FILE *fp2

[usmannazir]

Can some one tell me what exactly is following code doing in C, and what is its alternative in C++.

FILE *fp2;
fopen_s(&fp2, "lid driven cavity(%dx%d).plt", "w");
fprintf(fp2, "VARIABLES=x,y,u,v,p\n");
fprintf(fp2, "ZONE I=%d, J=%d, F=BLOCK, VARLOCATION=( 3,4,5]=CELLCENTERED)\n\n", Nx + 1, Ny + 1);

[Ganado]

Opens a file called "lid driven cavity(%dx%d).plt" (open for writing, without appending), and writes the next two lines to it.

Note on line 4, the first %d is replaced by the value of Nx+1, and the second %d is replaced by the value Ny + 1.

The issue I see with your code is that unlike line 4, line 2 uses fopen_s, which doesn't expect a formatted string. This means that the file name it expects literally contains "%dx%d", not e.g. "10x20".

In C++, something like

// Example program
#include <iostream>
#include <string>
#include <fstream>

int main()
{
    int Nx = 10;
    int Ny = 20;
    std::ofstream f2("lid driven cavity(%dx%d).plt"); // this line is probably an error
    f2 << "VARIABLES=x,y,u,v,p\n";
    f2 << "ZONE I=" << Nx + 1 << ", J=" << Ny + 1 << ", F=BLOCK, VARLOCATION=( 3,4,5]=CELLCENTERED)\n\n";
}

If you want a proper filename, I'd do something like this:

// Example program
#include <iostream>
#include <string>
#include <fstream>

int main()
{
    int a = 5;
    int b = 6;
    std::string filename = "lid driven cavity(" + std::to_string(a) + "x" + std::to_string(b) + ").plt";
    
    int Nx = 10;
    int Ny = 20;
    std::ofstream f2(filename);
    f2 << "VARIABLES=x,y,u,v,p\n";
    f2 << "ZONE I=" << Nx + 1 << ", J=" << Ny + 1 << ", F=BLOCK, VARLOCATION=( 3,4,5]=CELLCENTERED)\n\n";
}

Someone people that come from C don't like how verbosely the C++ stream's << operators work. You could write your own utility functions to make it look more like C.

[jonnin]

If you want to use the C style, use the C functions that are right there and fully functional in the c++. They work just fine. Wrapping up C++ style to look like C is going to add a layer of inefficiency and something else to debug and maintain. If you want both worlds, c++ error handling and such, then you would have to roll your own, but it would still add bloat just to change the code cosmetically, a questionable design pattern.

[dhayden]

Perhaps you just didn't show it, but you're leaking the file descriptor in your original post. Add fclose(fp2) sometime after the last write to close it.

[usmannazir]

@jonnin,
I would be happy to use the C style, but my compiler VISUAL STUDIO COMMUNITY considers fprintf and fopen as an error. Do I need to change settings of my compiler to run such codes?
Can you tell me the function of pointer in this line

 FILE *fp2

[jonnin]

probably. I don't recall … do they still have that managed code crap? If so, turn that off for your project; it blocks some perfectly legal code for no reason.

should just work... what error message?

that isn't a function pointer, its a file pointer. literally -- its a pointer to a construct that interacts with files, and its type is also FILE. Its just how you talk to files in C. Forget that its a pointer (you won't new, delete, [], *access, access->thing, or any of the usual pointer stuff here).

Microsoft has also been obsessed with renaming the core C and a few other functions. It may be trying to force you to use fprintf_s() or something like that which it thinks is more secure. This is nonstandard bs from M$, and I don't know how to turn it off in your version, but you can make it stop that with a clicky somewhere. It should just be throwing nonsense warnings, but some settings may make that an error.

[dhayden]

To get the c functions be sure you have :

#include <cstdio>
using std::fprintf;
using std::fopen;
using std::fclose;
// etc, or do using namespace std;, but that's the lazy way out. 

[Zivojin]

https://stackoverflow.com/questions/21873048/getting-an-error-fopen-this-function-or-variable-may-be-unsafe-when-complin
Try this what is explained here.

[usmannazir]

The error I am getting is shown in the figure
https://imgur.com/a/V67kLZs

[dhayden]

For each of those errors, the problem is the argument to fclose() not fclose() itself. For example, the first one says "Argument of type "FILE**" is incompatible with parameter of type "FILE *". That says that you're passing a FILE** (pointer to pointer to FILE), but fclose() expects you to pass a FILE* (pointer to FILE). It sounds like you declared fp2 as FILE **fp2 instead of FILE *fp2.

Just below that at line 317 you're calling fprintf(&fp3, ...) where fp3 is a FILE *. Just pass fp3 instead.

In general, when using the stdio functions, you declare a variable of type FILE * and the functions return a value of that type, or accept it as a parameter:

FILE *fp = fopen("somefile", "w")
fprintf(fp, "blah blah");
fclose(fp);