I am trying to design a small file system.
I have created a text file to store the files data in.
int kufs_create_disk(char* disk_name, int disk_size){
FILE* file_ptr = fopen(disk_name, "w");
if (file_ptr == NULL)
return -1;
fseek (file_ptr, disk_size * 1024-1, SEEK_SET);
fwrite("", 1, sizeof(char), file_ptr); // to make a size for the file
fclose(file_ptr);
DiskName=disk_name;
return 0;
}
After writing to the file I get a file with the size I determine when I call the function.
kufs_create_disk("test.txt", 5);
which creates a file with size of 5kbs with '\0' to fill this file to the size.
I have created another function to write to this file in different places of the file which works just fine and I won't paste the code for simplicity.
When I try to read from the file using fread(), I'm not getting all the data I have written into the memory; rather I get just some of the data.
My read implementation would be:
int kufs_read(int fd, void* buf, int n){
FILE *file_ptr= fopen("test.txt","a+");
fseek (file_ptr, FAT[fd].position, SEEK_SET); //where FAT[fd].position is where I want to start my read and fd is for indexing purposes
fread(buf, 1, n, file_ptr); //n is the number of bytes to be read
FAT[fd].position = FAT[fd].position + n;
}
The thing is the file reads some of the characters written and doesn't read the rest. I did a little test by looping all over the file and checking whether every thing is being read and fread reads every thing but in the buf I only get some of the characters I've written.
The text file looks something like this:
0\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00written string1written string2 0\00\00\00\00\00\00\00\00\00\00\00\000\00\00\00\00\00\00\00\00\00\00\00\00writtenstring 3 \00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00\00
I get writtenstring1 and writtenstring2 in the buffer but I don't get writtenstring 3 for example.
Can you explain why?
Related
I am trying to build antivirus in C.
I do that like this:
Read data of virus and picture file to scanned.
Check if virus data appear in picture data.
I read the data of scanned file and virus file like this: ( I read the file by binary mode because the file is picture(.png) )
// open file
file = fopen(filePath, "rb");
if (!file)
{
printf("Error: can't open file.\n");
return 0;
}
// Allocate memory for fileData
char* fileData = calloc(fileLength + 1, sizeof(char));
// Read data of file.
fread(fileData, fileLength, 1, file);
after i read the file data and the Virus data i check if the virus appear in the file like this:
char* ret = strstr(fileData, virusID);
if (ret != NULL)
printf("Infetecd file");
It does not work even though in my picture i have VirusID.
I want to check if the binary data of virus appear in binary data of picture.
For example: my binary data of my virus http://pastebin.com/xZbWA9qu
And the binary data of my picture(with the virus): http://pastebin.com/yjXr84kr
First, note the order of arguments of fread, fread(void *ptr, size_t size, size_t nmemb, FILE *stream); so to get the number of bytes, it's better to do fread(fileData, 1, fileLength, file);. Your code will return 0 or 1 depends on whether there is enough data to be read in the file, not the number of bytes it has read.
Second, strstr is to search for strings, not memory blocks, in order to search binary blocks, you need to write your own, or you can use the GNU extension function memmem.
// Allocate memory for fileData
char *fileData = malloc(fileLength);
// Read data of file.
size_t nread = fread(fileData, 1, fileLength, file);
void *ret = memmem(fileData, nread, virusID, virusLen);
if (ret != NULL)
printf("Infetecd file");
Search for the first byte of the virus signature, if you find it then see if the next byte is the second byte of the signature, and so on until you have checked and matched all bytes of the signature. Then the file is infected. If not all bytes matches then search again for the first byte of the signature.
I am currently working on a project in which I have to read from a binary file and send it through sockets and I am having a hard time trying to send the whole file.
Here is what I wrote so far:
FILE *f = fopen(line,"rt");
//size = lseek(f, 0, SEEK_END)+1;
fseek(f, 0L, SEEK_END);
int size = ftell(f);
unsigned char buffer[MSGSIZE];
FILE *file = fopen(line,"rb");
while(fgets(buffer,MSGSIZE,file)){
sprintf(r.payload,"%s",buffer);
r.len = strlen(r.payload)+1;
res = send_message(&r);
if (res < 0) {
perror("[RECEIVER] Send ACK error. Exiting.\n");
return -1;
}
}
I think it has something to do with the size of the buffer that I read into,but I don't know what it's the correct formula for it.
