I have been reading APUE recently when a basic problem turned up. My code is shown below
#include <apue.h>
#define BUFFSIZE 4096
int main()
{
int n;
char buf[BUFFSIZE];
while((n = read(STDIN_FILENO, buf, BUFFSIZE)) > 0)
{
printf("n is %d\n", n); //this line is added by me for testing
if(write(STDOUT_FILENO, buf, n) != n)
err_sys("write error"); //functions defined by the book to print error message
}
if(n < 0)
err_sys("read error");
exit(0);
}
After compiling, when I run the program as shown below
> $ ./mycat
123456[enter]
n is 7
123456
1234[enter]
n is 5
1234
It seems that it works according to the structure of my code. And I don't quite understand the function of the 'enter'. Every time I pressed 'enter', read function terminates and pass the characters including the '\n' produced by the 'enter' to the write function. So it goes inside the loop and firstly prints the number of characters read.
However, the following testing seems go against the above and against the structure of the code.
> $ ./mycat > data
123456[enter]
1234[enter]
^D
> $ cat data
123456
1234
n is 7
n is 5
It seems like that the program firstly writes all the characters to the file then prints the value of 'n', however according to my understanding, it should firstly prints as follows
n is 7
123456
n is 5
1234
I think again and again and just couldn't figure it out. Could you please help me?
There are several kinds of buffering going on. The input to the program is buffered by the pseudoterminal device's Line-buffering discipline. On the output side, there is a file-system cache (a buffer in the OS for the whole file), and extra buffering in the C program when printing to a FILE * type. But read and write bypass the FILE * buffering and move data more or less directly to/from the file-system cache.
So it appears that your stdout buffer is being flushed automatically when all output is going to the terminal, but not when redirected to a file. So I'd recommend adding a call to
fflush(stdout);
after the printf call. This should explicitly flush the buffer (and enforce the ordering of the output that you want).
The important thing to be aware of is when you're using FILE *s which are a C-level structure manipulated by library functions (like fopen), and when you're using the raw file descriptor (which is just an integer, but refers to the underlying operating-system file). The FILE datatype is a wrapper around this lower level Unix implementation detail. The FILE functions implement an additional layer of buffering so the lower level can operate on larger blocks, and you can efficiently perform byte-at-a-type processing without doing lots and lots I/O handshakes.