C I/O

Program input

There are 3 main sources of input for programs:

from the command line
- you get access to data on the command line by using argc and argv[][]
from standard input (also called stdin)
- stdin can be the keyboard, a data file, or the output of another program
from an 'internally-defined' file
- open a file, use fscanf(), and don't forget to close the file

1. command line

To read from the command line:

include argc and argv in your parameter list for main().
use sscanf() to read the arguments (it stands for 'string scanf()')
- the first argument of a sscanf() is a string

Here is a program that counts from 1 to num, where num is provided by the user on the command line

   1 // countc.c
   2 // reads an integer from the command line and counts
   3 #include <stdio.h>
   4 #include <stdlib.h>
   5 
   6 int main(int argc, char *argv[]) {
   7    int num = 0;
   8    if (argc < 2 || sscanf(argv[1], "%d", &num) != 1) { // num is defined here
   9       fprintf(stderr,"Usage: %s integer\n", argv[0]);
  10       return EXIT_FAILURE;
  11    }
  12    for (int i=1; i<=num; i++) {
  13       printf("%d ", i);
  14    }
  15    printf("\n");
  16    return EXIT_SUCCESS;
  17 }

Notice the program prints a 'Usage' message if an integer argument is missing (discussed in next session)

To execute the program:

prompt$ dcc -o countc countc.c

prompt$ ./count
Usage: ./count integer

prompt$ ./count !t#q
Usage: ./count integer

prompt$ ./count 10
1 2 3 4 5 6 7 8 9 10

2. standard input

To read from standard input (usually called simply stdin)

a scanf() is used (instead of sscanf())
- a scanf() misses the string argument of a sscanf()
so where does num comes from?
- ... the default 'channel' stdin

   1 //counts.c
   2 // reads an integer from stdin and counts
   3 #include <stdio.h>
   4 #include <stdlib.h>
   5 
   6 int main(void) {
   7    int num;
   8    if (scanf("%d", &num) != 1) {
   9       fprintf(stderr, "Usage: a number expected\n");
  10       return EXIT_FAILURE;
  11    }
  12    // the rest of the program is exactly the same as the command-line version
  13    for (int i=1; i<=num; i++) {
  14       printf("%d ",i);
  15    }
  16    printf("\n");
  17    return EXIT_SUCCESS;
  18 }

Notice the Usage message this time is simpler than the command-line version above

... because we did not declare argc and argv, and so cannot use argv[0] this time!!
- we could have if we wanted to of course

There are many ways to 'test' a program that reads stdin.

Using the keyboard
```
 prompt$ dcc -o counts counts.c
 prompt$ ./counts
 10
 1 2 3 4 5 6 7 8 9 10
```
where the integer 10 was typed on the keyboard by the user, and the program generates the count from 1 to 10.

Using a data file, input.txt say, which contains the integer 10 (followed by a newline).

 prompt$ more input.txt
 10

 prompt$ ./counts < input.txt
 1 2 3 4 5 6 7 8 9 10

Using a pipe command. A pipe command joins the stdout of a program to the stdin of another program. If we have a program called write10.c:
```
   1  // write10.c
   2  // just print the string 10
   3  #include <stdio.h>
   4  #include <stdlib.h>
   5 
   6  int main(void) {
   7     printf("10\n");
   8     return EXIT_SUCCESS;
   9  }
```
then we can pipe its stdout to the stdin of our counting program
```
 prompt$ dcc -o write10 write10.c
 prompt$ dcc -o counts counts.c
 prompt$ ./write10 | ./counts
 1 2 3 4 5 6 7 8 9 10
```
But you can actually generate a string much more easily in UNIX using echo
```
 prompt$ echo "10" | ./counts
 1 2 3 4 5 6 7 8 9 10
```

3. A file

A program can open and close, and read from, and write to, a file that is 'internally' defined.

This is generally done when you have large volumes of stored data, or complex data or non-printable data.

these rarely happen in this course

Input/output design considerations

The vast majority of program can be written just using these i/o system calls

- scanf() to read from stdin - sscanf() to read from the command line - printf() to write to stdout - fprintf() to write to stderr

Program output and error messages

There are two standard output 'channels', standard output stdout and standard error stderr.

both are normally defined as the screen

printf() writes to stdout, hence the screen

fprintf(), which stands for file-printf, writes to a file, which could be stderr'

for example, in countc.c the 'Usage' message went to stderr:
1 fprintf(stderr,"Usage: %s integer\n", argv[0]);
- you could argue over whether it should go to stderr (hence be just a printf)
- stderr is normally reserved for serious errors

There is convention here:

standard input is scanf(),
- from a string is sscanf(), and it has an 'extra' first argument that is a string
standard output is printf(),
- to a file is fprintf(), and it has an 'extra' first argument that is a 'channel' (file)

Like stdin, we can re-direct stdout to a file For example:

dcc -o counts counts.c
./counts > output.txt
10

(where the integer 10 is input by the user) will result in the count from 1 to 10 going to the file output.txt

The following does the same thing:

./counts < input.txt > output.txt

This also does the same thing:
echo "10" | ./counts > output.txt

Lec01IO