fwrite binary data from a string

Question

I would like to create a file and write "A" into it (ascii is 65 == 01000001). Strange fact, whatever the value of std::string binary, there is always writed the letter P in myfile.txt.

    std::string binary = "01000001";
    std::string file = "myfile.txt";
    FILE* f;
    f = fopen(file.c_str(), "wb");      
    fwrite(&binary, 1, 1, f);     
    fclose(f);

After execution of this code, I read binary data with the command xxd -b myfile and I get this :

00000000: 01010000

Do you see a problem on this code ?

Why not use C-style strings directly? (Joke - you're not using C++ streams) You did fopen(file.c_str()..., but did fwrite(&binary.... One of them is wrong. — LogicStuff
– LogicStuff, Commented Jan 13, 2017 at 12:39
The binary variable is a string that contains the ASCII representation of "01000001". This is not a number is TEXT. To write "A" into the file you should use binary = "A" and then `fwrite(binary.c_str(),1,1,f)!!! But I think you would convert binary in its ASCII value starting from the binary value. — Sir Jo Black
– Sir Jo Black, Commented Jan 13, 2017 at 12:53
This code is a cocktail between C and C++, but solves your problem: #include <iostream> #include <cstdio> #include <cstdlib> int main(void) { std::string binary = "01000001"; std::string file = "myfile.txt"; char c; FILE* f; c=strtol(binary.c_str(), NULL, 2); f = fopen(file.c_str(), "wb"); // fwrite(binary.c_str(),1, 1, f); fwrite(&c,1, 1, f); fclose(f); return 0; } ... You shall insert the right indentation! — Sir Jo Black
– Sir Jo Black, Commented Jan 13, 2017 at 12:55
My goal is to copy files (not only text files), I have to use binary. I guess std::string isn't the best way to do that. Do you have suggestions to do that without strings ? — Milodupipo
– Milodupipo, Commented Jan 13, 2017 at 12:59

Hatted Rooster · Accepted Answer · 2017-01-13 12:39:51Z

6

fwrite(&binary, 1, 1, f);

You pass a pointer of the std::string to fwrite, that's bad.

You will want to get a pointer to the internal buffer of the string by calling c_str():

fwrite(binary.c_str(), 1, 1, f);

This is another reason to not use C file handles, fwrite's first argument is a const void* which is why your compiler didn't give you an error in the first place.

answered Jan 13, 2017 at 12:39

Hatted Rooster

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1 Comment

pcarter Over a year ago

With his current code, this will write the character '0', not 'A' as he says he wants to write.

cpatricio · Accepted Answer · 2017-01-13 15:32:47Z

2

Strings are bad when you want to do a bitstream...

int8_t binary = 0b01000001;
std::string file = "myfile.txt";
FILE* f;
f = fopen(file.c_str(), "wb");      
fwrite(&binary, 1, 1, f);     
fclose(f);

But when you are using C++, you don't really need to use C libraries

#include <fstream>

int main()
{
    std::fstream f;
    std::string file = "myfile.txt";
    f.open(file, std::fstream::out | std::fstream::binary);
    int8_t binary = 0b01000001; // int8_t is required to only write 8 bits/1 byte
    f << binary;
    f.close();
    return 0;
}

edited Jan 13, 2017 at 15:32

answered Jan 13, 2017 at 13:09

cpatricio

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1 Comment

pcarter Over a year ago

Your fwrite call should be passed &binary instead of binary

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