How to go from fopen to fopen_s

Question

Visual Studio is complaining about fopen. I can't find the proper syntax for changing it. I have:

FILE *filepoint = (fopen(fileName, "r"));

to

FILE *filepoint = (fopen_s(&,fileName, "r"));

What is the rest of the first parameter?

`fopen_s` [is documented at MSDN](https://msdn.microsoft.com/en-us/library/z5hh6ee9.aspx). The first parameter is supposed to be a `FILE**`, and the return value is an `errno_t`. — Michael, Feb 24 '15 at 09:02
I would just set the compiler settings to gag the warning message. — Jive Dadson, Jan 17 '18 at 02:17

chqrlie · Answer 1 · 2017-11-25T11:36:48.827

46

fopen_s is a "secure" variant of fopen with a few extra options for the mode string and a different method for returning the stream pointer and the error code. It was invented by Microsoft and made its way into the C Standard: it is documented in annex K.3.5.2.2 of the most recent draft of the C11 Standard. Of course it is fully documented in the Microsoft online help. You do not seem to understand the concept of passing a pointer to an output variable in C. In your example, you should pass the address of filepoint as the first argument:

errno_t err = fopen_s(&filepoint, fileName, "r");

Here is a complete example:

#include <errno.h>
#include <stdio.h>
#include <string.h>
...
FILE *filepoint;
errno_t err;

if ((err = fopen_s(&filepoint, fileName, "r")) != 0) {
    // File could not be opened. filepoint was set to NULL
    // error code is returned in err.
    // error message can be retrieved with strerror(err);
    fprintf(stderr, "cannot open file '%s': %s\n",
            fileName, strerror(err));
    // If your environment insists on using so called secure
    // functions, use this instead:
    char buf[strerrorlen_s(err) + 1];
    strerror_s(buf, sizeof buf, err);
    fprintf_s(stderr, "cannot open file '%s': %s\n",
              fileName, buf);
} else {
    // File was opened, filepoint can be used to read the stream.
}

Microsoft's support for C99 is clunky and incomplete. Visual Studio produces warnings for valid code, forcing the use of standard but optional extensions but in this particular case does not seem to support strerrorlen_s. Refer to Missing C11 strerrorlen_s function under MSVC 2017 for more information.

edited Nov 25 '17 at 11:36

answered Feb 24 '15 at 13:12

chqrlie

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strerror has a similar problem to fopen. Consider revising to show use of strerror_s for a +1 from me? – Assimilater Apr 15 '16 at 20:34
1

@Assimilater: there is no real *problem* with `fopen`. Visual Studio might produce a warning, prompting users to write less portable code with `fopen_s()`. The added semantics in `fopen_s` regarding the default permissions and the exclusive mode are best addressed with `fdopen` when available. `strerror_s` requires the user to provide a buffer whose length should be computed first by calling `strerrorlen_s(err)`. A headache hardly worth the trouble. – chqrlie Apr 16 '16 at 19:15
@Assimilater: Nevertheless, I edited the answer and provided code for both approaches. – chqrlie Apr 16 '16 at 19:21
I meant that when I used strerror(err) I got the same warning from visual studio (which prevented me from compiling) that I did with fopen. :/ – Assimilater May 10 '16 at 03:54
I didn't downvote but it complains about strerrorlen_s is unknown identifier. using vs 2013 – nurp Nov 24 '17 at 14:54
1

@nurp: Microsoft's support for C99 is clunky and incomplete. VS produces warnings for valid code, forcing the use of optional standard extensions but in this particular case does not seem to support the `strerrorlen_s`. Refer to https://stackoverflow.com/questions/44430141/missing-c11-strerrorlen-s-function-under-msvc-2017 for more information. – chqrlie Nov 25 '17 at 11:35

How to go from fopen to fopen_s

1 Answers1