C fopen mode read+write+create - c

Question

I want:
file to be created if it does not exist, not overwritten if it does.
to read and write and fseek where ever I want
and I can not find out valid mode — "w+", "rw" "rwb+" "r+b" "w+b" "a+" or what?
The basic level 'open' that performs well is:
int fd =open("fname", O_RDWR | O_CREAT, 0666);
but I would like to know fopen alternative.
Every mode letters [w, r, a, +] combination I tried will either overwrite contents, or fseek-fwrite not write where it should. "a+" will always append no matter what fseek sets... "rw+" works ok, but does not create nonexistent file ...etc.
Update: to clarify why for example "a+" is NOT a solution:
#include <stdio.h>
int main()
{ FILE *fp =fopen("aaa.txt", "a+");
fwrite("aaaaaaaaaaaaaaaaaaa", 1, 10, fp);
fseek(fp, 5, SEEK_SET);
fwrite("AAA", 1, 3, fp);
fclose(fp);
return 0;
}
runned with: $ rm aaa.txt; gcc test.c && ./a.out && cat aaa.txt && echo .
produces wrong result: aaaaaaaaaaAAA.
result should be: aaaaaAAAaa
Update2: summary... minimal functions that i end up with:
FILE *fopenrwc(char*n) {FILE*f=fopen(n,"a");if(f)fclose(f),f=fopen(n,"r+");return f;}
or:
FILE *fopenrwc(char*n) {return fdopen(open(n,O_RDWR|O_CREAT,0666),"r+");}

Answer 1

If you review the manual page for fopen(), none of the standard open mode strings meets your requirements.
If you're on a sufficiently POSIX-like machine to be able to use open(), don't underestimate the benefits of fdopen() which would allow you to use open() with the options you showed and then create a file stream to use that file.
Note that "rw+" is not a valid mode. If you're (un)lucky, it gets treated as r+.
If you can't use fdopen() for some reason, you may be best off trying r+ and if that fails using w+; that opens a small window of vulnerability where someone might create a file that you then clobber with the w+ option — or creates a symlink so you end up creating a file where you didn't intend to do so.
This is the way it used to be necessary to work with open(); originally, you called open() — in the days before there was an O_CREAT — and if that failed, then you used creat() instead. That's a long time ago, though — see 'UNIX Programming' in 7th Edition UNIX Programmer's Manual Vol 2.
In general, testing with access() doesn't help. It leaves open a window of vulnerability because there is a TOCTOU — Time of Check, Time of Use — gap between the use of access() and open() (or fopen()). This is also the trouble with open() and creat(), or two calls to fopen().
If you want the finer controls, such as O_EXCL, or specialized properties such as O_DSYNC or O_NOCTTY, or even control over the permissions on the created file other than the default as modified by umask(), then open() plus fdopen() is practically the only way to go.

Answer 2

There is no direct way to meet your requirement with a simple fopen. IMHO, you best choice is to first use a low level open to create the file, and then use a fdopen (as suggested by Jonathan Leffler) to get a FILE * that can then be used with all the C library IO functions:
int fd =open("fname", O_RDWR | O_CREAT, 0666);
FILE *fp = fdopen(fd, "r+");
/* Ok, you can do what you want with fp */