Partial line from cpp file ending up in output file - haunted code? - c++

Question

I'm sorry, it would be extremely difficult to make a fully reproducible version of the error --- so please bare with my schematic code.
This program retrieves information from a web page, processes it, and saves output to an ASCII file. I also have a 'log' file (FILE *theLog---contained within a Manager object) for reporting errors, etc.
Some background methods:
// Prints string to log file
void Manager::logEntry(const string lstr) {
if( theLog != NULL ) { fprintf(theLog, "%s", lstr.c_str()); }
}
// Checks if file with given name already exists
bool fileExists(const string fname) {
FILE *temp;
if( temp = fopen(fname.c_str(), "r") ) {
fclose(temp);
return true;
} else { return false; }
}
// Initialize file for writing (some components omitted)...
bool initFile(FILE *&oFile, const string fname) {
if(oFile = fopen(fname.c_str(), "w") ) { return true; }
else { return false; }
}
The stuff causing trouble:
// Gets data from URL, saves to file 'dataFileName', input control flag 'foreCon'
// stu is some object that has string which i want
bool saveData(Manager *man, Stuff *stu, string dataFileName, const int foreCon) {
char logStr[CHARLIMIT_LARGE]; // CHARLIMIT_LARGE = 2048
sprintf(logStr, "Saving Data...\n");
man->logEntry( string(logStr) ); // This appears fine in 'theLog' correctly
string data = stu->getDataPrefixStr() + getDataFromURL() + "\n"; // fills 'data' with stuff
data += stu->getDataSuffixStr();
if( fileExists(dataFileName) ) {
sprintf(logStr, "save file '%s' already exists.", dataFileName.c_str() );
man->logEntry( string(logStr) );
if( foreCon == -1 ) {
sprintf(logStr, "foreCon = %d, ... exiting.", foreCon); // LINE 'A' : THIS LINE ENDS UP IN OUTPUT FILE
tCase->logEntry( string(logStr) );
return false;
} else {
sprintf(logStr, "foreCon = %d, overwriting file.", foreCon); // LINE 'B' : THIS LINE ENDS UP IN LOG FILE
tCase->logEntry( string(logStr) );
}
}
// Initialize output file
FILE *outFile;
if( !initFile(outFile, dataFileName) ) {
sprintf(logStr, "couldn't initFile '%s'", dataFileName.c_str());
tCase->logEntry( string(logStr) );
return false;
}
fprintf(outFile, "%s", data.c_str()); // print data to output file
if( fclose(outFile) != EOF) {
sprintf(logStr, "saved to '%s'", dataFileName.c_str());
tCase->logEntry( string(logStr) );
return true;
}
return false;
}
If the file already exists, AND 'int foreCon = -1' then the code should print out line 'A' to the logFile. If the file exists and foreCon != -1, the old file is overwritten with data. If the file doesn't exist, it is created, and the data is written to it.
The result however, is that a broken up version of line 'A' appears in the data file AND line 'B' is printed in the log file!!!!
What the data file looks like:
.. exiting.20130127 161456
20130127 000000,55,17,11,0.00
20130127 010000,54,17,11,0.00
... ...
The second line and onward look correct, but there is an extra line that contains part of line 'A'.
Now, the REALLY WEIRD PART. If I comment out everything in the if( foreCon == -1) { ... } block, then the data file looks like:
%d, ... exiting.20130127 161456
20130127 000000,55,17,11,0.00
20130127 010000,54,17,11,0.00
... ...
There is still an extra line, but it is the LITERAL CODE copied into the data file.
I think there is a poltergeist in my code. I don't understand how any of this could happen.
Edit: I've tried printing to console the data string, and it gives the same messed up values: i.e. %d, ... exiting.20130127 161456 - so it must be something about the string instead of the FILE *

Answer 1

Answer based on your latest comment:
getDataPrefixStr() ends up returning a string which starts with
something like string retStr = COMCHAR + " file created on ..."; such
that const char COMCHAR = '#';. Could the COMCHAR be the problem??
You can't add characters and string literals (which are arrays of char, not strings) like that.
You're adding 35 (the ASCII for "#") to the address of " file created on ... ", i.e. getDataPrefixStr() is whatever starts 35 characters from the start of that string. Since all literal strings are stored together in the same data area, you'll get strings from the program in the output.
Instead, you cold do
const string COMCHAR = "*";
string retStr = COMCHAR + " file created on ...";

Answer 2

It could be that logStr is too short and that it is causing data to be overwritten in other buffers (did you double check CHARLIMIT_LARGE?). You can diagnose this by commenting all writes to logStr (sprintf) and see if data is still corrupted. In general, your code is vulnerable to this if a user can set dataFileName (to be a very long string); use snprintf or ostringstream instead.
Otherwise, I would guess that either stu->getDataPrefixStr() or getDataFromURL() are returning corrupted results or return type char* instead of string. Try printing these values to the console directly to see if they are corrupted or not. If they return a char*, then data = stu->getDataPrefixStr() + getDataFromURL() will have undefined behavior.

Answer 3

if( temp = fopen(fname.c_str(), 'r') ) {
should be
if( temp = fopen(fname.c_str(), "r") ) {