Custom Scanf Routine

[Tarique]

I have tried to write my custom scanf function on the lines of minprintf
provided in K&R2.In the function myscanf() i access memory directly
using the address passed to the function .Can it be dangerous ?

I am getting the correct output though.Any help is appreciated.

/*Include Files*/

/*Assisting Functions*/
int flushln(FILE *f){ /*Code*/}
char *input(char *message){/*Code*/}
static int getInt(void){/*Code*/}
/*My Custom Scanf Routine*/

int myscanf(const char* format,...)
{
va_list ap;
const char *p;
int count = 0;
int temp;

va_start(ap,for mat);
for( p = format ; *p ;p++) {
if( *p != '%'){
continue;
}
switch(*++p){
case 'd':
if(temp = getInt()){
*(long *)va_arg(ap,int ) = temp;/*Direct Memory Access*/
count ++;
}
else{ puts("Input Error"); }
break;
}
}
va_end(ap);
return count;
}

int main(void)
{
int a = 0,b = 0;
myscanf("%d %d",&a,&b);
printf("%d %d",a,b);

return 0;
}

Thanks!

[viza]

Hi

On Fri, 04 Jul 2008 03:12:09 +0530, Tarique wrote:

I have tried to write my custom scanf function

*(long *)va_arg(ap,int ) = temp;

That is a very bad idea, it might work on certain systems where pointers
are the same size as integers, but that is often not the case.

Even if this does work once, the next operation on ap might fail.

va_arg needs to be told the type of the argument that was written in
place of the ... in when the function was called. You should always put
the correct type in the second argument. In most cases that means you
won't need to cast the return.

* va_arg(ap,long* ) = temp;

will work perfectly, as long as a long pointer was really written at the
current place in the argument list.

HTH
viza

[Tarique]

Tarique wrote:
....snip...

This is my code for a minimal custom scanf function(for entering valid
ints n longs with error checking)
I would be really grateful if someone can review it.Comments awaited!

Thank You

#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <limits.h>
#include <stdarg.h>

#define BUFFSIZE 98 + 2 /*Used by input()*/

/*Clear The Input stream if required*/
int flushln(FILE *f) {
int ch;
while (('\n' != (ch = getc( f ))) && (EOF != ch))
continue;
return ch;
}

/*Accept a string from user (parse later on)*/
char *input(const char* message,char *buff)
{
char buffer[ BUFFSIZE ];
char *p = &buffer[ BUFFSIZE-1 ];

if(message != "")
puts(message);

buffer[BUFFSIZE -1] = '$';

if( fgets(buffer,BU FFSIZE,stdin) == NULL) {
if(ferror(stdin )) {
perror("Input Stream Error");
clearerr( stdin );
return "I/O";
}
if(feof(stdin)) {
perror("EOF Encountered");
clearerr( stdin );
return "EOF";
}
}
else{
if(!((*p == '$') || ( (*p == '\0') && (*--p == '\n') ))) {
flushln(stdin);
puts("Too long input!\n\n");
return NULL;
}
}
strcpy(buff,buf fer);
return buff;
}

/*Accept a string and pass a valid long int if possible,else return 0*/
long getInt(void)
{
char intbuff[100];
char *buffptr = intbuff;
char *end_ptr;
long int lVal;
int trial = 1; /*No of tries on wrong input*/
int dist=0;
errno = 0;

while(trial-- != 0)
{
buffptr = input("Enter :",intbuff);
if (buffptr == "I/O" || buffptr == "EOF" || buffptr == NULL)
return 0;

lVal= strtol(buffptr, &end_ptr, 0);
if (ERANGE == errno){
perror("Out of Range");
}
else if (lVal INT_MAX){
perror("Too Large!");
}
else if (lVal < INT_MIN){
perror("Too Small");
}
else if (end_ptr == buffptr){
printf("Not a Valid Integer\n\n");
}
else
return lVal;
}
return 0;
}

int myscanf(const char* format,...)
{
va_list ap;
const char *p;
int count = 0;
int temp;

va_start(ap,for mat);
for( p = format ; *p ;p++) {
if( *p != '%'){
continue;
}
switch(*++p){
case 'd':
if(temp =(int)getInt()) {
*va_arg(ap,long *) = temp;/* Direct Memory Access to MEM */
count ++;
}
else break;
break;

case 'l':
if((*++p) == 'd'){
if(temp =getInt()){
*va_arg(ap,long *) = temp;/* Direct Memory Access to MEM */
count ++;
p--;
}
else {
*va_arg(ap,long *) = 0;/*Invalid input ,so set variable to zero*/
p--;
break;
}
}
break;
default:
break;
}
}
va_end(ap);
return count;
}

int main(void)
{
long int a = 0,b = 0;
int c = 0,d = 0;

myscanf("%ld %ld",&a,&b);
printf("%ld %ld \n\n",a,b);

myscanf("%d %d",&c,&d);
printf("%d %d",c,d);

return 0;
}

[santosh]

Tarique wrote:

Tarique wrote:
...snip...

