Programming Tutorials

Standard Input and Output in C

By: Ivan Lim in C Tutorials on 2007-09-26  

The C library implements a simple model of text input and output. A text stream consists of a sequence of lines; each line ends with a newline character. If the system doesn't operate that way, the library does whatever necessary to make it appear as if it does. For instance, the library might convert carriage return and linefeed to newline on input and back again on output.

The simplest input mechanism is to read one character at a time from the standard input, normally the keyboard, with getchar:

   int getchar(void)

getchar returns the next input character each time it is called, or EOF when it encounters end of file. The symbolic constant EOF is defined in <stdio.h>. The value is typically -1, bus tests should be written in terms of EOF so as to be independent of the specific value.

In many environments, a file may be substituted for the keyboard by using the < convention for input redirection: if a program prog uses getchar, then the command line

   prog <infile

causes prog to read characters from infile instead. The switching of the input is done in such a way that prog itself is oblivious to the change; in particular, the string ``<infile'' is not included in the command-line arguments in argv. Input switching is also invisible if the input comes from another program via a pipe mechanism: on some systems, the command line

   otherprog | prog

runs the two programs otherprog and prog, and pipes the standard output of otherprog into the standard input for prog.

The function

   int putchar(int)

is used for output: putchar(c) puts the character c on the standard output, which is by default the screen. putchar returns the character written, or EOF is an error occurs. Again, output can usually be directed to a file with >filename: if prog uses putchar,

   prog >outfile

will write the standard output to outfile instead. If pipes are supported,

   prog | anotherprog

puts the standard output of prog into the standard input of anotherprog.

Output produced by printf also finds its way to the standard output. Calls to putchar and printf may be interleaved - output happens in the order in which the calls are made.

Each source file that refers to an input/output library function must contain the line

   #include <stdio.h>

before the first reference. When the name is bracketed by < and > a search is made for the header in a standard set of places (for example, on UNIX systems, typically in the directory /usr/include).

Many programs read only one input stream and write only one output stream; for such programs, input and output with getchar, putchar, and printf may be entirely adequate, and is certainly enough to get started. This is particularly true if redirection is used to connect the output of one program to the input of the next. For example, consider the program lower, which converts its input to lower case:

   #include <stdio.h>
   #include <ctype.h>

   main() /* lower: convert input to lower case*/
   {
       int c

       while ((c = getchar()) != EOF)
           putchar(tolower(c));
       return 0;
   }

The function tolower is defined in <ctype.h>; it converts an upper case letter to lower case, and returns other characters untouched. As we mentioned earlier, ``functions'' like getchar and putchar in <stdio.h> and tolower in <ctype.h> are often macros, thus avoiding the overhead of a function call per character. Regardless of how the <ctype.h> functions are implemented on a given machine, programs that use them are shielded from knowledge of the character set.