[SOLVED] how to set flags for non valid arguments getopt?

astrogeek · 08-04-2017, 10:40 PM

Quote:

Originally Posted by BW-userx

unsigned char and just char what I read is the same thing I googled it.

Code:

there is no difference. Actually on many platforms unqualified char is signed.
 The same way -- e.g. if you have an 8-bit char, 7 bits can be used for magnitude
  and 1 for sign. So an unsigned char might range from 0 to 255,
  whilst a signed char might range from -128 to 127 (for example)

neither one is a digit so isdigit should return a non zero

But you missed the point, or I did not make it clear.

First, isdigit() does not check the type (i.e. signed, unsigned, int or char).

Code:

DESCRIPTION
       These functions check whether c, which must have the value of an unsigned char or EOF,
       falls into a certain character class according to the current locale.

It checks whether the encoded character represents a digit in the locale, usually ascii for our purposes.

My point was not signed vs unsigned char or int, it was char or int, vs pointer to char.

argv[optind] is always a pointer to a character array, so with...

Code:

isdigit(argv[optind])

...you are passing a pointer, not a char or an int. char is defined as the smallest representation of a character on any system, and pointer is always the largest integer type. Hence the source of memory corruption and segfault.

This may also be obscured a bit if you are not including ctype.h. Then isdigit is probably inlined using a gcc macro so it results in no errors. Using -Wall as previously mentioned may show a warning at compile time.

Next, when you call it this way, you are passing it an integer, that is a number, not a digit encoded as a character.

Code:

isdigit(atoi(argv[optind]))

So it does not segfault, but it is no longer a valid test of the character encoding and your test fails.

Dereferencing argv[optind] passed the character (i.e. ascii value) and tests whether the first character of the argument is an encoded digit...

Code:

isdigit(*argv[optind])

Note that it does not test the numeric value of multi-character arguments, only the first character of the argument string. So if the argument is "32", this only tests the "3". If the argument is "3X" it passes the test too!

BW-userx · 08-05-2017, 08:55 AM

Quote:

Originally Posted by astrogeek

But you missed the point, or I did not make it clear.

First, isdigit() does not check the type (i.e. signed, unsigned, int or char).

Code:

DESCRIPTION
       These functions check whether c, which must have the value of an unsigned char or EOF,
       falls into a certain character class according to the current locale.

It checks whether the encoded character represents a digit in the locale, usually ascii for our purposes.

My point was not signed vs unsigned char or int, it was char or int, vs pointer to char.

argv[optind] is always a pointer to a character array, so with...

Code:

isdigit(argv[optind])

...you are passing a pointer, not a char or an int. char is defined as the smallest representation of a character on any system, and pointer is always the largest integer type. Hence the source of memory corruption and segfault.

This may also be obscured a bit if you are not including ctype.h. Then isdigit is probably inlined using a gcc macro so it results in no errors. Using -Wall as previously mentioned may show a warning at compile time.

Next, when you call it this way, you are passing it an integer, that is a number, not a digit encoded as a character.

Code:

isdigit(atoi(argv[optind]))

So it does not segfault, but it is no longer a valid test of the character encoding and your test fails.

Dereferencing argv[optind] passed the character (i.e. ascii value) and tests whether the first character of the argument is an encoded digit...

Code:

isdigit(*argv[optind])

Note that it does not test the numeric value of multi-character arguments, only the first character of the argument string. So if the argument is "32", this only tests the "3". If the argument is "3X" it passes the test too!

I went back and looked in to isdigit then put I to the test myself as just the name in itself is misleading.

when I read on it I was only looking for return value making sure true was zero so I could use the return value as a means for error checking, handling not input has to be an int to begin with, which pretty much makes it useless.

so even using isdigit( atoi(char) ) is valid because it is now giving isdigit what it needs for input with is an int. But still will not tell if it is a a - z or 1 - 0 ..

needless to say I as just running when I should have been walking is all.

pan64 · 08-07-2017, 10:01 AM

for checking how isdigit really works you can use ddd (The Data Display Debugger). Just compile your code with -g and without -O and execute your code inside the debugger.

Code:

// splittting this: isdigit(atoi(argv[optind])) into:
a = argv[optind]
b = atoi(a)
c = isdigit(b)

will make easier to understand what is passed to which function and what is the result.
And of course if you don't want to debug you can printf all these variables to stdout (just need to take care about the real types).

BW-userx · 08-07-2017, 12:44 PM

Quote:

Originally Posted by pan64

for checking how isdigit really works you can use ddd (The Data Display Debugger). Just compile your code with -g and without -O and execute your code inside the debugger.

Code:

// splittting this: isdigit(atoi(argv[optind])) into:
a = argv[optind]
b = atoi(a)
c = isdigit(b)

will make easier to understand what is passed to which function and what is the result.
And of course if you don't want to debug you can printf all these variables to stdout (just need to take care about the real types).

What??? GDB has a gui now -> The Data Display Debugger.

that will be interesting to check in to. Because yeah I've been using printf everywhere to see if the flow of data is going where it needs to be and what is the values of them when and if it gets there.

.... mod:

... and its already installed, I didn't even know I had it until I did a ddd on the cli -- wow -- thanks!