Question

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How to get file descriptor of buffer in memory?

I’m trying to force a library that uses a FILE class to use a buffer in memory instead of a file. I tried fmemopen, however, the library uses fileno which returns -1 and causes the library to crash. So I read that I need a file descriptor and I could use the pipe command to make one. I don’t fully understand what I did but I got it to work. However, I don’t know if what I did is actually safe. So here is the code:

int fstr[2];
pipe(fstr);
write(fstr[1], src, strlen(src));
close(fstr[1]);
FILE* file = fdopen( fstr[0], "r" );

So is this safe?

Answer 1

If you really need a file handle, this is as good as anything I can think of.

One possible issue you may run into is that the buffer size for a pipe is limited. If your string is larger than the buffer size, the write(...) to the pipe will block until some data is read(...) from the other end.

Ideally you would have a worker thread writing to the pipe, but if this is not possible/too hard, you can possibly adjust the pipe buffer size through fcntl(fd, F_SETPIPE_SZ, ...).