1.5. Input and Output

File Descriptors

File descriptors are normally small non-negative integers that the kernel uses to identify the files being accessed by a particular process. Whenever it opens an existing file or creates a new file, the kernel returns a file descriptor that we use when we want to read or write the file.

Standard Input, Standard Output, and Standard Error

By convention, all shells open three descriptors whenever a new program is run: standard input, standard output, and standard error. If nothing special is done, as in the simple command

   ls

then all three are connected to the terminal. Most shells provide a way to redirect any or all of these three descriptors to any file. For example,

   ls > file.list

executes the ls command with its standard output redirected to the file named file.list.

Unbuffered I/O

Unbuffered I/O is provided by the functions open, read, write, lseek, and close. These functions all work with file descriptors.

Example

If we're willing to read from the standard input and write to the standard output, then the program in Figure 1.4 copies any regular file on a UNIX system.

The <unistd.h> header, included by apue.h, and the two constants STDIN_FILENO and STDOUT_FILENO are part of the POSIX standard (about which we'll have a lot more to say in the next chapter). In this header are function prototypes for many of the UNIX system services, such as the read and write functions that we call.

The constants STDIN_FILENO and STDOUT_FILENO are defined in <unistd.h> and specify the file descriptors for standard input and standard output. These values are typically 0 and 1, respectively, but we'll use the new names for portability.

In Section 3.9, we'll examine the BUFFSIZE constant in detail, seeing how various values affect the efficiency of the program. Regardless of the value of this constant, however, this program still copies any regular file.

The read function returns the number of bytes that are read, and this value is used as the number of bytes to write. When the end of the input file is encountered, read returns 0 and the program stops. If a read error occurs, read returns -1. Most of the system functions return 1 when an error occurs.

If we compile the program into the standard name (a.out) and execute it as

   ./a.out > data

standard input is the terminal, standard output is redirected to the file data, and standard error is also the terminal. If this output file doesn't exist, the shell creates it by default. The program copies lines that we type to the standard output until we type the end-of-file character (usually Control-D).

If we run

   ./a.out < infile > outfile

then the file named infile will be copied to the file named outfile.

Figure 1.4. List all the files in a directory

#include "apue.h"

#define BUFFSIZE    4096

int
main(void)
{
    int     n;
    char    buf[BUFFSIZE];

    while ((n = read(STDIN_FILENO, buf, BUFFSIZE)) > 0)
        if (write(STDOUT_FILENO, buf, n) != n)
            err_sys("write error");
        if (n < 0)
            err_sys("read error");

        exit(0);
}

In Chapter 3, we describe the unbuffered I/O functions in more detail.

Standard I/O

The standard I/O functions provide a buffered interface to the unbuffered I/O functions. Using standard I/O prevents us from having to worry about choosing optimal buffer sizes, such as the BUFFSIZE constant in Figure 1.4. Another advantage of using the standard I/O functions is that they simplify dealing with lines of input (a common occurrence in UNIX applications). The fgets function, for example, reads an entire line. The read function, on the other hand, reads a specified number of bytes. As we shall see in Section 5.4, the standard I/O library provides functions that let us control the style of buffering used by the library.

The most common standard I/O function is printf. In programs that call printf, we'll always include <stdio.h>normally by including apue.has this header contains the function prototypes for all the standard I/O functions.

Example

The program in Figure 1.5, which we'll examine in more detail in Section 5.8, is like the previous program that called read and write. This program copies standard input to standard output and can copy any regular file.

The function getc reads one character at a time, and this character is written by putc. After the last byte of input has been read, getc returns the constant EOF (defined in <stdio.h>). The standard I/O constants stdin and stdout are also defined in the <stdio.h> header and refer to the standard input and standard output.

Figure 1.5. Copy standard input to standard output, using standard I/O

#include "apue.h"

int
main(void)
{
    int     c;

    while ((c = getc(stdin)) != EOF)
        if (putc(c, stdout) == EOF)
            err_sys("output error");

    if (ferror(stdin))
        err_sys("input error");

    exit(0);
}