fgets() question

A friend of mine is trying to write this simple C program and he's having a problem with the fgets() function. He's also brazilian but he doesn't speak english, that's why I'm asking for him. Take a look at his code, please:

Code:

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

void flush_newline(char *);
int process_choice(char *);
int valid_menu_choice(char *);

int main()
{
   char line[5];

   do{
      printf("[1] Cadastrar cliente\n");
      printf("[2] Visualizar ficha\n");
      printf("[3] Sair\n\n");

      printf("> ");
      fgets(line, sizeof line, stdin);
      flush_newline(line);
   }while(process_choice(line) != 3);

   return 0;
}




/*____________________________________________________________________
| flush_newline(): retira o \n do final da string se o mesmo existir. |
|                                                     |
| Parâmetros: a string a ser testada.                          |
|_____________________________________________________________________|*/
void flush_newline(char *line)
{
   char *pointer;

   if((pointer = strchr(line, '\n')) != NULL)
      *pointer = '\0';
}





/*______________________________________________________________________________________________
| process_choice(): Valida e, se tudo estiver certo, processa a opção escolhida pelo usuário    |
|                   no menu principal.                                                          |
|                                                                                               |
| Parâmetros:                                                                                   |
|     line: texto digitado pelo usuário no prompt do menu principal.                            |
|                                                                                               |
| Retorna:                                                                                      |
|     Se o input do usuário não for uma opção válida, a função retorna o número 0. Dessa forma, |
|     o menu e o prompt vão ser mostrados outra vez na tela por causa do loop na função main(). |
|_______________________________________________________________________________________________|*/
int process_choice(char *line)
{
   int menu_choice;
   
   if((menu_choice = valid_menu_choice(line)) == 0)
      return 0;

   if(menu_choice == 3)
      exit(1);
}




/*_________________________________________________________________________________________________
| valid_menu_choice(): Valida o input do usuário de acordo com as opções do menu principal.         |
|                                                                           |
| Parâmetros:                                                                  |
|     line: texto digitado pelo usuário no prompt do menu principal.                        |
|                                                                           |
| Retorna:                                                                     |
|     Se o input do usuário não for válido, a função retorna o número 0, o que vai causar a função |
|     process_choice() retornar 0 também. O menu e o prompt vão ser mostrados outra vez na tela.   |
|                                                                           |
|     Se o input for válido, ele será convertido para int e retornado.                        |
|__________________________________________________________________________________________________|*/
int valid_menu_choice(char *line)
{
   size_t i;
   int menu_choice;

   if(line[0] == '\0'){
      printf("Comando desconhecido.\n\n");
      return 0;
   }

   for(i = 0; i < strlen(line); i++){
      if(!isdigit(line[i])){
         printf("Comando desconhecido.\n\n");
         return 0;
      }
   }

   sscanf(line, "%d", &menu_choice);

   return menu_choice;
}

At the prompt, if you type something with more than four characters, look what happens:

Code:

[1] Cadastrar cliente
[2] Visualizar ficha
[3] Sair

> qqqqqqqqqqqqqqq
Comando desconhecido.

[1] Cadastrar cliente
[2] Visualizar ficha
[3] Sair

> Comando desconhecido.

[1] Cadastrar cliente
[2] Visualizar ficha
[3] Sair

> Comando desconhecido.

[1] Cadastrar cliente
[2] Visualizar ficha
[3] Sair

> Comando desconhecido.

[1] Cadastrar cliente
[2] Visualizar ficha
[3] Sair

> Comando desconhecido.

[1] Cadastrar cliente
[2] Visualizar ficha
[3] Sair

> Comando desconhecido.

[1] Cadastrar cliente
[2] Visualizar ficha
[3] Sair

>

Wasn't fgets() supposed to take care of that? I couldn't help him but I hope one of you can. Thanks in advance.

fgets() will remove num - 1 characters from the input stream and put them into the buffer. The problem is that if you type more then that number of characters then the rest will be left in the buffer and will immediatly be picked up by the next call to fgets(). What needs to be done is to remove the excess characters from the input stream if they exceeded the max size of your buffer. Something like:

Code:

void flush_newline(char *line)
{
   char *pointer;

   if((pointer = strchr(line, '\n')) != NULL)
      *pointer = '\0';
   else
     /* Flush the input buffer */
}

Looking at it everything looks like its working fine, I didn't try to run it though. But this code:

Code:

for(i = 0; i < strlen(line); i++){
      if(!isdigit(line[i])){
         printf("Comando desconhecido.\n\n");
         return 0;
      }
   }

Checks the input to see if the first character is a digit, since its not it (you typed q's i nthe example) it prints that message and returns.

fgets() then grabs the remainig characters in the buffer (the q's past the first 5) and moves on. You need t oclear out those extra characters in the buffer.

I think you can do that by calling fflush() right before the fgets() call. But I've been reading a lot of bad stuff about fflush() and its uses so I dunno if thats the best route to take.

