`fgetpos` Not Returning the Correct Position - c++

Question

Update: To get around the problem below, I have done
if (ftell(m_pFile) != m_strLine.size())
fseek(m_pFile, m_strLine.size(), SEEK_SET);
fpos_t position;
fgetpos(m_pFile, &position);
this then returns the correct position for my file. However, I would still like to understand why this is occurring?
I want to get the position in a text file. For most files I have been reading the first line, storing the position, doing some other stuff and returning to the position afterwards...
m_pFile = Utils::OpenFile(m_strBaseDir + "\\" + Source + "\\" + m_strFile, "r");
m_strLine = Utils::ReadLine(m_pFile);
bEOF = feof(m_pFile) != 0;
if (bEOF)
{
Utils::CompilerError(m_ErrorCallback,
(boost::format("File '%1%' is empty.") % m_strFile).str());
return false;
}
// Open.
pFileCode = Utils::OpenFile(strGenCode + "\\" + m_strFile, options.c_str());
m_strLine = Utils::Trim(m_strLine);
Utils::WriteLine(pFileCode, m_strLine);
// Store location and start passes.
unsigned int nLineCount = 1;
fpos_t position;
fgetpos(m_pFile, &position);
m_strLine = Utils::ReadLine(m_pFile);
...
fsetpos(m_pFile, &position);
m_strLine = Utils::ReadLine(m_pFile);
With all files provided to me the storage of the fgetpos and fsetpos works correctly. The problem is with a file that I have created which looks like
which is almost identical to the supplied files. The problem is that for the file above fgetpos(m_pFile, &position); is not returning the correct position (I am aware that the fpos_t position is implementation specific). After the first ReadLine I get a position of 58 (edited from 60) so that when I attempt to read the second line with
fsetpos(m_pFile, &position);
m_strLine = Utils::ReadLine(m_pFile);
I get
on 700
instead of
Selection: Function ADJEXCL
Why is fgetpos not returning the position of the end of the first line?
_Note. The Utils.ReadLine method is:
std::string Utils::ReadLine(FILE* file)
{
if (file == NULL)
return NULL;
char buffer[MAX_READLINE];
if (fgets(buffer, MAX_READLINE, file) != NULL)
{
if (buffer != NULL)
{
std::string str(buffer);
Utils::TrimNewLineChar(str);
return str;
}
}
std::string str(buffer);
str.clear();
return str;
}
with
void Utils::TrimNewLineChar(std::string& s)
{
if (!s.empty() && s[s.length() - 1] == '\n')
s.erase(s.length() - 1);
}
Edit. Following the debugging suggestions in the comments I have added the following code
m_pFile = Utils::OpenFile(m_strBaseDir + "\\" + Source + "\\" + m_strFile, "r");
m_strLine = Utils::ReadLine(m_pFile);
// Here m-strLine = " Logic Definition Report Chart Version: New Version 700" (64 chars).
long vv = ftell(m_pFile); // Here vv = 58!?
fpos_t pos;
vv = ftell(m_pFile);
fgetpos(m_pFile, &pos); // pos = 58.
fsetpos(m_pFile, &pos);
m_strLine = Utils::ReadLine(m_pFile);

Answer 1

Sorry, but your Utils functions have clearly been written by an incompetent. Some issues are just a matter of style. For trimming:
void Utils::TrimNewLineChar(std::string& s)
{
if (!s.empty() && *s.rbegin() == '\n')
s.resize(s.size() - 1); // resize, not erase
}
or in C++11
void Utils::TrimNewLineChar(std::string& s)
{
if (!s.empty() && s.back() == '\n')
s.pop_back();
}
ReadLine is even worse, replace it with:
std::string Utils::ReadLine(FILE* file)
{
std::string str;
char buffer[MAX_READLINE];
if (file != NULL && fgets(buffer, MAX_READLINE, file) != NULL)
{
// it is guaranteed that buffer != NULL, since it is an automatic array
str.assign(buffer);
Utils::TrimNewLineChar(str);
}
// copying buffer into str is useless here
return str;
}
That last str(buffer) in the original worries me especially. If fgets reaches a newline, fills the buffer, or reaches end of file, you're guaranteed to get a properly terminated string in your buffer. If some other I/O error occurs? Who knows? It might be undefined behavior.
Best not to rely on the value of buffer when fgets fails.