Proper methods to Copy files/folders programmatically in C using POSIX functions - c

Question

These terms may not be 100% accurate, but I'm using the GCC compiler and POSIX library. I have C code compiled with the SQLite amalgamation file to a single executable.
In the user interface that exchanges JSON messages with the C program, I'd like to make it possible for users to copy the SQLite database files they create through the C program, and copy a full directory/folder.
Thus far, I've been able to rename and move files and folders programmatically.
I've read many questions and answers here, at Microsoft's C runtime library, and other places but I must be missing the fundamental points. I'm using regular old C, not C++ or C#.
My question is are there POSIX functions similar to rename(), _mkdir(), rmdir(), remove(), _stat(), that allow for programmatic copying of files and folders in Windows and Linux?
If not, can one just make a new folder and/or file and fread/fwrite the bytes from the original file to the new file?
I am primarily concerned with copying SQLite database files, although I wouldn't mind knowing the answer in general also.
Is this answer an adequate method?
Is the system() function a poor method? It seems to work quite well. However, it took awhile to figure out how to stop the messages, such as "copied 2 files" from being sent to stdout and shutting down the requesting application since it's not well-formed JSON. This answer explains how and has a link to Microsoft "Using command redirection operators". A /q in xcopy may or may not be necessary also, but certainly didn't do the job alone.
Thank you very much for any direction you may be able to provide.
The question that someone suggested as an answer and placed the little submission box on this question is one that I had already linked to in my question. I don't mean to be rude but, if it had answered my question, I would not have written this one. Thank you whoever you are for taking the time to respond, I appreciate it.
I don't see how that would be a better option than using system() because with the right parameters all the sub-directories and files of a single parent folder can be copied in one statement without having to iterate through all of them manually. Is there any reason why it would not be better to use system() apart from the fact that code will need to be different for each OS?
Handling errors are a bit different because system() doesn't return an errno but an exit code; however, the errors can be redirected from stderr to a file and pulled from there, when necessary

Answer 1

rename(): posix
_mkdir(): not posix. You want mkdir which is. mkdir takes two arguments, the second of which should usually be 077.
rmdir(): posix
remove(): posix
_stat(): not posix, you want stat() which is.
_stat and _mkdir are called as such on the Windows C library because they're not quite compatible with the modern Unix calls. _mkdir is missing an argument, and _stat looks like a very old version of the Unix call. You'll have trouble on Windows with files larger than 2GB.
You could do:
#ifdef _WIN32
int mkdir(const char *path, int mode) { return _mkdir(path); } /* In the original C we could have #defined this but that doesn't work anymore */
#define stat _stat64
#endif
but if you do so, test it like crazy.
In the end, you're going to be copying stuff with stdio; this loop works. (beware the linked answer; it has bugs that'll bite ya.)
int copyfile(const char *src, const char *dst)
{
const int bufsz = 65536;
char *buf = malloc(bufsz);
if (!buf) return -1; /* like mkdir, rmdir, return 0 for success, -1 for failure */
FILE *hin = fopen(src, "rb");
if (!hin) { free(buf); return -1; }
FILE *hout = fopen(dst, "wb");
if (!hout) { free(buf); fclose(hin); return -1; }
size_t buflen;
while ((buflen = fread(buf, 1, bufsz)) > 0) {
if (buflen != fwrite(buf, 1, buflen)) {
fclose(hout);
fclose(hin);
free(buf);
return -1; /* IO error writing data */
}
}
free(buf);
int r = ferror(hin) ? -1 : 0; /* check if fread had indicated IO error on input */
fclose(hin);
return r | (fclose(hout) ? -1 : 0); /* final case: check if IO error flushing buffer -- don't omit this it really can happen; calling `fflush()` won't help. */
}