I would like to validate the user input. For example, if I enter a character in the place of a integer, then the program should terminate.

CodePudding user response：

The simplest way to read an integer from the user with basic input validation would be the following:

#include <stdio.h>
#include <stdlib.h>

int main( void )
{
    int num;

    printf( "Enter a number: " );

    switch ( scanf( "%d", &num ) )
    {
    case 1:
        //input is valid
        break;
    case 0:
        printf( "That was not a valid number!\n" );
        exit( EXIT_FAILURE );
    default:
        printf( "Unexpected input error occurred!\n" );        
        exit( EXIT_FAILURE );
    }

    printf( "You entered the following valid input: %d\n", num );
}

However, this solution is not perfect, as it will accept for example 6sdfh4q as valid input for the number 6, although that input should probably be rejected. This is because scanf is not designed for line-based user input.

For line-based user input, it is probably better to use fgets instead of scanf. In contrast to scanf, fgets will generally read one line at a time, so it is the perfect function for reading user input. After reading one line of user input, you can attempt to convert the input to a number using the function strtol or sscanf.

Performing full input validation is rather complex in C. This is how I would write it:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <errno.h>

int main( void )
{
    char buffer[1024], *p;
    long num;

    //prompt user for input
    printf( "Enter a number: " );

    //get one line of input from input stream
    if ( fgets( buffer, sizeof buffer, stdin ) == NULL )
    {
        fprintf( stderr, "Unrecoverable error reading from input!\n" );
        exit( EXIT_FAILURE );
    }

    //make sure that entire line was read in (i.e. that
    //the buffer was not too small)
    if ( strchr( buffer, '\n' ) == NULL )
    {
        printf( "Line input was too long!\n" );
        exit( EXIT_FAILURE );
    }

    //attempt to convert string to number
    errno = 0;
    num = strtol( buffer, &p, 10 );
    if ( p == buffer )
    {
        printf( "Error converting string to number\n" );
        exit( EXIT_FAILURE );
    }

    //make sure that no range error occurred
    if ( errno == ERANGE )
    {
        printf( "Range error!\n" );
        exit( EXIT_FAILURE );
    }

    //make sure that remainder of line contains only whitespace,
    //so that input such as "6sdfh4q" gets rejected
    for ( ; *p != '\0'; p   )
    {
        if ( !isspace( (unsigned char)*p ) )
        {
            printf( "Unexpected input encountered!\n" );
            exit( EXIT_FAILURE );
        }
    }

    //number was successfully converted, so we print it
    printf( "You entered the following valid number: %ld\n", num );
}

Example output of this program when entering a valid number:

Enter a number: 65
You entered the following valid number: 65

Example output of this program when entering invalid input:

Enter a number: 6sdfh4q
Unexpected input encountered!

CodePudding user response：

The normal way to terminate a program is to call ‘exit’ function.

exit(ERROR_CODE);