I have to read the contents of a file, and store them in a char* array. I have the following, but when I output my char* array I don't get the full output of the file I read in. Am I incorrectly calculating the file size of my output array?
Note: I must use POSIX calls to read the contents of the file.
#include "iostream"
#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>
using namespace std;
int main() {
const char* name = "test.txt";
int fd = open(name, O_RDONLY);
off_t length = lseek(fd, 0, SEEK_END);
cout << "file size: " << length << endl;
char* array = new char[length];
lseek(fd, 0, SEEK_SET); // resets position to 0
read(fd, array, sizeof(array)-1);
cout << array << "finished" << endl;
close(fd);
delete[] array;
return 0;
}
Contents of test.txt are This is a string of sample text
But this is my output when running:
file size: 34
This isfinished
Related
I'm working on a project where I need to read a binary file and then operate on the data(count how many times 3 appears). My approach was to use read() to store data into a char buffer. Then operate on the buffer. My biggest issue is using read() correctly, I don't believe it is storing any data into char buffer. char buffer remains empty after read() is called.
// Program reads how many times the integer 3 occurs in a binary file
#include <iostream>
#include <fstream> // needed to use files
using namespace std;
int main()
{
int count = 0;
char buffer[32];
int size = sizeof(buffer) / sizeof(buffer[0]);
// open file "example.bin"
fstream file("example.bin", ios::in | ios::ate | ios::binary);
if(!file)
{
cout << "Error opening file" << endl;
return 0;
}
cout << "opened file" << endl;
// reading the data into memory
file.read(reinterpret_cast<char *>(buffer), sizeof(buffer));
for(int i = 0; i < size; i++)
{
if (buffer[i] == '3')
count++;
}
cout << "There exists " << count << " threes in this file." << endl;
// close file
file.close();
return 0;
}
I am reading binary file cmd.exe into unsigned chars array. Total bytes read into bytes_read are 153. I converted it to base64 string and then decode this string back (code from 2nd answer base64 decode snippet in c++) into vector<'BYTE>. Here BYTE is unsigned char.
decodedData.size() is also 153. But when I write this vector to file in binary mode to get my cmd.exe file again I get only 1 KB file. What thing I missed?
// Reading size of file
FILE * file = fopen("cmd.exe", "r+");
if (file == NULL) return 1;
fseek(file, 0, SEEK_END);
long int size = ftell(file);
fclose(file);
// Reading data to array of unsigned chars
file = fopen("cmd.exe", "r+");
unsigned char * myData = (unsigned char *)malloc(size);
int bytes_read = fread(myData, sizeof(unsigned char), size, file);
fclose(file);
std::string encodedData = base64_encode(&myData[0], bytes_read);
std::vector<BYTE> decodedData = base64_decode(encodedData);
////write data to file
ofstream outfile("cmd.exe", ios::out | ios::binary);
outfile.write((const char *)decodedData.data(), decodedData.size());
Update:
Thanks #chux for suggesting "r+" --> "rb+" Problem resolved.
You marked this as C++.
This is one C++ approach using fstream to read a binary file. To simplify for this example, I created a somewhat bigger m_buff than needed. From the comments, it sounds like your fopen("cmd.exe", "r+") was in error, so I'm only providing a C++ binary read.
Method tReader() a) opens a file in binary mode, b) reads the data into m_buff, and c) captures gCount for display.
It also demonstrates one possible use of chrono to measure duration.
#include <chrono>
// 'compressed' chrono access --------------vvvvvvv
typedef std::chrono::high_resolution_clock HRClk_t;
typedef HRClk_t::time_point Time_t;
typedef std::chrono::microseconds US_t;
using namespace std::chrono_literals; // suffixes 100ms, 2s, 30us
#include <iostream>
#include <fstream>
#include <cassert>
class T516_t
{
enum BuffConstraints : uint32_t {
Meg = (1024 * 1024),
END_BuffConstraints
};
char* m_buff;
int64_t m_gCount;
public:
T516_t()
: m_buff (nullptr)
, m_gCount (0)
{
m_buff = new char[Meg];
}
~T516_t() = default;
int exec()
{
tReader();
return(0);
}
private: // methods
void tReader()
{
std::string pfn = "/home/dmoen/.wine/drive_c/windows/system32/cmd.exe";
// open file in binary mode
std::ifstream sIn (pfn, std::ios_base::binary);
if (!sIn.is_open()) {
std::cerr << "UNREACHABLE: unable to open sIn " << pfn
<< " priviledges? media offline?";
return;
}
Time_t start_us = HRClk_t::now();
do
{
// perform read
sIn.read (m_buff, Meg);
// If the input sequence runs out of characters to extract (i.e., the
// end-of-file is reached) before n characters have been successfully
// read, buff contains all the characters read until that point, and
// both eofbit and failbit flags are set
m_gCount = sIn.gcount();
if(sIn.eof()) { break; } // exit when no more data
if(sIn.failbit ) {
std::cerr << "sIn.faileBit() set" << std::endl;
}
}while(1);
auto duration_us = std::chrono::duration_cast<US_t>(HRClk_t::now() - start_us);
sIn.close();
std::cout << "\n " << pfn
<< " " << m_gCount << " bytes"
<< " " << duration_us.count() << " us"
<< std::endl;
} // int64_t tReader()
}; // class T516_t
int main(int , char**)
{
Time_t start_us = HRClk_t::now();
int retVal = -1;
{
T516_t t516;
retVal = t516.exec();
}
auto duration_us = std::chrono::duration_cast<US_t>(HRClk_t::now() - start_us);
std::cout << " FINI " << duration_us.count() << " us" << std::endl;
return(retVal);
}
One typical output on my system looks like:
/home/dmoen/.wine/drive_c/windows/system32/cmd.exe 722260 bytes 1180 us
FINI 1417 us
Your results will vary.