This is my code for a minimal custom scanf function(for entering valid
ints n longs with error checking)
I would be really grateful if someone can review it.Comments awaited!

Thank You

Just a note. Haven't seen it in detail, sorry.

Also in future please use spaces in place of tabs when posting to
Usenet. As you can see below, some software on the Usenet stripped out
all your tabs, rendering unformatted code, which is very difficult to
read.

#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <limits.h>
#include <stdarg.h>

#define BUFFSIZE 98 + 2 /*Used by input()*/

/*Clear The Input stream if required*/
int flushln(FILE *f) {
int ch;
while (('\n' != (ch = getc( f ))) && (EOF != ch))
continue;
return ch;
}

/*Accept a string from user (parse later on)*/
char *input(const char* message,char *buff)
{
char buffer[ BUFFSIZE ];
char *p = &buffer[ BUFFSIZE-1 ];

if(message != "")
puts(message);

buffer[BUFFSIZE -1] = '$';

if( fgets(buffer,BU FFSIZE,stdin) == NULL) {
if(ferror(stdin )) {
perror("Input Stream Error");
clearerr( stdin );
return "I/O";
}
if(feof(stdin)) {
perror("EOF Encountered");
clearerr( stdin );
return "EOF";
}
}
else{
if(!((*p == '$') || ( (*p == '\0') && (*--p == '\n') ))) {
flushln(stdin);
puts("Too long input!\n\n");
return NULL;
}
}
strcpy(buff,buf fer);
return buff;
}

/*Accept a string and pass a valid long int if possible,else return
0*/ long getInt(void)
{
char intbuff[100];
char *buffptr = intbuff;
char *end_ptr;
long int lVal;
int trial = 1; /*No of tries on wrong input*/
int dist=0;
errno = 0;

while(trial-- != 0)
{
buffptr = input("Enter :",intbuff);
if (buffptr == "I/O" || buffptr == "EOF" || buffptr == NULL)
return 0;

lVal= strtol(buffptr, &end_ptr, 0);
if (ERANGE == errno){
perror("Out of Range");
}
else if (lVal INT_MAX){
perror("Too Large!");
}
else if (lVal < INT_MIN){
perror("Too Small");
}

What will you do on implementations where INT_MAX == LONG_MAX and
INT_MIN == LONG_MIN?

<snip rest>

[viza]

On Fri, 04 Jul 2008 18:10:51 +0530, Tarique wrote:

I would be really grateful if someone can review it.Comments awaited!

#define BUFFSIZE 98 + 2 /*Used by input()*/

Generally it's not smart to use arbitrary fixed length buffers. Are you
certain that the input won't be bigger? If your code is well written
then it that would cause it to fail, and if it is not well written then
it could FEYC (f***ing explode your computer).

/*Clear The Input stream if required*/ int flushln(FILE *f) {
int ch;
while (('\n' != (ch = getc( f ))) && (EOF != ch))
continue;
return ch;
}

Gah! ugly. It's best that you don't use assignments as truth values,
especially as a beginner. That continue is superfluous too. I would
also ungetc() the character and return void.

/*Accept a string from user (parse later on)*/ char *input(const char*
message,char *buff) {
char buffer[ BUFFSIZE ];
char *p = &buffer[ BUFFSIZE-1 ];

char *p= buffer + BUFSIZE - 1; might be easier to read.

if(message != "")

You can't do that in C. Well, you can, but it doesn't mean what you
think. Lookup the strcmp() function.

puts(message);
buffer[BUFFSIZE -1] = '$';
if( fgets(buffer,BU FFSIZE,stdin) == NULL) {
if(ferror(stdin )) {
perror("Input Stream Error");
clearerr( stdin );
return "I/O";

It's not usual to return strings like that one. In case of error perhaps
you might want to return NULL.

