In c, you could use fopen, and getch. Usually, if you can't be exactly sure of the length of the longest line, you could allocate a large buffer (e.g. 8kb) and almost be guaranteed of getting all lines.
If there's a chance you may have really really long lines and you have to process line by line, you could malloc a resonable buffer, and use realloc to double it's size each time you get close to filling it.
#include <stdio.h>
#include <stdlib.h>
void handle_line(char *line) {
printf("%s", line);
}
int main(int argc, char *argv[]) {
int size = 1024, pos;
int c;
char *buffer = (char *)malloc(size);
FILE *f = fopen("myfile.txt", "r");
if(f) {
do { // read all lines in file
pos = 0;
do{ // read one line
c = fgetc(f);
if(c != EOF) buffer[pos++] = (char)c;
if(pos >= size - 1) { // increase buffer length - leave room for 0
size *=2;
buffer = (char*)realloc(buffer, size);
}
}while(c != EOF && c != '\n');
buffer[pos] = 0;
// line is now in buffer
handle_line(buffer);
} while(c != EOF);
fclose(f);
}
free(buffer);
return 0;
}