C fgetc weird behavior when simultaneously used as parameter to same function

Question

I'm using fgetc to read and validate a stream from text file, one character at a time, but for some reason it reorders the characters. The text file contains something like "abc"

void newFunction(int i, int j, int k);

int main()
{
    FILE *fp;
    /...opens text file.../
    newFunction(fgetc(fp), fgetc(fp), fgetc(fp));
}

void newFunction(int i, int j, int k)
{
    printf("%d %d %d", i, j, k,);
}

I'm expecting a b c but why does my code give c b a?

It is not defined in which order the arguments are evaluated. — Some programmer dude, Apr 23 '17 at 05:58
@Someprogrammerdude you mean precedence? I'll try enclosing in parentheses. — reiallenramos, Apr 23 '17 at 06:01
*Operator* precedence has nothing to do with it. It's simply that the arguments can be evaluated in any order. You will not know which `fgetc` call will be called first. Please read e.g. [this evaluation order reference](http://en.cppreference.com/w/c/language/eval_order) for more information. — Some programmer dude, Apr 23 '17 at 06:04
No, @Someprogrammerdude means that you can't predict which `fgetc()` will be evaluated first. Also, it's terrible practice to "*shorten*" the code like this, and it's too specific to a given input file, you should not write a program to work with a given input file, a "*file format*" is more appropriate and in that case I recommend an array, `char abc[3]` and `fread()` perhaps? — Iharob Al Asimi, Apr 23 '17 at 06:05
Possible duplicate of [Why are these constructs (using ++) undefined behavior?](http://stackoverflow.com/questions/949433/why-are-these-constructs-using-undefined-behavior) — too honest for this site, Apr 23 '17 at 06:16
You should get numbers as output since you used `%d`. Always copy-paste the _exact_ code, Preferably a [mcve] — Spikatrix, Apr 23 '17 at 06:48
@Olaf Isn't this code an example for _unspecified_ behavior and not _undefined_ behavior? And since the linked question is about UB with `++` mania, I don't think this is a dupe of that one. — Spikatrix, Apr 23 '17 at 06:52

score 0 · Answer 1 · edited Apr 23 '17 at 08:49

0

void newFunction(int i, int j, int k);

int main()
{
    FILE *fp;
    /...opens text file.../
    int i = fgetc(fp);
    int j = fgetc(fp);
    int k = fgetc(fp);
    newFunction(i,j,k);
}

void newFunction(int i, int j, int k)
{
    printf("%d %d %d", i, j, k);
}

You can do above to avoid ambiguity in order evaluating parameters.

edited Apr 23 '17 at 08:49

M.M

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answered Apr 23 '17 at 06:10

Austin

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Yes. It also helps maintain the sanity of anyone who had to debug, maintain or enhance the code. – ThingyWotsit Apr 23 '17 at 08:29

C fgetc weird behavior when simultaneously used as parameter to same function

1 Answers1