Smart-write to arbitrary file descriptor from C/C++ [duplicate]

问题

This question already has answers here:

In C how do you redirect stdin/stdout/stderr to files when making an execvp() or similar call? (3 answers)

Redirecting exec output to a buffer or file (5 answers)

Rerouting stdin and stdout from C (8 answers)

Closed 4 days ago.

THE QUESTION

I mean to write C/C++ code such that I am able to specify to a user of the program how to redirect various message categories (each one being output at a file descriptor number that I should know) to different user-selected files. E.g., 3: messages from mild warnings, 4: messages from numeric routines (and not type 3), etc.

EDIT #1 (Top posting)

I managed to redirect stdout to my fd file descriptor number from within C with dup2 (I learned today about it). In this way, I can write to fd with all the same goodies that I would do with stdout. I simply do

int retval = dup2(fd, STDOUT_FILENO);

Now I have a couple of subsequent questions:

How can I remove the redirection from stdout to fd at a later stage in my code? I have found here that I need to first "clone" stdout, and use the clone as a backup for later restoring. This looks somewhat cumbersome.
What if I want to write alternately to fd and stdout in nearby sentences? Moreover, I would likely want to capture both with something like
```
mycomm > out.txt 3> myout.txt
```
From my current understanding of the use of dup2, I would have to repeatedly apply step 1 above, which seems clumsy.

ORIGINAL ATTEMPT

I would need:

Within the code I can write to predefined file descriptor numbers (say, 3)

const int fd = 3;
<something that relates fd with an ofstream or FILE *>
fp << <whatever stuff>;   // C++ style
fprintf(...)              /* C style */

If the executable generated is mycomm, at the command line I can write
```
mycomm 3> myout.txt
```

I put together code below. It partially does the job, except for at least one undesirable effect: I have to tell the number of bytes to be written. I want something that is smart enough to guess that, in particular when chaining several outputs, as in

cout << myintvar << mystringvar << ", that's it";

I can always write to an ostringstream, get the length, and use it as an argument, but there might be something neater already available.

I am thinking about creating a class, whose single member is an int for the file descriptor, but adding the capability to easily smart-write to it.

Is this a correct approach? Is there an alternative, easier, more fool-proof way of achieving this?

Besides, What is a safe way to first check if the "hardcoded" fd number is available?

My code:

#include <iostream>
#include <unistd.h>
#include <stdio.h>
#include <string.h>

using namespace std;

int main() {
    const int fd = 3;
    char c[100] = "Testing";
    ssize_t nbytes = write(fd, (void *) c, strlen(c));
    cout << "Wrote " << nbytes << " bytes to file descriptor " << fd << endl;
    return 0;
}

来源：https://stackoverflow.com/questions/61852010/smart-write-to-arbitrary-file-descriptor-from-c-c

标签

c++

fwrite