I have written a binary file using a struct as follows:
struct block{
char data[32];
};
so what I end up with is basically a large binary file full of char[32]. The data is formatted in specific positions so grabbing specific pieces of information is not difficult. However, I tried to read the file like so:
int lines=0;
std::ifstream inputFile("file.bin",std::ios::binary);
while (!inputFile.eof())
{
inputFile.read(blocks[lines].data, sizeof(block));
lines++;
}
inputFile.close();
lines--;
and then displaying it like this:
std::cout<<"block 1: "<<blocks[0].data<<std::endl;
// etc ...
I thought that blocks[i].data should just give me the char[32] that belongs to index i, but it instead gives me every "data" element in the struct from that index to the end of the struct. I'm sure that it is my misunderstanding of how that works. My question is: how do I just get the char[32] represented by blocks[i].data?
The problem is your std::cout output statement. When you try to output blocks[0].data, what operator<< gets is not the array of 32 chars, but a pointer to the first char. This is interpreted as pointer to a C string, and therefore it outputs all characters found in memory from there on until it finds a '\0'. Since each array element contains just the corresponding characters from the file, all characters of the file are output (unless there's a '\0' in the file, then output stops there). Also, you seem to be (un-)lucky that a '\0' follows your data in memory, so the output stops there (instead of continuing to output whatever is in memory afterwards, and possibly giving a segmentation fault when the end of the process' memory is reached).
To just output the 32 charactes as characters, use std::cout.write(blocks[0].data,32). Otherwise to output them as ints just loop through them and convert each one to int:
for (int i = 0; i < 32; ++i)
std::cout << static_cast<int>(blocks[0].data[i]) << ' ';
Of course you can use all the stream manipulators to get the numbers in the form you want (e.g. std::hex for hexadecimal output, and/or std::setw and std::setfill to get fixed width numbers).
std::cout<<"block 1: "<<blocks[0].data<<std::endl;
You're sending a char[] to the stream, which gets promoted to a char*, so it thinks it's a NULL terminated string, and attempts to display it as such. It's hard to tell what it is that you want, but this will display it in hexidecimal:
std::cout << std::setfill('0') << std::hex;
for(int i=0; i<25; ++i)
std::cout << std::setw(2) << blocks[0].data[i];
std::cout << std::setfill(' ') << std::dec;
Your input section needs to be changed:
while (inputFile.read(blocks[lines].data, sizeof(block))
{
lines++;
}
The reason is that the EOF condition is not determined until AFTER a read operation occurs. One side effect of your use of EOF checking is that an extra line may be read.
Related
I am working on a C++ project and I am having an issue.
Below is my code
tempfingerprint = libssh2_hostkey_hash(session, LIBSSH2_HOSTKEY_TYPE_RSA);
char temp[48];
memset(temp, 0, sizeof(temp));
for (i = 0; i < 16; i++)
{
//fingerprintstream << (unsigned char)tempfingerprint[i] << ":";
if (temp[0] == 0)
{
sprintf(temp, "%02X:", (unsigned char)tempfingerprint[i]);
}
else
{
//sprintf(temp, "%s:%02X", temp, (unsigned char)tempfingerprint[i]);
char characters[3];
memset(characters, 0, sizeof(characters));
//If less than 16, then add the colon (:) to the end otherwise don't bother as we're at the end of the fingerprint
sprintf(characters, "%02X:", (unsigned char)tempfingerprint[i]);
strcat(temp, characters);
}
}
//Remove the end colon as its not needed. 48 Will already be null terminated, so the previous will contain the last colon
temp[47] = 0;
return string(temp);
When I run my app, I get the following error from visual studio
Run-Time-Check Failure #2 - Stack around the variable 'temp' was corrupted.
I've ran the same code on Linux through Valgrind and no errors were shown so I'm not sure what the problem is with Windows.
Here's an approach using on what Paul McKenzie's talking about (though he might implement it differently) based on it looks like you were trying to do with the stream
#include <iostream>
#include <sstream>
#include <iomanip> // output format modifiers
using namespace std;
int main()
{
stringstream fingerprintstream;
// set up the stream to print uppercase hex with 0 padding if required
fingerprintstream << hex << uppercase << setfill('0');
// print out the first value without a ':'
fingerprintstream << setw(2) << 0;
for (int i = 1; i < 16; i++) // starting at 1 because first has already been handled.