One more thing,is the sprintf done correctly?
If you are reading binary files, a NUL character may appear anywhere in the file.
Thus, using string functions like sprintf and strlen is a bad idea.
If you really need to use a second buffer (buffer), you could use memcpy.
You could also directly read into r.payload (if r.payload is already allocated with sufficient size).
You are looking for fread for a binary file.
The return value of fread tells you how many bytes were read into your buffer.
You may also consider to call fseek again.
See here How can I get a file's size in C?
Maybe your code could look like this:
#include <stdint.h>
#include <stdio.h>
#define MSGSIZE 512
struct r_t {
uint8_t payload[MSGSIZE];
int len;
};
int send_message(struct r_t *t);
int main() {
struct r_t r;
FILE *f = fopen("test.bin","rb");
fseek(f, 0L, SEEK_END);
size_t size = ftell(f);
fseek(f, 0L, SEEK_SET);
do {
r.len = fread(r.payload, 1, sizeof(r.payload), f);
if (r.len > 0) {
int res = send_message(&r);
if (res < 0) {
perror("[RECEIVER] Send ACK error. Exiting.\n");
fclose(f);
return -1;
}
}
} while (r.len > 0);
fclose(f);
return 0;
}
No, the sprintf is not done correctly. It is prone to buffer overflow, a very serious security problem.
I would consider sending the file as e.g. 1024-byte chunks instead of as line-by-line, so I would replace the fgets call with an fread call.
Why are you opening the file twice? Apparently to get its size, but you could open it only once and jump back to the beginning of the file. And, you're not using the size you read for anything.
Is it a binary file or a text file? fgets() assumes you are reading a text file -- it stops on a line break -- but you say it's a binary file and open it with "rb" (actually, the first time you opened it with "rt", I assume that was a typo).
IMO you should never ever use sprintf. The number of characters written to the buffer depends on the parameters that are passed in, and in this case if there is no '\0' in buffer then you cannot predict how many bytes will be copied to r.payload, and there is a very good chance you will overflow that buffer.
I think sprintf() would be the first thing to fix. Use memcpy() and you can tell it exactly how many bytes to copy.
I'm trying to write a simple block of code that writes the contents of an array to a position in a file. The file is a simple, text only file. It seems to do everything write, but file contents end up in hex (with a bunch of zeroes at the beginning). I'm using the following:
FILE * fp; // file to write to
void fwritel(long offset, char * data) {
fp = fopen(filename, "w");
fseek(fp, offset, SEEK_SET);
fwrite(data, 1, strlen(data) - 1, fp);
fclose(fp);
}
I've also used a variation of pwrite that didn't work: pwrite(fp, data, strlen(data) - 1, offset); The code I'm looking for would write data at position offset only modifying the file from offset to offset + strlen(data).
This is a multiple part fix:
Joachim Pilebord is right. Opening the file using fopen(file, "w") will destroy the contents of the file. So each time it would open the file, zero the file, then write the new contents.
M Oehm was also correct, the new content would be padded with zeros - thus Sublime treated it as hex.
So, opening the file once, somewhere at the beginning of the program, using only
fseek(fp, offset, SEEK_SET);
fwrite(data, 1, strlen(data), fp);
and closing the file somewhere near the end of the program produces the correct output. Using fputc(' ') would work but you'd need to make sure I'm not replacing characters that you need (i.e. previously placed data in those parts of the file).
I am trying to write data to a binary file and then read back the data from the file. The data consists of a single number (an integer) nrows. Below is my code for writing the data to a binary file. However, when I try to read back the data and print the result using printf, I obtain a nonsensical result: -2.
