Whenever I encounter a substitute character http://en.wikipedia.org/wiki/Substitute_character while reading a file in C++ using getline(), it is interpreted as a EOF so I cannot advance with my reading in order to get the file entire content. So my question is, how can I skip the substitute characters and read the content of the file until the "real" EOF?
Open the file in binary mode instead of text mode. If you're using fopen, make open it in one of the "b" modes, e.g. "rb". If you're using a C++ ifstream object, open it with the ios::binary flag.
For example:
// C method
FILE *f = fopen("filename", "rb");
// C++ method
std::ifstream f("filename", std::ios::in | std::ios::binary);
Related
I want to open a file for reading, close it, and then open it again for writing using the same std::fstream. But when I reopen the file with writing permissions all the data inside is being cleared and it becomes 0 bytes long.
Here's my code:
char* data = new char[5];
std::fstream fs("myFile", std::ios::binary | std::ios::in);//opening with read
fs.read(data, 5);
fs.clear();
fs.close();
//Do some stuff
fs.open("myFile", std::ios::binary | std::ios::out);//opening with write
//The file is already empty at this point
When opening a file with the default write flag, i.e., std::ios::out with the <iostream> facility or "w" with the <cstdio> functions, there is a combination of POSIX flags happening behind your back - a truncate flag is added. This means that upon opening in write mode, the file content is discarded. To circumvent this, open the file the second time with
fs.open("myFile", std::ios::binary | std::ios::app);
The append mode moves the file cursor to the end of the file at each write operation and is not combined with the truncate flag, see here. Note that when you want to write to arbitrary positions in the existing file, you need to open it with
fs.open("myFile", std::ios::binary | std::ios::in | std::ios::out);
which does not truncate its content and allows for cursor positioning with the seek* functions.
I've a file on which I require multiple operations. Sometimes I just want to append data at the end of the file, sometimes I just want to read from the file, and sometimes, I want to erase all the data and write form the beginning of the file. And then, I again need to append data at the end of file.
I'm using following code:
ofstream writeToTempFile;
ifstream readFromTempFile;
writeToTempFile.open("tempFile.txt", ios::app | ios::out);
readFromTempFile.open("tempFile.txt", ios::in);
// Reading and Appending data to the file
// Now it is time to erase all the previous data and start writing from the beginning
writeToTempFile.open("tempFile.txt", std::ofstream::trunc); // Here I'm removing the contents.
// Write some data to the file
writeToTempFile.open("tempFile.txt", std::ofstream::app); // Using this, I'm again having my file in append mode
But what I've done doesn't work correctly. Please suggest me some solution in C++. ( Not in C)
The problem with the code is:
I wasn't closing the file before I called the method open again on it.
So, close the file before you re-open it with some different permissions.
I am developing in C++ using NetBeans 6.9 on Ubuntu 11.04. I have declared the input and output file name strings and file streams thus
ifstream fpInputFile, fpOutputFile;
string inputFileName="", outputFileName="";
The input file name is assigned the name of an existing file as an input argument to the application. The output file name is given the same as the input name except that "_output" is inserted before the final period. So the output is written to the same directory as the input is located. Also I start netbeans with
su netbeans
so the IDE has root privileges to the directory. I try to open the files, and check whether they are opened thus.
fpInputFile.open(inputFileName.c_str(), ifstream::in);
fpOutputFile.open(outputFileName.c_str(), ifstream::out);
if (!(fpInputFile.is_open())) throw ERROR_OPENING_FILE;
if (!(fpOutputFile.is_open())) throw ERROR_OPENING_FILE;
The input file opens successfully but the output file does not.
Any help in determining why the output file is not opening for writing would be most appreciated.
Declare the output file as an ofstream rather than a ifstream.
You could also use a fstream for both input and output files.
The obvious problem is that you probably meant to open the file using a std::ofstream rather than an std::ifstream. This helps with actually writing to the stream although there are ways to write to an std::ifstream as long as it is opened for reading. For example, you could use the std::streambuf interface directly or use the std::streambuf to construct and std::ostream.