}
if(feof(stdin)) {
perror("EOF Encountered");
clearerr( stdin );
return "EOF";
}
}
else{
if(!((*p == '$') || ( (*p == '\0') && (*--p == '\n') )))

{

flushln(stdin);
puts("Too long input!\n\n");
return NULL;
}
}
strcpy(buff,buf fer);

If you know that buff is at least as big as buffer, why do you need
buffer in the first place? Read to buff.

return buff;
}

/*Accept a string and pass a valid long int if possible,else return 0*/
long getInt(void)
{
char intbuff[100];
char *buffptr = intbuff;
char *end_ptr;
long int lVal;
int trial = 1; /*No of tries on wrong input*/ int dist=0;
errno = 0;

while(trial-- != 0)
{
buffptr = input("Enter :",intbuff);
if (buffptr == "I/O" || buffptr == "EOF" || buffptr == NULL)
return 0;

lVal= strtol(buffptr, &end_ptr, 0);
if (ERANGE == errno){

You didn't set errno to zero. Well you did, but it might have changed
many times since then. Do it immediately before each conversion.

I haven't gome through it all, but HTH
viza

[Tarique]

viza wrote:
...snip...

>
I haven't gome through it all, but HTH
viza

@Santosh and Viza

Thanks for the quick review.Ive pasted the code here :
http://phpfi.com/329176
Hope it is easier to read.

[Barry Schwarz]

On Fri, 04 Jul 2008 18:10:51 +0530, Tarique <pe*****@yahoo. comwrote:

>This is my code for a minimal custom scanf function(for entering valid
ints n longs with error checking)
I would be really grateful if someone can review it.Comments awaited!

Thank You

#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <limits.h>
#include <stdarg.h>

#define BUFFSIZE 98 + 2 /*Used by input()*/

/*Clear The Input stream if required*/
int flushln(FILE *f) {
int ch;
while (('\n' != (ch = getc( f ))) && (EOF != ch))
continue;
return ch;
}

/*Accept a string from user (parse later on)*/
char *input(const char* message,char *buff)
{
char buffer[ BUFFSIZE ];
char *p = &buffer[ BUFFSIZE-1 ];

if(message != "")
puts(message);

buffer[BUFFSIZE -1] = '$';

if( fgets(buffer,BU FFSIZE,stdin) == NULL) {
if(ferror(stdin )) {
perror("Input Stream Error");
clearerr( stdin );
return "I/O";
}
if(feof(stdin)) {
perror("EOF Encountered");
clearerr( stdin );
return "EOF";
}
}
else{
if(!((*p == '$') || ( (*p == '\0') && (*--p == '\n') ))) {

Please don't use tabs, especially within a line.

This test is backwards. If *p is '$', then a short line was entered
and there is nothing to flush. If the second half is true, exactly
BUFFSIZE-1 characters were entered (including the Enter key) and there
is still nothing to flush.

An easier test would be to set buffer[BUFFSIZE-2] to '\n'. If it is a
different non-zero value after the fgets, then too much data was
entered.

> flushln(stdin);
puts("Too long input!\n\n");
return NULL;
}
}
strcpy(buff,buf fer);
return buff;
}

/*Accept a string and pass a valid long int if possible,else return 0*/
long getInt(void)
{
char intbuff[100];
char *buffptr = intbuff;
char *end_ptr;
long int lVal;
int trial = 1; /*No of tries on wrong input*/
int dist=0;
errno = 0;

while(trial-- != 0)
{
buffptr = input("Enter :",intbuff);
if (buffptr == "I/O" || buffptr == "EOF" || buffptr == NULL)
return 0;

lVal= strtol(buffptr, &end_ptr, 0);

You need to indent consistently. This statement is not part of the
range of the preceding if. It should be indented to the same level as
that if.

> if (ERANGE == errno){

You should reset errno before calling strtol so that you don't process
residual data. strtol is not allowed to reset errno so you must do
it.

> perror("Out of Range");
}
else if (lVal INT_MAX){
perror("Too Large!");
}
else if (lVal < INT_MIN){
perror("Too Small");
}
else if (end_ptr == buffptr){
printf("Not a Valid Integer\n\n");

This is insufficient. Input of the form "123abc" will appear valid.
To be sure that strtol processed all the characters, make sure end_ptr
points to either '\n' or '\0'.

> }
else
return lVal;

How do you distinguish between input of 0 and one of the "error"
conditions such as "I/O"?

> }
return 0;
}

int myscanf(const char* format,...)
{
va_list ap;
const char *p;
int count = 0;
int temp;

va_start(ap,for mat);
for( p = format ; *p ;p++) {
if( *p != '%'){
continue;
}
switch(*++p){

These two statements are within the range of the for but you indenting
makes them look independent.

> case 'd':
if(temp =(int)getInt()) {

The cast is not needed.

