C++: Store read binary file into buffer

问题

I'm trying to read a binary file and store it in a buffer. The problem is, that in the binary file are multiple null-terminated characters, but they are not at the end, instead they are before other binary text, so if I store the text after the '\0' it just deletes it in the buffer.

Example:

char * a = "this is a\0 test";
cout << a;

This will just output: this is a

here's my real code:

this function reads one character

bool CStream::Read  (int * _OutChar)
{
    if (!bInitialized)
        return false;

    int iReturn = 0;

     *_OutChar = fgetc (pFile);

    if (*_OutChar == EOF)
        return false;

    return true;
}

And this is how I use it:

    char * SendData = new char[4096 + 1];

    for (i = 0; i < 4096; i++)
    {
        if (Stream.Read (&iChar))
            SendData[i] = iChar;
        else
            break;
    }

回答1:

I just want to mention that there is a standard way to read from a binary file into a buffer.

Using <cstdio>:

char buffer[BUFFERSIZE];

FILE * filp = fopen("filename.bin", "rb"); 
int bytes_read = fread(buffer, sizeof(char), BUFFERSIZE, filp);

Using <fstream>:

std::ifstream fin("filename.bin", ios::in | ios::binary );
fin.read(buffer, BUFFERSIZE);

What you do with the buffer afterwards is all up to you of course.

Edit: Full example using <cstdio>

#include <cstdio>

const int BUFFERSIZE = 4096;    

int main() {
    const char * fname = "filename.bin";
    FILE* filp = fopen(fname, "rb" );
    if (!filp) { printf("Error: could not open file %s\n", fname); return -1; }

    char * buffer = new char[BUFFERSIZE];
    while ( (int bytes = fread(buffer, sizeof(char), BUFFERSIZE, filp)) > 0 ) {
        // Do something with the bytes, first elements of buffer.
        // For example, reversing the data and forget about it afterwards!
        for (char *beg = buffer, *end=buffer + bytes; beg < end; beg++, end-- ) {
           swap(*beg, *end);
        }
    }

    // Done and close.
    fclose(filp);

    return 0;
}

回答2:

The problem is definitievely the writing of your buffer, because you read a byte at a time.

If you know the length of the data in your buffer, you could force cout to go on:

char *bf = "Hello\0 world"; 
cout << bf << endl;
cout << string(bf, 12) << endl;

This should give the following output:

Hello
Hello  world

However this is a workaround, as cout is foreseent to output printable data. Be aware that the output of non printable chars such as '\0' is system dependent.

Alternative solutions:

But if you manipulate binary data, you should define ad-hoc data structures and printing. Here some hints, with a quick draft for the general principles:

struct Mybuff {   // special strtucture to manage buffers of binary data
    static const int maxsz = 512; 
    int size;
    char buffer[maxsz]; 
    void set(char *src, int sz)  // binary copy of data of a given length
    { size = sz; memcpy(buffer, src, max(sz, maxsz)); }
} ;

Then you could overload the output operator function:

ostream& operator<< (ostream& os, Mybuff &b)
{
    for (int i = 0; i < b.size; i++) 
        os.put(isprint(b.buffer[i]) ? b.buffer[i]:'*');  // non printables replaced with *
    return os;
}

ANd you could use it like this:

char *bf = "Hello\0 world"; 
Mybuff my; 
my.set(bf, 13);   // physical copy of memory
cout << my << endl;   // special output

回答3:

I believe your problem is not in reading the data, but rather in how you try to print it.

char * a = "this is a\0 test";
cout << a;

This example you show us prints a C-string. Since C-string is a sequence of chars ended by '\0', the printing function stops at the first null char. This is because you need to know where the string ends either by using special terminating character (like '\0' here) or knowing its length.

So, to print whole data, you must know the length of it and use a loop similar to the one you use for reading it.

回答4:

Are you on Windows? If so you need to execute _setmode(_fileno(stdout), _O_BINARY);

Include <fcntl.h> and <io.h>

回答5:

static std::vector<unsigned char> read_binary_file (const std::string filename)
{
    // binary mode is only for switching off newline translation
    std::ifstream file(filename, std::ios::binary);
    file.unsetf(std::ios::skipws);

    std::streampos file_size;
    file.seekg(0, std::ios::end);
    file_size = file.tellg();
    file.seekg(0, std::ios::beg);

    std::vector<unsigned char> vec(file_size);
    vec.insert(vec.begin(),
               std::istream_iterator<unsigned char>(file),
               std::istream_iterator<unsigned char>());
    return (vec);
}

and then

auto vec = read_binary_file(filename);
auto src = (char*) new char[vec.size()];
std::copy(vec.begin(), vec.end(), src);

来源：https://stackoverflow.com/questions/24716250/c-store-read-binary-file-into-buffer

标签

c++

buffer

fgets

null-terminated