{
// print out the rest prepending the ':'
fingerprintstream << ":" << setw(2) << i;
}
// print results
std::cout << fingerprintstream.str();
return 0;
}
Output:
00:01:02:03:04:05:06:07:08:09:0A:0B:0C:0D:0E:0F
Just realized what I think OP ran up against with the garbage output. When you output a number, << will use the appropriate conversion to get text, but if you output a character << prints the character. So fingerprintstream << (unsigned char)tempfingerprint[i]; takes the binary value at tempfingerprint[i] and, thanks to the cast, tries to render it as a character. Rather than "97", you will get (assuming ASCII) "a". A large amount of what you try to print will give nonsense characters.
Example: If I change
fingerprintstream << ":" << setw(2) << i;
to
fingerprintstream << ":" << setw(2) << (unsigned char)i;
the output becomes
0?:0?:0?:0?:0?:0?:0?:0?:0?:0?:0 :0
:0?:0?:0
:0?:0?
Note the tab and the line feeds.
I need to know the definition of tempfingerprint to be sure, but you can probably solve the garbage output problem by removing the cast.
Based on new information, tempfingerprint is const char *, so tempfingerprint[i] is a char and will be printed as a character.
We want a number, so we have to force the sucker to be an integer.
static_cast<unsigned int>(tempfingerprint[i]&0xFF)
the &0xFF masks out everything but the last byte, eliminating sign extension of negative numbers into huge positive numbers when displayed unsigned.
There are, as far as I see, two issues in the code which lead to exceeding array boundaries:
First, with char temp[48] you reserve exactly 48 characters for storing results; However, when calling strcat(temp, characters) with the 16th value, and characters comprises at least the characters including the colon, then temp will comprise 16*3 digits/colons + one terminating '\0'-character, i.e. 49 characters (not 48). Note that strcat automatically appends a string terminating char.
Second, you define char characters[3] such that you reserve place for two digits and the colon, but not for the terminating '\0'-character. Hence, an sprintf(characters, "%02X:",...) will exceed characterss array bounds, as sprintf also appends the string terminator.
So, if you do not want to rewrite your code in general, changing your definitions to char temp[49] and char characters[4] will solve the problem.
I have a function print_string which takes in a character array as its argument.
For some reason I can't get it to print out if I use a while loop like
int i = 0;
while(str[i]!= '\0'){
cout << str[i];
i++;
}
but if I use a for loop and specify the length of the array it can.
Thanks
The code works for me. My guess: you're probably experience the problem with buffering. std::cout is buffered, so characters first go into buffer, and then, at certain points, the contents of the buffer are put on screen.
The most typical way to force this is: std::cout << std::endl;
Also, tip: std::cerr is unbuffered, so you could use it for debugging this issue.
----------------- EDIT -----------------------
Based on juanchopanza's comment : I edit the title
Based on jrok's comment : I'm using ofstream to write, and ifstream to read.
I'm writing 2 programs, first program do the following tasks :
Has a vector of integers
convert it into array of string
write it in a file
The code of the first program :
vector<int> v = {10, 200, 3000, 40000};
int i;
stringstream sw;
string stringword;
cout << "Original vector = ";
for (i=0;i<v.size();i++)
{
cout << v.at(i) << " " ;
}
cout << endl;
for (i=0;i<v.size();i++)
{
sw << v[i];
}
stringword = sw.str();
cout << "Vector in array of string : "<< stringword << endl;
ofstream myfile;
myfile.open ("writtentext");
myfile << stringword;
myfile.close();
The output of the first program :
Original vector : 10 200 3000 40000
Vector in string : 10200300040000
Writing to File .....
second program will do the following tasks :
read the file
convert the array of string back into original vector
----------------- EDIT -----------------------
Now the writing and reading is fine, thanks to Shark and Jrok,I am using a comma as a separator. The output of first program :
Vector in string : 10,200,3000,40000,
Then I wrote the rest of 2nd program :
string stringword;
ifstream myfile;
myfile.open ("writtentext");
getline (myfile,stringword);
cout << "Read From File = " << stringword << endl;
cout << "Convert back to vector = " ;
for (int i=0;i<stringword.length();i++)
{
if (stringword.find(','))
{
int value;
istringstream (stringword) >> value;
v.push_back(value);
stringword.erase(0, stringword.find(','));
}
}
for (int j=0;j<v.size();i++)
{
cout << v.at(i) << " " ;
}
But it can only convert and push back the first element, the rest is erased. Here is the output :
Read From File = 10,200,3000,40000,
Convert back to vector = 10
What did I do wrong? Thanks
The easiest thing would be to insert a space character as a separator when you're writing, as that's the default separator for operator>>
sw << v[i] << ' ';
Now you can read back into an int variable directly, formatted stream input will do the conversion for you automatically. Use vector's push_back method to add values to it as you go.
