How do I use a #defined constant as the max field width in fscanf?

Question

All of this is in C89, not C99.

I have a constant.

#define MAX_NAME_LEN 256

I want to use it as the max field width in fscanf, sort of like this.

fscanf(input, "%256s", name);

But I want to use the MAX_NAME_LEN instead of the literal 256 for sake of good style. I have tried all of

fscanf(input, "%MAX_NAME_LENs", name);

char* max_name_len_str = malloc(16 * sizeof *max_name_len_str);
sprintf(max_name_len_str, "%d", MAX_NAME_LEN);
fscanf(input, "%" max_name_len_str "s", name);
free(max_name_len_str);

//works with printf, but has different meaning in scanf
fscanf(input, "%*s", MAX_NAME_LEN, name);

fscanf(input, "%%ds", MAX_NAME_LEN, name);

without success.

char* nameFormat = malloc(16 * sizeof *nameFormat); //I assume I don't ever want more than 10^13 characters in a name
sprintf(nameFormat, "%s%ds", "%", MAX_NAME_LEN);
fscanf(input, nameFormat, name);
free(nameFormat);

does work, but is ungainly as all get out. Is there a more elegant solution?

@weather-vane Your deleted answer was off by 1. size vs length issue. — chux - Reinstate Monica, Jun 21 '17 at 20:59
@chux: it was missing a cast too: `(rsize_t)(MAX_NAME_LEN + 1)`, or better `sizeof(name)`. — chqrlie, Jun 21 '17 at 21:00
@WeatherVane: your deleted answer is useful: `fscanf_s` is not C89, but it is obviously available on the most common non C99 compiler: MSVC. — chqrlie, Jun 21 '17 at 21:05
agreed, it might be useful for others looking for the same answer. — blackghost, Jun 21 '17 at 21:10
[How to prevent `scanf()` causing a buffer overflow?](https://stackoverflow.com/questions/1621394/) recommends the 'ungainly as all get out' solution because it works, more or less regardless of how `MAX_NAME_LEN` expands. Almost any other solution puts limits on the permissible/acceptable values (expansions) of `MAX_NAME_LEN`. It depends on how general and robust you need the code to be. If you only need it to work with your code at the moment, you can hack together a string with the macro value. If you need a general solution, then you probably need the `sprintf()` method. — Jonathan Leffler, Jun 21 '17 at 21:21

mch · Answer 1 · 2017-06-21T21:17:48.930

5

You can use this macro:

#define STRINGIFY(X) INDIRECT(X)
#define INDIRECT(X) #X

like this:

#define MAX 10
puts("%"STRINGIFY(MAX)"d");

which will print %10d.

In your case it would be

char name[MAX_NAME_LEN + 1];
fscanf(input, "%"STRINGIFY(MAX_NAME_LEN)"s", name);

The # in the macro stringifies (makes " around it) whatever is after it. So the macro must be the decimal number only.
The level of indirection is needed to expand MAX_NAME_LEN to 256. INDIRECT(MAX_NAME_LEN) would expand to "MAX_NAME_LEN".

edited Jun 21 '17 at 21:17

answered Jun 21 '17 at 20:44

mch

9,424
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Curious, is `#X` valid for C89? – chux - Reinstate Monica Jun 21 '17 at 20:50
1

@chux : Yes, `str(s)` and `xstr(s)` (defined in the same way as [here](https://gcc.gnu.org/onlinedocs/gcc-3.4.3/cpp/Stringification.html)) are given as examples in section 6.8.3.5. – Daniel Kamil Kozar Jun 21 '17 at 20:58
4

One sharp stick -- you need to ensure that `MAX_NAME_LEN` defines a literal number (it cannot be a calculation, so `#define MAX_NAME_LEN (255-2)` would not work). – blackghost Jun 21 '17 at 21:08
1

Furthermore, `MAX_NAME_LEN` must be written in base 10: `0x100` will not work either, nor will `20L`... – chqrlie Jun 21 '17 at 21:10
Suggest showing `name[]` declaration like `name[MAX_NAME_LEN + 1];` as the +1 is important. – chux - Reinstate Monica Jun 21 '17 at 21:12
Yes, putting `()` around it constitutes a calculation... in my mind anyways :-). – blackghost Jun 21 '17 at 21:15

chux - Reinstate Monica · Accepted Answer · 2017-06-21T21:10:36.663

1st simplification.

// Unlikely need to malloc short array
char nameFormat[16]; // I assume I don't ever want more than 10^13 characters  
sprintf(nameFormat, "%%%ds", MAX_NAME_LEN);  // %% prints a %
fscanf(input, nameFormat, name);

2nd offering or better, use stringify @DevilaN.
Note that the size of the buffer needs to be at least 1 more than the "%s" width.

#define MAX_NAME_LEN 256
#define MAX_NAME_LEN_STR "256"

char name[MAX_NAME_LEN + 1];
fscanf(input, "%" MAX_NAME_LEN_STR "s", name);

3rd, use fgets() (my preference), Of course this reads a line and not a sequence of non-white-space characters. This often does meet that higher level goal though.

#define MAX_NAME_LEN 256
char name[MAX_NAME_LEN + 2];
fgets(name, sizeof name, input);

score 0 · Answer 3 · answered Jun 21 '17 at 20:48

0

You will struggle with fscanf. I don't know better solution than proposed by you if you are bound with fscanf.

Personally I would switch to fgets like in this simple example:

#define MAX_NAME_LEN 25

int main()
{
        char string[MAX_NAME_LEN];
        fgets(string,MAX_NAME_LEN,stdin);

        printf("%s", string);

}

answered Jun 21 '17 at 20:48

DevilaN

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There is a perfectly valid solution posted in mch's answer. – Daniel Kamil Kozar Jun 21 '17 at 20:48
2

@DanielKamilKozar 2 issues to cope when switching to `fgets()` that are not mentioned/addressed here: A `+ 2` buffer needed (`\n` and `\0`) and that `fgets()` saves white-space, unlike `"%s"`. – chux - Reinstate Monica Jun 21 '17 at 20:54

score 0 · Answer 4 · answered Jun 21 '17 at 22:51

As a reference, following is a fscanf_s() approach to prevent overflow as used by earlier version of VS that did not attain C99 compliance. An additional unsigned argument follows name[] indicator the size of the buffer. This appears to apply to VS 2015 too.

char name[MAX_NAME_LEN + 1];
fscanf_s(input, "%s", name, (unsigned) sizeof name);

C11 includes fscanf_s() in its Annex K which is normative, for information only, so a compliant compiler need not implement it. Yet in C11, it is specified as needing a rsize_t/size_t argument .

fscanf_s(input, "%s", name, sizeof name);

If input is more than 256 non-white-space characters, a matching failure occurs. In this case, fscanf() return EOF. I think the VS version does the same. I think both will consume all the non-white-space in the stream, even if there is not enough room to store them all.

Either of these is a bit different than the below in that extra text is not read into name[], so no buffer overflow. Any extra text remains for the next input function call. fscanf() returns 1.

fscanf(input, "%256s", name);

How do I use a #defined constant as the max field width in fscanf?

4 Answers4