Checking the file I/O status in Open C++
Contents |
Overview
This code snippet shows how to get information about file I/O status in Open C++. The member function rdstate() returns the current internal error state flags of the stream (goodbit, eofbit, failbit, badbit). More than one error state could be set at the same time and it is possible to check for a specific state with the & operator and a specific bitmask. The member function rdstate() returns value goodbit if there are no errors. The other way to check the file I/O status is to use the member functions good(), eof(), fail(), and bad(). The member function clear() can be used to reset the state flags.
Note: In order to use this code, you need to install the Open C/C++ plug-in.
This snippet can be self-signed.
MMP file
The following libraries are required:
STATICLIBRARY libcrt0.lib
LIBRARY libstdcpp.lib
LIBRARY libc.lib
LIBRARY euser.lib
Source file
#include <fstream> //fstream
#include <string> //string, getline()
#include <iostream> //cout
using namespace std;
void check_stream_state1(const fstream &s)
{
int state = s.rdstate();
/* State flags:
goodbit = 0x00, //file has been opened without problems
badbit = 0x01, //e.g. writing to a file that is not open / no space left
eofbit = 0x02, //file opened for reading has reached the end
failbit = 0x04, //e.g. format error: alphabetical character is extracted
//from the stream but integer is expected
*/
//goodbit is a constant with the value 0
if (!state)
{
cout << "goodbit - no error occurred" << endl;
}
else
{
if (state & ios::badbit)
cout << "badbit - a fatal I/O error has occurred" << endl;
if (state & ios::eofbit)
cout << "eofbit - end of file is encountered" << endl;
if (state & ios::failbit)
cout << "failbit - a nonfatal I/O error has occurred" << endl;
}
}
void check_stream_state2(const fstream &s)
{
//true if no error indicators are set
if(s.good())
{
cout << "good() returned true" << endl;
}
else
{
//true if a stream is marked as unusable (badbit)
if (s.bad())
cout << "bad() returned true" << endl;
//true if end of file is reached on the stream
if (s.eof())
cout << "eof() returned true" << endl;
//true if some operation failed (failbit) or
//the stream is marked as unusable (badbit)
if(s.fail())
cout << "fail() returned true" << endl;
}
}
int main()
{
fstream output("stream.txt", ios::out);
//using member function good()
if(output.good())
{
//write text to the file
output << "Text To File" << endl;
}
//check that goodbit is set
check_stream_state1(output);
check_stream_state2(output);
//close the output stream
output.close();
//try to write text to a file that is not open
output << "Some More Text" << endl;
//check that badbit and failbit are set
check_stream_state1(output);
check_stream_state2(output);
fstream input("stream.txt", ios::in);
string text;
//using member function eof()
while(!input.eof())
{
getline (input, text);
cout << text << endl;
}
//reset/set state flag to be goodbit
input.clear();
//try to write to input-only stream
input << "Some More Text" << endl;
//check that badbit and failbit are set
check_stream_state1(input);
check_stream_state2(input);
//reset/set state flag to be goodbit
input.clear();
//move read pointer back to the beginning of the file
input.seekg(0,ios::beg);
int number = 0;
//try to extract integer from the stream
input >> number;
//check that failbit is set
check_stream_state1(input);
check_stream_state2(input);
//close the input stream
output.close();
//getchar();
return 0;
}
Postconditions
The example application has created the file stream.txt containing one line of text. check_stream_state() functions have displayed state flag information about file I/O statuses as standard output.