Yes, this question is over a year old, and probably completely irrelevant to the original asker, but Google led me here so it might lead others here too.
When posting, please post a complete minimal working example, having to add #include and main and stuff is time better spent helping. It's also important because of your very problem.
Why your second code isn't working is all in this block
for (int i=0;i<stringword.length();i++)
{
if (stringword.find(','))
{
int value;
istringstream (stringword) >> value;
v.push_back(value);
stringword.erase(0, stringword.find(','));
}
}
istringstream (stringword) >> value interprets the data up to the comma as an integer, the first value, which is then stored.
stringword.find(',') gets you the 0-indexed position of the comma. A return value of 0 means that the character is the first character in the string, it does not tell you whether there is a comma in the string. In that case, the return value would be string::npos.
stringword.erase deletes that many characters from the start of the string. In this case, it deletes 10, making stringword ,200,3000,40000. This means that in the next iteration stringword.find(',') returns 0.
if (stringword.find(',')) does not behave as wished. if(0) casts the integer to a bool, where 0 is false and everything else is true. Therefore, it never enters the if-block again, as the next iterations will keep checking against this unchanged string.
And besides all that there's this:
for (int j=0;j<v.size();i++)
{
cout << v.at(i) << " " ;
}
it uses i. That was declared in a for loop, in a different scope.
The code you gave simply doesn't compile, even with the added main and includes. Heck, v isn't even defined in the second program.
It is however not enough, as the for condition stringword.length() is recalculated every loop. In this specific instance it works, because your integers get an extra digit each time, but let's say your input file is 1,2,3,4,:
The loop executes normally three times
The fourth time, stringword is 4, stringword.length() returns 2, but i is already valued 3, so i<stringword.length() is invalid, and the loop exits.
If you want to use the string's length as a condition, but edit the string during processing, store the value before editing. Even if you don't edit the string, this means less calls to length().
If you save length beforehand, in this new scenario that would be 8. However, after 4 loops string is already empty, and it executes the for loop some more times with no effect.
Instead, as we are editing the string to become empty, check for that.
All this together makes for radically different code altogether to make this work:
while (!stringword.empty())
{
int value;
istringstream (stringword) >> value;
v.push_back(value);
stringword.erase(0, stringword.find(',')+1);
}
for (int i = 0; i < v.size(); i++)
{
cout << v.at(i) << " " ;
}
A different way to solve this would have been to not try to find from the start, but from index i onwards, leaving a string of commas. But why stick to messy stuff if you can just do this.
And that's about it.
#define ECHOMAX 100
struct tDataPacket
{
int iPacket_number;
char sData[ECHOMAX];
};
int main () {
tDataPacket packet;
packet.iPacket_number=10;
strcpy(packet.sData,"Hello world");
char buffer[sizeof(tDataPacket)];
memcpy(buffer,&packet.iPacket_number,sizeof(int));
memcpy(buffer+sizeof(int),packet.sData,ECHOMAX);
std::cout<<"Buffer = "<<buffer<<"END";
return 0;
}
In the above code I am trying to pack my structure in a char[] buffer so that I can send it to a UDP socket. But the output of the program is "" string. So nothing is getting copied to 'buffer'. Am I missing anything??
When you copy the int, at least one of the first "n" characters of the buffer will be zero (where "n" is the size of an int on your platform). For example for a 4-byte int:
x00 x00 x00 x0a or x0a x00 x00 x00
Depending on the endianness of your processor.
Printing out the zero will have the effect of terminating the output string.
You have no code to sensibly print the contents of the buffer, so you are expecting this to work by magic. The stream's operator << function expects a pointer to a C-style string, which the buffer isn't.
It's "" because int iPacket_number is probably laid out in memory as:
0x00 0x00 0x00 0x0a
which is an empty string (nul-terminator in the first character).
