Converting the output of a void method to String in C

Question

I have written a void function which takes an unsigned int. Then I call this in the main function. I want the output to be a string so that I can include it in the printf statement for formatting reasons. But since I'm calling a void function, I can't tell printf that its a string (%s) or an int (%d). How can I take the output from this called void function in my main and turn it to some kind of string or integer?

Show the function declaration. If it is as you say, `void fn(....)`, then you can't do what you're trying. `void` means no return value. — WhozCraig, Jun 23 '14 at 06:11
`void something (unsigned int x){}` which returns nothing. Then in my `main`, I call `something(some value)`. — user3754974, Jun 23 '14 at 06:12
`void` means that you're not returning any data http://stackoverflow.com/a/3487716/2265932 — nervosol, Jun 23 '14 at 06:13
We'll I understand that a void function returns nothing. I was just wondering if there was another way around it so that I can keep the void function but somehow convert its output to a string or an integer in the main. Is that possible? — user3754974, Jun 23 '14 at 06:17
@user3754974 What do you mean by "its output"? Something it's calculating? It's `printf` output? — Jonathon Reinhart, Jun 23 '14 at 06:18
@JonathonReinhart The void function does have an output which are numbers. But I want to include the output of numbers from the void function to be a string or int. Does this make sense? — user3754974, Jun 23 '14 at 06:21
Not really. You should read much more about C programming. You look very confused. — Basile Starynkevitch, Jun 23 '14 at 06:24
@user3754974 The "output" of a function is its return value. See my example answer. — Jonathon Reinhart, Jun 23 '14 at 06:34

Basile Starynkevitch · Answer 1 · 2014-06-23T06:52:06.633

A function declared as e.g. void foo(int); has no result. So you cannot convert its inexistent result to an integer or to a string.

^{notice that C does not have stricto sensu a string datatype. You use pointer to memory zones made of char, with the convention to zero-terminate strings.}

Perhaps you want to get the output (done using printf, fprintf, fputs, and other stdio(3) standard library functions) of a function as a string. On Linux (and GNU libc) systems, you could use open_memstream(3)

^{Do not confuse the output of a function with its result!}

So suppose you have written some outputing function like e.g.

void show_x(FILE*out, int x) { fprintf(out, "x=%d\n", x); }

Then you could get that into a string like ptrbuf (on Linux) using:

char*ptrbuf=NULL;
size_t sizbuf=0;
FILE* outmem = open_memstream(&ptrbuf, &sizbuf);
if (!outmem) { perror("open_memstream"); exit(EXIT_FAILURE); };
show_x(outmem, 42);
fflush(outmem);
printf ("now ptrbuf is %s\n", ptrbuf);
fclose(outmem);
// you later should free the `malloc`-ed `ptrbuf`
free (ptrbuf), ptrbuf=NULL;
sizbuf=0;

Considering the OP's obvious n00bish-ness, `open_memstream` seems a bit heavy here. — Jonathon Reinhart, Jun 23 '14 at 06:14
I would hardly think I'm a noob. Perhaps a beginner as you were at one point. But I agree, this seems a bit hardcore for me. Maybe I don't understand it yet. — user3754974, Jun 23 '14 at 06:25
Sorry if you took offense to that, "beginner" was all I really meant. We were all noobs at one point :-) — Jonathon Reinhart, Jun 23 '14 at 06:32

score 1 · Answer 2 · answered Jun 23 '14 at 06:13

A void function returns nothing. Change its return type to be const char* to return a C-string:

#include <stdio.h>

const char* func_that_returns_string(unsigned int x)
{
    switch (x) {
    case 0:  return "zero";
    case 1:  return "one";
    default: return "who needs any other numbers?";
    }
}

int main(void)
{
    int i = 3;
    const char* str;

    str = func_that_returns_string(i);

    printf("%d: %s\n", i, str);

    return 0;
}

Converting the output of a void method to String in C

2 Answers2