FILE *fout;
FILE *file_pointer;
int nrows = 5;
fout = fopen("matrixB.bin", "wb") //Writing to a binary file.//
fwrite(&nrows, sizeof(int), 1, fout); //Writing the number nrows to the binary file "matrixB.bin"//
file_pointer = fopen("matrixB.bin", "rb"); //Reading a binary file.//
fread(&nrows, sizeof(int), 1, file_pointer);
printf("%d", nrows); //Here -2 is printed, instead of 5.//
What is the problem with my code?
One of the characteristics of the C file functions (fread, fwrite, etc) is that they normally perform buffered I/O.
Your program doesn't flush and/or close the file before reopening it. It should.
I am new to C and was trying to write a program just to copy a file so that I could learn the basics of files. My code takes a file as input, figures out its length by subtracting its start from its end using fseek and ftell. Then, it uses fwrite to write, based on what I could get from its man page, ONE element of data, (END - START) elements long, to the stream pointed to by OUT, obtaining them from the location given by FI. The problem is, although it does produce "copy output," the file is not the same as the original. What am I doing wrong? I tried reading the input file into a variable and then writing from there, but that didn't help either. What am I doing wrong?
Thanks
int main(int argc, char* argv[])
{
FILE* fi = fopen(argv[1], "r"); //create the input file for reading
if (fi == NULL)
return 1; // check file exists
int start = ftell(fi); // get file start address
fseek(fi, 0, SEEK_END); // go to end of file
int end = ftell(fi); // get file end address
rewind(fi); // go back to file beginning
FILE* out = fopen("copy output", "w"); // create the output file for writing
fwrite(fi,end-start,1,out); // write the input file to the output file
}
Should this work?
{
FILE* out = fopen("copy output", "w");
int* buf = malloc(end-start); fread(buf,end-start,1,fi);
fwrite(buf,end-start,1,out);
}
This isn't how fwrite works.
To copy a file, you'd typically allocate a buffer, then use fread to read one buffer of data, followed by fwrite to write that data back out. Repeat until you've copied the entire file. Typical code is something on this general order:
#define SIZE (1024*1024)
char buffer[SIZE];
size_t bytes;
while (0 < (bytes = fread(buffer, 1, sizeof(buffer), infile)))
fwrite(buffer, 1, bytes, outfile);
The first parameter of fwrite is a pointer to the data to be written to the file not a FILE* to read from. You have to read the data from the first file into a buffer then write that buffer to the output file. http://www.cplusplus.com/reference/cstdio/fwrite/
Perhaps a look through an open-source copy tool in C would point you in the right direction.
Here is How It can be done:
Option 1: Dynamic "Array"
Nested Level: 0
// Variable Definition
char *cpArr;
FILE *fpSourceFile = fopen(<Your_Source_Path>, "rb");
FILE *fpTargetFile = fopen(<Your_Target_Path>, "wb");
// Code Section
// Get The Size Of bits Of The Source File
fseek(fpSourceFile, 0, SEEK_END); // Go To The End Of The File
cpArr = (char *)malloc(sizeof(*cpArr) * ftell(fpSourceFile)); // Create An Array At That Size
fseek(fpSourceFile, 0, SEEK_SET); // Return The Cursor To The Start
// Read From The Source File - "Copy"
fread(&cpArr, sizeof(cpArr), 1, fpSourceFile);
// Write To The Target File - "Paste"
fwrite(&cpArr, sizeof(cpArr), 1, fpTargetFile);
// Close The Files
fclose(fpSourceFile);
fclose(fpTargetFile);
// Free The Used Memory
free(cpArr);
Option 2: Char By Char
Nested Level: 1
// Variable Definition
char cTemp;
FILE *fpSourceFile = fopen(<Your_Source_Path>, "rb");
FILE *fpTargetFile = fopen(<Your_Target_Path>, "wb");
// Code Section
// Read From The Source File - "Copy"
while(fread(&cTemp, 1, 1, fpSourceFile) == 1)
{
// Write To The Target File - "Paste"
fwrite(&cTemp, 1, 1, fpTargetFile);
}
// Close The Files
fclose(fpSourceFile);
fclose(fpTargetFile);