get string in input and print it folded in C - c

Question

I'm trying to write a program which gets one or more input lines, and if one line is too long, it gets folded at a maximum number of chars. My approach would be to write the input chars in a first array, of a given length. If Maximum length is reached or '\n' as input, i copy the content in a bigger array which will be the final string to print and get the second line in input. Problem is: it doesn't work and I can't figure out why. Thanks for the help
#include <stdio.h>
#define MAXCOL 10
#define FINAL_LENGTH 300
char line[MAXCOL];
char final_string[FINAL_LENGTH];
extern int fnlstr_pos = 0;
int main()
{
int pos, c;
pos = 0;
while(c=getchar() != EOF)
{
line[pos] = c;
if (pos + 1 >= MAXCOL || c == '\n'){
to_printandi(pos);
pos = 0;
}
++pos;
}
printf("%s", final_string);
}
to_printandi(pos)
int pos;
{
int i;
for(i = 0; i <= pos; ++i){
final_string[fnlstr_pos] = line[i];
++fnlstr_pos;
}
if (final_string[fnlstr_pos] != '\n'){
final_string[++fnlstr_pos] = '\n';
}
++fnlstr_pos;
}

Answer 1

There are several problems in the code. Others have already pointed out the bug in the getchar() line.
More variables and more functions and more code only twist one around in knots. If you take some time to think about what you want to achieve, go slowly, you can get your results with much less effort. Less code full of helpful comments, make for better programs.
EDIT
Looking at code with fresh eyes, I realised that the two lines explicitly setting the 'trailing' byte to '\0' were write 0 overtop of bytes already initialised to 0. Have commented out those two lines as they are superfluous.
#include <stdio.h>
int main () {
char buf[ 1024 ] = { 0 }; // buffer initialised
int ch, cnt = 0, ccnt = 0; // input char and counters
while( ( ch = getchar() ) != EOF ) { // get a character
ccnt++; // count this character
buf[ cnt++ ] = (char)ch; // assign this character
// buf[ cnt ] = '\0'; // string always terminated
if( buf[ cnt-1 ] == '\n' ) // user supplied LF?
ccnt = 0; // reset the counter (for width)
else
if( ccnt == 10 ) { // reached max output width?
buf[ cnt++ ] = '\n'; // inject a LF
// buf[ cnt ] = '\0'; // string always terminated
ccnt = 0; // reset the counter (for width)
}
}
puts( buf ); // output the whole shebang
return 0;
}
0123456789abcdefghijklmnop
qrs
tuv
wxyz
^D // end of input
0123456789
abcdefghij
klmnop
qrs
tuv
wxyz
Like the OP code, this does not test for overrunning the buffer. An easy addition left as an exercise for the reader.
EDIT2:
Then again, why have a buffer to overrun?
#include <stdio.h>
void main( void ) {
for( int ch, n = 0; ( ch = getchar() ) != EOF; /**/ )
if( (n = putchar( ch ) == '\n' ? 0 : n + 1) == 10 )
putchar( '\n' ), n = 0;
}

Answer 2

This question is very broad and it would helpful if you said what the problem is but I can see one issue -- you don't null terminate the final_string variable. add
final_string[fnlstr_pos] = '\0';
before the printf.
Maybe that fixes the problem you are having.

Answer 3

For starters this statement
while(c=getchar() != EOF)
is equivalent to
while( c = ( getchar() != EOF ) )
So c is always equal to1 if EOF is not encountered.
You need to write
while( ( c=getchar() ) != EOF)
And you need to append the input sequence with the terminating zero character '\0' tp form a string.
Another problem is these code snippet with for loop
for(i = 0; i <= pos; ++i){
final_string[fnlstr_pos] = line[i];
++fnlstr_pos;
}
As within the loop the variable fnlstr_pos was increased then this if statement
if (final_string[fnlstr_pos] != '\n'){
final_string[++fnlstr_pos] = '\n';
}
invokes undefined behavior because the variable points to a non-initialized element of the array.

Answer 4

The main problem is here:
while(c=getchar() != EOF)
Given the operator precedence, this will result in the same as:
while(c= (getchar() != EOF))
So c will be 1 (true) inside the loop.
Change this to:
while((c=getchar()) != EOF)
This is the main problem. Just as #Hogan suggested, there are other issues such as not null terminating the strings.
As you declare them global, they will be zeroed so you can get away with that though not in the case the user provides a string with maximum length.
Also it would greatly improve the code if you could use string manipulation functions from <string.h> instead of copying byte by byte.