff &&
buffer[1] == 0xd8 &&
buffer[2] == 0xff &&
(buffer[3] & 0xf0) == 0xe0)
{
if(counter != 0)
{
fclose(outfile); //If this is not the first JPG opened, close previous file
}
sprintf(filename, "%03d.jpg", counter); //print stream to 'filename' the value of 'counter' in 3 digits
outfile = fopen(filename, "w"); //Open file named outfile in write mode
counter++; //Add 1 to counter
}
if(counter != 0) //Don't start writing on a file until the first jpeg is found
{
fwrite(buffer, sizeof(buffer), 1, outfile); /* - Write function that takes buffer data
(aka the array of elements to be written) ,
- Write a block of 512 bytes of elements
(aka the size of buffer),
- 1 block of 512 bytes at a time,
- And it writes it to the output, aka 'outfile' */
}
}
fclose(outfile); //Remember to close the last file once we get out of the while-loop
return 0;
}
The program only opens the output file if the header looks okay, but writes to the output irregardless. If you read a file that doesn't have a jpeg header, it'll break.