Why are you using %d to describe a long and confuse everyone.

Why do you not accept 0 as valid input?

> *va_arg(ap,long *) = temp;/* Direct Memory Access to MEM */
count ++;
}
else break;

This is superfluous. If you remove it, the code will behave the same.

> break;

case 'l':
if((*++p) == 'd'){
if(temp =getInt()){
*va_arg(ap,long *) = temp;/* Direct Memory Access to MEM */
count ++;
p--;
}
else {
*va_arg(ap,long *) = 0;/*Invalid input ,so set variable to zero*/

I still don't understand why 0 is illegal but it sure seems like a
deliberate decision.

> p--;
break;
}
}
break;
default:
break;
}

If the value processed for %d is 0, you do not store any value for the
corresponding argument AND you do not pop the argument off the list.

If the value processed for %ld is 0, you store 0 and pop the argument
but don't increment count.

There could be an inconsistency between the value of count returned
and which arguments contain valid data.

> }
va_end(ap);
return count;
}

int main(void)
{
long int a = 0,b = 0;
int c = 0,d = 0;

myscanf("%ld %ld",&a,&b);
printf("%ld %ld \n\n",a,b);

myscanf("%d %d",&c,&d);

Here you invoke undefined behavior. myscanf will treat &c and &d as
long int* when they aren't. I think you meant for myscanf to extract
an int*, not a long*, for the %d case.

> printf("%d %d",c,d);

return 0;
}

Remove del for email

[Barry Schwarz]

On Fri, 04 Jul 2008 14:30:08 -0700, Barry Schwarz <sc******@dqel. com>
wrote:

>On Fri, 04 Jul 2008 18:10:51 +0530, Tarique <pe*****@yahoo. comwrote:

>>This is my code for a minimal custom scanf function(for entering valid
ints n longs with error checking)
I would be really grateful if someone can review it.Comments awaited!

Thank You

#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <limits.h>
#include <stdarg.h>

#define BUFFSIZE 98 + 2 /*Used by input()*/

/*Clear The Input stream if required*/
int flushln(FILE *f) {
int ch;
while (('\n' != (ch = getc( f ))) && (EOF != ch))
continue;
return ch;
}

/*Accept a string from user (parse later on)*/
char *input(const char* message,char *buff)
{
char buffer[ BUFFSIZE ];
char *p = &buffer[ BUFFSIZE-1 ];

if(message != "")
puts(message);

buffer[BUFFSIZE -1] = '$';

if( fgets(buffer,BU FFSIZE,stdin) == NULL) {
if(ferror(stdin )) {
perror("Input Stream Error");
clearerr( stdin );
return "I/O";
}
if(feof(stdin)) {
perror("EOF Encountered");
clearerr( stdin );
return "EOF";
}
}
else{
if(!((*p == '$') || ( (*p == '\0') && (*--p == '\n') ))) {

Please don't use tabs, especially within a line.

This test is backwards.

Obviously I missed the !.
Remove del for email

[CBFalconer]

viza wrote:

Tarique wrote:

.... snip ...

>

>/*Clear The Input stream if required*/ int flushln(FILE *f) {
int ch;
while (('\n' != (ch = getc( f ))) && (EOF != ch))
continue;
return ch;
}

Gah! ugly. It's best that you don't use assignments as truth
values, especially as a beginner. That continue is superfluous
too. I would also ungetc() the character and return void.

Not in the least ugly. Apart from the awkward location of the
initial comment. Works like a charm, too. The continue prevents
misreading an isolated semi. Your ungetc recommendation would make
the routine fail to perform the function indicated by the name.

--
[mail]: Chuck F (cbfalconer at maineline dot net)
[page]: <http://cbfalconer.home .att.net>
Try the download section.

[viza]

On Fri, 04 Jul 2008 19:48:08 -0400, CBFalconer wrote:

viza wrote:

>Tarique wrote:

>>/*Clear The Input stream if required*/ int flushln(FILE *f) {
int ch;
while (('\n' != (ch = getc( f ))) && (EOF != ch))
continue;
return ch;
}

Gah! ugly. It's best that you don't use assignments as truth values,
especially as a beginner. That continue is superfluous too. I would
also ungetc() the character and return void.

Not in the least ugly.

I wouldn't take it home.

Apart from the awkward location of the initial comment. Works like a
charm, too. The continue prevents misreading an isolated semi.

Your ungetc recommendation would make the routine fail to perform the
function indicated by the name.

True. I misread the ugly loop construct and thought it read one-past the
newline.