Firstly you probably want some sort of marshalling so that the on-the-wire representation is well established and portable (think endian differences between platforms).
Secondly you shouldn't need to "print" the resulting string; it makes no sense.
Thirdly you want unsigned char, not (signed) char.
You can't print an integer as text, because it's not text.
You will need to do a loop (or something like that) to print the actual contents of the buffer:
std::cout << "Buffer=";
for(size_t i = 0; i < sizeof(tDataPacket); i++)
{
std::cout << hex << (unsigned int)buffer[i] << " ";
if ((i & 0xf) == 0xf) std::cout << endl; // Newline every 16.
}
std::cout << "END" << endl;
You can do this but it's not really relevant to display binary data like that:
std::cout<<"Buffer = "; for each (auto c in buffer)
{
std::cout<< c;
}
std::cout <<"END";
I've come this far without asking for help, but I've got a problem that I can't seem to fix. I like cryptology, so now that I am learning C++, I want to make programs to encrypt and decrypt strings. I read that the best way is to convert the text to ASCII and go from there, so here is a simple program I made in C++ to try and convert a char variable to ASCII:
#include <iostream>
#include <string>
#include <math.h>
using namespace std;
int main()
{
char strString[1000];
cout<<"Enter you message:"<<endl;
cin>>strString[1000];
string strEncrypt;
int a = 0;
while (strString != '\0')
{
int b = (int)strString[a];
strEncrypt.at(a) = b; //This is where I'm getting an error.
a++;
}
cout<<"Encrypted message:"<<endl;
cout<<strEncrypt<<endl;
}
So, I've tried all 3 things I know to do to troubleshoot (Google, check for missing simicolons, and make sure I'm doing == not =, but this is just something I don't know how to do, not something I'm forgetting (I hope). So, any help would great!
You don't have to change the characters to ASCII they already are. Chars are basically the same as integers in memory.
Now to your question; . If you want to set a character in a string you can do that like this
string[index] = b;
Another thing to be careful for in your code. You are using cin to read the string from the user. This will not let you read messages that have spaces in them and will only read the first word. For example, if the user enters "Love Crypto" cin will only read "Love" and "Crypto" will be ignored. To get the entire line, use getline instead.
As for looping over characters in a string, it's better to do it as follows:
for(int i = 0; i < strString.length(); i++)
{
strString[i] = bla;
}
Again, you're code isn't actually doing anything. It is only reading a letter and then storing a "letter" in another string.
string::at() throws exception if the index passed to at() is out of range. So, if you are getting runtime error then it's expected. Because, your string strEncrypt is initialized to "" and thus the size is 0.
You may try
strEncrypt.reserve(strlen(strString));
Easiest way to actually make the code you have work is change this line strEncrypt.at(a) = b; to this strEncrypt += b; Which will add the characters to the empty string strEncrypt.
Your code doesn't make much sense though as char types are already ascii. You'll have to explain more about what kind of encrypting you are trying to do and maybe we can point you in the right direction.
EDIT: After thinking about what you're trying to do a bit more based on the code you have it seems like you want to print the numeric ascii value of characters. You can do that with just a cast like this:
string input;
cout << "Enter you message:" << endl;
// handle spaces in the message
getline(cin, input);
cout << "String chars as ascii values:" << endl;
cout << "Char: " << "ASCII Code:" << endl;
for (int i = 0; i < input.length(); ++i)
{
// casting the char to an int with (int) will print the ascii code
cout << input[i] << " " << (int)input[i] << endl;
}
On top of the fact that your input is already in ASCII, keep in mind that doing cin >> strString[1000] doesn't limit the input captured to the length of your buffer unless you specifically specify the number of characters to capture for the stream object using setw() or setting it's ios_base::width data member. So your method right now risks buffer overflows.
Secondly, the form of cin >> that you're using will not capture the entire line of input. Instead it will stop at the first white-space or any other delimiting character (or end-of-file if that is reached first). In your case, if you are entering a line like "Hello World", then the syntax you're using will only capture "Hello" and drop "World".
A much better idea would be to use the getline() function with a std::string object if you are wanting to capture a line of input to a string and remove the delimiting newline character without risking buffer overflows ... for instance:
string strString;
getline(cin, strString);
Apart from advises given, when receiving this kind of run-time errors use Cppcheck utility.
It will give you the answer: "Message: Array 'strString[1000]' index 1000 out of bounds".