how can i save the output of putchar to a variable - c

Question

I am working on some code that filters text before it is sent further into a program (this code removes everything but all alphanumeric characters and and underscores), the code itself works perfectly except for the fact that I cannot find a way to store the output of of it for use in other parts of the program, If i had to guess, this probably involves saving the stdout from putchar into a variable, but i cannot find much info for doing so online, if someone could point me in the right direction for this I would really appreciate it, thanks!
#include <stdio.h>
#include <string.h>
#include <ctype.h>
int main(void) {
int i;
char *p;
char stg[] = "hello";
for (p = &stg[0]; *p != '\0'; p++) {
if (isalnum(*p) || *p == '_') {
putchar (*p);
}
}
putchar ('\n');
return 0;
}

Answer 1

Perhaps I don't understand your "need" to use putchar() while doing the filtering, but you can filter the input into an output array of chars to use however needed after the filtering as shown below.
#include <stdio.h>
#include <string.h>
#include <ctype.h>
int main(void) {
int i;
char *p;
char stg[] = "hel123*^_lo";
char output[200] = {0x00};
int index = 0;
p = stg;
while( *p )
{
if (isalnum(*p) || *p == '_')
{
output[index++] = (char)putchar(*p);
}
p++;
}
putchar('\n');
printf("[%s]\n", output);
return 0;
}
Output:
hel123_lo
[hel123_lo]
EDIT:
And if you want to just filter the string into an array without displaying the string using putchar() you'd do something like this:
#include <stdio.h>
#include <string.h>
#include <ctype.h>
int main(void) {
int i;
char *p;
char stg[] = "hel123*^_lo";
char output[200] = {0x00};
int index = 0;
p = stg;
while( *p )
{
if (isalnum(*p) || *p == '_')
{
output[index++] = *p;
}
p++;
}
printf("[%s]\n", output);
return 0;
}
And what exactly are you trying to do with the output of the filtered text?

Answer 2

putchar -- int putchar( int ch ); -- returns the character you wrote if successful, EOF if failure.
Nothing prevents you from declaring an int variable, whether a scalar variable, an array element, or a field in a struct, and saving what you wrote. Take note, the return is an int, not a char.
Based on what you've written, you'll probably have to write some code to manage all the outputs you save. In other words, if I'm interpreting your question correctly, you'll be doing more than just saving what you wrote to a variable. You'll probably want to say where in the code you executed the putchar and perhaps even the time. Just guessing though.