converting from string to int using strtol() : \0' or '\n'? - c

Question

I have a doubt with conversion from string to int.
I got a string through function fgets then I used strtol function to convert it to int.
This is the code :
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <limits.h>
int main(void)
{
char buf[BUFSIZ];
char *p = NULL;
long int val;
int numero;
int temp;
do
{
temp=0;
printf ("Enter a number: ");
if (fgets(buf, sizeof(buf), stdin) != NULL)
{
val = strtol(buf, &p, 10);
if(buf==p)
{
printf(" no digits \n");
temp=1;
}
if ((errno == ERANGE && (val == LONG_MAX || val == LONG_MIN)) || (errno != 0 && val == 0))
{
perror("strtol");
temp=1;
}
if (*p != '\0')
{
printf("you have insert any char character \n");
temp=1;
}
}
else
{
printf("Error\n");
temp=1;
}
}
while(temp == 1);
/* If we got here, strtol() successfully parsed a number */
numero=(int)val;
printf("***** The number is : %d ******* \n",numero);
return 0;
}
and this code doen't work, but it work if I replace this control
if (*p != '\0')
{
printf("you have insert any char character \n");
temp=1;
}
with this one :
if (*p != '\n')
{
printf("you have insert any char character \n");
temp=1;
}
Do you know why ? :)
EDIT :
This is the code of my final function version :) Thanks to all :) Now it seem that all correctly works :
int readIN(int *numero)
{
long int val;
char buf[BUFSIZ];
char *p = NULL;
if (fgets(buf, sizeof(buf), stdin) != NULL)
{
val = strtol(buf, &p, 10);
if(buf==p)
{
return 1;
}
if ( (val > INT_MAX || val < 0) || (errno != 0 && val == 0))
{
return 1;
}
if (*p != '\n' && *p != '\r' && *p != '\0')
{
return 1;
}
}
else
{
return 1;
}
*numero=(int)val;
return 0;
}

Answer 1

Your code works perfectly. It has correctly identified that there are trailing characters after the number. As Matt says, gets returns a whole line, including the trailing line feed character (aside: why do people insist on posting the actual answer as a comment?).
Given that your string will almost always have this line feed, your simple fix is probably the right thing to do.
There are two exceptions though:
end of file
operating systems that use carriage returns at the end of line.
A more thorough solution would be this:
if (*p != '\n' && *p != '\r' && *p != '\0')
{
printf("error: unexpected trailing characters in input\n");
temp=1;
}