The more interesting question is: why isn't the file opened for writing when std::ios_base::in | std::ios_base::out is used for the open mode? std::ifstream automatically adds std::ios_base::in. It turns out, that the mode std::ios_base::in | std::ios_base::out doesn't create a file but it would successfully open an existing file. If you really want use an std::ifstream to open a file for output which potentially doesn't exist you would need to use either std::ios_base::out | std::ios_base::trunc or std::ios_base::out | std::ios_base::app:
the former would force the file to be created or truncated if it exists
the latter would force writes to append to the file in all cases
My personal guess is, however, that you are best off just using std::ofstream or, if you want to open the file for both reading and writing std::fstream (which, however, would also need to have std::ios_base::trunc or std::ios_base::app added to create a file if none exists).
I'm writing a method to replace a specified string from a binary file and it writes NULLs before the position I set with seekp, then writes the string and closes the stream. I only want to replace some bytes in the file. Before this piece of code I tried out with ofstream with ios::binary and ios::out flags. What's wrong to destroy all data in the file?
Before this piece of code, I open the file with an instance of ifstream to read the same position verifyng the first byte in the string. I only comment this for information.
Thank you all!
The code:
fstream ofs();
ofs.open(nomArchBin,ios::in | ios::out | ios::binary);
if (!ofs.good()) {
cout << "...";
return;
}
ofs.seekp(despEnArchivo,ios::beg);
char* registroChar = registroACadena(reg);
ofs.write(registroChar,cabecera.tamanioReg);
I know this sounds silly, but the only way to open a file for writing
and not to truncate it is to open it for reading as well: if you're
really doing ios::in | ios::out | ios::binary, it should work. (But
since you obviously reentered the code, and didn't copy/paste it, I'm
not sure if this is really what you did.)
Other points you have to pay attention to when trying to seek:
The file must be open in binary mode, and imbued with the "C"
locale. (IMHO, a file opened in binary mode should ignore the locale,
but this isn't what the standard says.)
Both `seekg` and `seekp` have the same effect; using either changes the
position of the other.
The only function which allows seeking to an arbitrary location is
the two argument seek; the one argument form can only be used to seek to
a position previously obtained by a tell.
On MacOS with gcc4.2 should the following code create a new file if none exists?
#include <fstream>
void test () {
std::fstream file ("myfile.txt", std::ios::in | std::ios::out);
}
By my logic it should, either open up an existing file for read/writing or create a new empty file for read/writing. But the behaviour I get is that it will not create a new file if 'myfile.txt' does not exist.
How do I get the same behavior as fopen("myfile.txt", "r+"); ?
Furthermore,
#include <fstream>
void test () {
std::ofstream file ("myfile.txt", std::ios::in | std::ios::out);
}
Will always truncate an existing file...
Is this the standard behavior?
First of all, I have no idea why you think that fopen("r+") creates a file if it doesn't exist - according to ISO C & C++, it does not, it just opens an existing file for read/write. If you want to create a file with fopen, you use "w+".
For streams, you just specify trunc:
std::ofstream file ("myfile.txt",
std::ios::in | std::ios::out | std::ios::trunc);
However, both this and fopen("w+") will truncate the file. There's no standard way to open the file without truncating if it exists, but create it if it does not exist in a single call. At best you can try to open, check for failure, and then try to create/truncate; but this may lead to a race condition if file is created by another process after the check but before truncation.
In POSIX, you can use open with O_CREAT and without O_TRUNC.
For the first case, you have specified that you are BOTH reading from and writing to the file. Hence, it will fail, if the file does not already exist. You could try to use two separate streams (one for reading and the other for writing), and construct the output stream, first. As for the second case, you can use "std::ios::app" (open in append mode) to prevent truncation. [Tested with Mac OS X 10.4.11, GCC 4.0.1]