Segmentation Fault in vsprintf() - c++

Question

#define printm(p) writeToAnalyticsFile p
void writeToAnalyticsFile (const char* format, ...)
{
std::string errorLog;
std::string headerLine = "\"errorText\":";
FILE *fp = fopen( "Analyse.txt", "w" );
if( 0 == fp )
{
}
else
{
va_list args;
va_start (args, format);
vsprintf (const_cast<char*>(errorLog.c_str()),format, args);
va_end (args);
fputs(headerLine.c_str(),fp);
fputs("\"",fp);
fputs( errorLog.c_str(), fp );
fputs("\"",fp);
fclose(fp);
}
}
class abc
{
public:
static const char * func()
{
std::string str = "john";
return str.c_str();
}
};
int main()
{
printm(("My Name is %s and surname is %s and age is %d",abc::func(),"john",25));
return 0;
}
I basically want to replace printm((--- )) with writeToAnalytics(--) which writes data to file.
I am getting Segmentation Fault with below code.
Problem is with calling abc::func() in main. Is it because of local storage of str in func()?

Answer 1

The func function is incorrect, as it is returning an invalid pointer:
static const char * func()
{
std::string str = "john";
return str.c_str();
}
The string is destroyed at the end of the function, and therefore the pointer str.c_str() is a dangling pointer - pointing to a string that doesn't exist any more.
Solution is to just return "john". Literals last forever.
static const char * func()
{
return "john";
}
Or, better still, just return the string,
static string func()
{
std::string str = "john";
return str;
}
and use abc::func().c_str() at the call site.
(There may be other problems, but this is the most obvious to me)