Your ofstream use looks good (so did not replicate).
I'm trying to read in an .exe and write it back out. My code works with .txt files but for some reason it is breaking executables. What am I doing wrong?
I'm not sure if I am reading it wrong or writing it wrong..
#include <string>
#include <vector>
#include <iostream>
#include <filesystem>
#include <unordered_set>
#include <Windows.h>
unsigned char *ReadFileAsBytes(std::string filepath, DWORD &buffer_len)
{
std::ifstream ifs(filepath, std::ofstream::binary | std::ifstream::ate);
if (!ifs.is_open())
{
return(nullptr);
}
// Go To End
ifs.seekg(0, ifs.end);
// Get Position (Size)
buffer_len = static_cast<DWORD>(ifs.tellg());
// Go To Beginning
ifs.seekg(0, ifs.beg);
// Allocate New Char Buffer The Size Of File
PBYTE buffer = new BYTE[buffer_len];
ifs.read(reinterpret_cast<char*>(buffer), buffer_len);
ifs.close();
return buffer;
}
void WriteToFile(std::string argLocation, unsigned char *argContents, int argSize)
{
std::ofstream myfile;
myfile.open(argLocation);
myfile.write((const char *)argContents, argSize);
myfile.close();
}
int main()
{
// Config
static std::string szLocation = "C:\\Users\\Admin\\Desktop\\putty.exe";
static std::string szOutLoc = "C:\\Users\\Admin\\Desktop\\putty2.exe";
DWORD dwLen;
unsigned char *szBytesIn = ReadFileAsBytes(szLocation, dwLen);
std::cout << "Read In " << dwLen << " Bytes" << std::endl;
// Write To File
WriteToFile(szOutLoc, szBytesIn, dwLen);
system("pause");
}
You open input file in binary mode, but in this code
std::ofstream myfile;
myfile.open(argLocation);
you open output file without binary mode. And there is no reason to call open separately:
std::ofstream myfile( argLocation, std::ios::out | std::ios::binary | std::ios::trunc);
I tried to read the EDID of my monitor connected to LVDS1. I use ArchLinux and C++/clang. My problem is: the file size always returns 0. I don't know if this is a programming problem or something OS specific, other files return a proper file size. Is it a special file? Is the symlink directory /sys/class/drm/card0-DP-1 the problem?
file: /sys/class/drm/card0-LVDS-1/edid
code:
#include <fstream>
#include <iostream>
using namespace std;
typedef unsigned char BYTE;
long
get_file_size(FILE *f)
{
long pos_cursor, pos_end;
pos_cursor = ftell(f);
fseek(f, 0, 2);
pos_end = ftell(f);
fseek(f, pos_cursor, 0);
return pos_end;
}
int
main()
{
const char *filepath = "/sys/class/drm/card0-LVDS-1/edid";
FILE *file = NULL;
if((file = fopen(filepath, "rb")) == NULL)
{
cout << "file could not be opened" << endl;
return 1;
}
else
cout << "file opened" << endl;
long filesize = get_file_size(file);
cout << "file size: " << filesize << endl;
fclose(file);
return 0;
}
output:
file opened
file size: 0
===
as suggested by MSalters, I tried stat for the file size. Also returns 0. I assume the code is correct, so it is somehow just not possible to access the file?
I also tried the symlink target path (/sys/devices/pci0000:00/0000:00:02.0/drm/card0/card0-LVDS-1/edid), just in case that was the problem - but still 0.
code:
#include <iostream>
#include <sys/stat.h>
using namespace std;
int
main()
{
const char *filepath = "/sys/class/drm/card0-LVDS-1/edid";
struct stat results;
if (stat(filepath, &results) == 0)
cout << results.st_size << endl;
else
cout << "error" << endl;
return 0;
}
output
0
===
I tried other files in the same directory (dpms edid enabled i2c-6 modes power status subsystem uevent). They all return a filesize of 4096 except edid.
I suspect that fseek(f, 0, 2); might mean fseek(f, 0, SEEK_CUR); which obviously doesn't do anything. You'd want SEEK_END which isn't portable, but then again /sys/ isn't either. (Of course, do #include <stdio.h>)
But considering it's already Linux-specific, why not use stat ?
Hey just try to compile an image in code and put it out as jpg again but fwrite returns 0 and img.jpg keeps empty :(
#include <stdio.h>
#include <string>
#include <iostream>
size_t count = 76830;//=length of data
const uint8_t data[] = {0xff,0xd8,0xff,0xe0,0x0,0x10,0x4a,0x46,0x49,0x46.....
0x0,0x7f,0xff,0xd9};
using namespace std;
//use this to saveToFile...
void saveToFile(const char * filename) {
FILE * file = fopen(filename, "rb");
if (file == NULL) {
std::cout << "error opening file" << std::endl;
return;
}
cout << endl << fwrite(data, sizeof(uint8_t), sizeof(data), file); //this line returns 0!!!
fclose(file);
}
int main () {
saveToFile("img.jpg");
}
fopen(filename, "rb");
This opens the file for reading, not writing. You want
fopen(filename, "wb");
// ^