In C programming, the main difference between low-level I/O functions (open/close/read/write) and stream-level I/O functions (fopen/fclose/fread/fwrite) is that stream-level functions are buffered. Presumably, low-level I/O functions will incur a disk operation on each read(). Although the kernel may cache this, we cannot rely too much on it. Disk operations are expensive and so low-level I/O does not provide fgetc equivalent.
Stream-level I/O functions have a buffer. On reading, they load a block of data from disk to memory. If at a fgetc() call the data have been retrieved to memory, it will not incur a disk operation, which greatly improves the efficiency.
Stream-level I/O functions are part of the standard C library. Why do we need a new wrapper? Three reasons. First, when you work with an alternative I/O library (such as zlib or libbzip2) which do not come with buffered I/O routines, you probably need a buffered wrapper to make your code efficient. Second, using a generic wrapper makes your code more flexible when you want to change the type of input stream. For example, you may want to write a parser that works on a normal stream, a zlib-compressed stream and on a C string. Using a unified stream wrapper will simplify coding. Third, my feeling is most of steam-level I/O functions in stdio.h are not conventient given that they cannot enlarge a string automatically. In a lot of cases, I need to read one line but I do not know how long a line can be. Managing this case is not so hard, but doing this again and again is boring.
In the end, I come up with my own buffered wrapper for input streams. It is generic in that it works on all types of I/O steams with a read() call (or equivalent), or even on a C string. I show an example here without much explanation. I may expand this post in future. Source codes can be found in my programs page.
#include <fcntl.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include "kstream.h"
// arguments: type of the stream handler,
// function to read a block, size of the buffer
KSTREAM_INIT(int, read, 10)
int main()
{
int fd;
kstream_t *ks;
kstring_t str;
bzero(&str, sizeof(kstring_t));
fd = open("kstream.h", O_RDONLY);
ks = ks_init(fd);
while (ks_getuntil(ks, '\n', &str, 0) >= 0)
printf("%s\n", str.s);
ks_destroy(ks);
free(str.s);
close(fd);
return 0;
}