Ok, I changed his code to this:

Code:

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

void flush_newline(char *);
int process_choice(char *);
int valid_menu_choice(char *);

int main()
{
    char line[5];

    do{
        printf("[1] Cadastrar cliente\n");
        printf("[2] Visualizar ficha\n");
        printf("[3] Sair\n\n");

        printf("> ");
        fgets(line, (sizeof line) - 1, stdin);
        flush_newline(line);
    }while(process_choice(line) != 3);

    return 0;
}




/*______________________________________________________________________________________
| flush_newline(): retira o \n do final da string se o mesmo existir ou limpa o buffer. |
|                                                                                        |
| Parâmetros: a string a ser testada.                                                    |
|_______________________________________________________________________________________|*/
void flush_newline(char *line)
{
    char ch;
    char *pointer;

    if((pointer = strchr(line, '\n')) != NULL)
        *pointer = '\0';

    else{
        while((ch = getchar()) != '\n')
            ch = getchar();
    }
}





/*______________________________________________________________________________________________
| process_choice(): Valida e, se tudo estiver certo, processa a opção escolhida pelo usuário    |
|                   no menu principal.                                                          |
|                                                                                               |
| Parâmetros:                                                                                   |
|     line: texto digitado pelo usuário no prompt do menu principal.                            |
|                                                                                               |
| Retorna:                                                                                      |
|     Se o input do usuário não for uma opção válida, a função retorna o número 0. Dessa forma, |
|     o menu e o prompt vão ser mostrados outra vez na tela por causa do loop na função main(). |
|                                                                                                |
|      Se o input for válido e igual ao número 3, o número 3 é retornado e o programa termina.   |
|_______________________________________________________________________________________________|*/
int process_choice(char *line)
{
    int menu_choice;
    
    if((menu_choice = valid_menu_choice(line)) == 0)
        return 0;

    else if(menu_choice == 1){
        printf("Cadastrar cliente.\n\n");
        return 0;
    }

    else if(menu_choice == 2){
        printf("Visualizar ficha.\n\n");
        return 0;
    }

    else
        return 3;
}




/*_________________________________________________________________________________________________
| valid_menu_choice(): Valida o input do usuário de acordo com as opções do menu principal.           |
|                                                                                                   |
| Parâmetros:                                                                                       |
|     line: texto digitado pelo usuário no prompt do menu principal.                               |
|                                                                                                   |
| Retorna:                                                                                           |
|     Se o input do usuário não for válido, a função retorna o número 0, o que vai causar a função |
|     process_choice() retornar 0 também. O menu e o prompt vão ser mostrados outra vez na tela.   |
|                                                                                                   |
|     Se o input for válido, ele será convertido para int e retornado.                               |
|__________________________________________________________________________________________________|*/
int valid_menu_choice(char *line)
{
    size_t i;
    int menu_choice;

    if(line[0] == '\0'){
        printf("Comando desconhecido.\n\n");
        return 0;
    }

    for(i = 0; i < strlen(line); i++){
        if(!isdigit(line[i])){
            printf("Comando desconhecido.\n\n");
            return 0;
        }
    }

    sscanf(line, "%d", &menu_choice);

    if((menu_choice <= 0) || (menu_choice > 3)){
        printf("Comando desconhecido.\n\n");
        return 0;
    }

    return menu_choice;
}

It worked, but now there's just a little detail. Look at this:

Code:

[1] Cadastrar cliente
[2] Visualizar ficha
[3] Sair

> 111111111111111
Comando desconhecido.

[1] Cadastrar cliente
[2] Visualizar ficha
[3] Sair

> 111111111111111111111111

Comando desconhecido.

[1] Cadastrar cliente
[2] Visualizar ficha
[3] Sair

>

If I start the program, type a bunch of ones and press enter, the error message will be displayed ("Comando desconhecido"). If I do the same thing on the second prompt, I have to hit enter twice in order to get the error message, and it stays like that until I input something good. Why is it happening? Sorry about the silly questions

Nevermind. We got it...

Code:

void flush_newline(char *line)
{
	char *pointer;

	if((pointer = strchr(line, '\n')) != NULL)
		*pointer = '\0';

	else
		while(!feof(stdin) && getchar() != '\n');
}

remove the

Code:

 ch = getchar();

from the body of the loop.

>> I have to hit enter twice in order to get the error message, and it stays like that until I input something good. <<

Consider what is happening here:

Code:

        while((ch = getchar()) != '\n')
            ch = getchar();

Let's unroll it:

Code:

// Input left over in buffer is "a\n"

// First time in the loop...
ch = getchar(); // get 'a'
if (ch != '\n') getchar(); // get '\n'

// Second time in the loop...
ch = getchar(); // Whoops, we're trying to get an extra character!
if (ch != '\n') getchar(); // Not run if you pressed enter.

Code:

fgets(line, sizeof(line), stdin);   /* RIGHT */

fgets(line, sizeof line, stdin);    /* WRONG */
fgets(line, (sizeof line), stdin);  /* WRONG */

>>char ch;
>>ch = getchar()
getchar returns and int, not a char. Get it wrong, and you'll run into trouble at some point!

http://faq.cprogramming.com/cgi-bin/...&id=1043284351

Originally Posted by sand_man

Code:

fgets(line, sizeof(line), stdin);   /* RIGHT */

fgets(line, sizeof line, stdin);    /* WRONG */
fgets(line, (sizeof line), stdin);  /* WRONG */

() are only required with sizeof when you are refering to a type.
So

Code:

int x;
printf("%d\n", sizeof x);
printf("%d\n", sizeof(int);

are both correct but

Code:

printf("%d\n", sizeof int);

Would be incorrect.