1. Line at a time I/O with fgets() and fputs()
The fgets(buffer_address, buf_size, file) function can be used to read from a stream until either:
a newline character '\n' = 10 = 0x0a is encountered, or
the specified number of characters - 1 has been read, or
end of file is reached.
fgets() will append the NULL character to whatever it reads in.

2. Line at a time input and output
Since fgets() will read in multiple characters it is not possible to assign what it reads to a single variable of type char. Thus a pointer to a buffer must be passed (as was the case with scanf()).
The fputs() function writes a NULL terminated string to a stream (after stripping off the NULL). The following example is yet another cat program, but this one works one line at a time.

3. Line at a time input and output
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main() {
unsigned char *buff = NULL;
int line = 0;
buff = malloc(1024); // alloc storage to hold data
if (buff == 0)
exit(1);
while (fgets(buff, 1024, stdin) != 0)
fputs(buff, stdout);
fprintf(stderr, "%d %d \n", line, strlen(buff));
line += 1; }
free(buff); // release the storage malloc allocated

4. Line at a time input and output
Here buff is declared a pointer and the storage which will hold the data is allocated with malloc()
Alternatively we could have declared unsigned char buff[1024] and not used malloc()

5. Line at a time input and output
Here is the output that went to stderr. Note that each line that appeared empty has length 1 (the newline character itself) and the lines that appear to have length 1 actually have length 2.
0 12
1 1
2 19
3 20
4 1
5 7
6 2
7 24
8 18
9 1
10 24
11 18
12 15
13 1
14 41
15 5
16 27
17 55
18 17
19 4
20 2