Comments on my code?

[Platonic Solid]

Hi-

I am learning C from some old lecture notes, but they don't have
solutions so I'd like some feedback on my code if anyone has the time.

This is an exercise: "Write a program to trim any leading whitespace
from a string and return a newly allocated buffer containing the trimmed
string. Don't forget to handle errors."

main(argc, argv)
int argc; char **argv;
{
char *strtrim(), *r=0;
puts(argc>1 && (r=strtrim(*++a rgv)) ? r : "Unspecifie d error");
free(r);
}

/* trims initial whitespace */
char *strtrim(s)
char *s;
{
char *r, *malloc();
for( ; isspace(*s); s++);
r=malloc(strlen (s)+1);
if(r)
strcpy(r,s);
return r;
}

This looks OK to me and works correctly, but the compiler produces some
mysterious warnings about conflicting definitions that I don't really
understand.
--
How come we never talk anymore?

[CBFalconer]

Platonic Solid wrote:

>
I am learning C from some old lecture notes, but they don't have
solutions so I'd like some feedback on my code if anyone has the
time.

This is an exercise: "Write a program to trim any leading whitespace
from a string and return a newly allocated buffer containing the
trimmed string. Don't forget to handle errors."

main(argc, argv)
int argc; char **argv;
{
char *strtrim(), *r=0;
puts(argc>1 && (r=strtrim(*++a rgv)) ? r : "Unspecifie d error");
free(r);

.... snip ...

>
This looks OK to me and works correctly, but the compiler produces
some mysterious warnings about conflicting definitions that I
don't really understand.

To start with, you are failing to use prototypes (using obsolete
K&R 1 function headers), omitting necessary #includes, and using
system reserved names. The latter includes anything of the form
"strx..." where x is a lowercase letter.

You can get a satisfactory approximation to the standard in N869 or
N1124. You can find a suitable bzip2 compressed version of N869
at:

<http://cbfalconer.home .att.net/download/>

--
Chuck F (cbfalconer at maineline dot net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home .att.net>

--
Posted via a free Usenet account from http://www.teranews.com

[CBFalconer]

Platonic Solid wrote:

>

.... snip ...

>
OK, so that explains why strcpy produces a warning as it returns
char * and not int. But the compiler also complains about malloc
(I specifically supply the return type for that) and strlen, which
returns int so should be OK by Q 1.25.

You didn't give code, so we can't make suggestions. If you have
problems with malloc you are probably a) casting the return or b)
failing to include <stdlib.hor c) capturing the return value in
other than a pointer or d) using a C++ compiler.

--
Chuck F (cbfalconer at maineline dot net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home .att.net>

--
Posted via a free Usenet account from http://www.teranews.com

[Platonic Solid]

On 1 Sep 2007 at 16:42, CBFalconer wrote:

Platonic Solid wrote:

>>

... snip ...

>>
OK, so that explains why strcpy produces a warning as it returns
char * and not int. But the compiler also complains about malloc
(I specifically supply the return type for that) and strlen, which
returns int so should be OK by Q 1.25.

You didn't give code, so we can't make suggestions.

On the contrary, I posted my code, but here it is again:

main(argc, argv)
int argc; char **argv;
{
char *strtrim(), *r=0;
puts(argc>1 && (r=strtrim(*++a rgv)) ? r : "Unspecifie d error");
free(r);
}

/* trims initial whitespace */
char *strtrim(s)
char *s;
{
char *r, *malloc(), *strcpy();
for( ; isspace(*s); s++);
r=malloc(strlen (s)+1);
if(r)
strcpy(r,s);
return r;
}

(I've made one fix, by putting in the correct return type for strcpy)

If you have problems with malloc you are probably a) casting the
return or b) failing to include <stdlib.hor c) capturing the return
value in other than a pointer or d) using a C++ compiler.

On the contrary, I've declared malloc correctly and store its return
value in a char *.

I'm not sure what to do about people's advice that some of the syntax in
the lecture notes is now old-fashioned - as it still seems to compile
just fine, I'm tempted to stick with what I know.

--
Chuck F (cbfalconer at maineline dot net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home .att.net>

--
How come we never talk anymore?

[Harald van =?UTF-8?B?RMSzaw==?=]

Platonic Solid wrote:

char *r, *malloc(), *strcpy();

On the contrary, I've declared malloc correctly and store its return
value in a char *.

No, you haven't declared malloc correctly.

[Keith Thompson]

Harald van Dijk <tr*****@gmail. comwrites:

Platonic Solid wrote:

> char *r, *malloc(), *strcpy();

On the contrary, I've declared malloc correctly and store its return
value in a char *.

No, you haven't declared malloc correctly.

Specifically, malloc returns void*, not char*, and it takes an
argument of type size_t. Since you didn't declare the argument type,
something as simple as malloc(42) invokes undefined behavior, since
you're passing an argument of type int.

If you had declared it correctly as
void *malloc(size_t) ;
then a call like malloc(42) would implicitly convert the argument to
size_t -- and a call with a non-numeric argument would trigger an
error message.

But the best way to provide the correct declaration is not to
manually re-write it yourself. It's to provide
#include <stdlib.h>
at the top of your source file. There's simply no good reason not to
do it that way.

The more correct information you can provide for the compiler, the
more it can help you. A lot of the facilities for this, particularly
function prototypes, were introduced with the ANSI C standard in 1989.

Prototypes were introduced into C 18 years ago. There are no longer
any significant compilers still being used that don't support them, so
there's no good reason not to use them, along with a number of other
features introduced by ANSI. (The latest ISO C standard, C99, hasn't
caught on as well, so there are still good reasons to avoid
C99-specific features.)

Your code is archaic. You can still write working archaic code if
you're careful enough, but you really should learn the language as it
is today. Reading K&R2 is probably the best way to do this.

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <* <http://users.sdsc.edu/~kst>
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[Army1987]

On Sat, 01 Sep 2007 20:35:10 +0200, Platonic Solid wrote:

On the contrary, I've declared malloc correctly and store its return
value in a char *.

I'm not sure what to do about people's advice that some of the syntax in
the lecture notes is now old-fashioned - as it still seems to compile
just fine, I'm tempted to stick with what I know.

But, in ANSI/ISO C, which has been around since 1989 and radically
revised in 1999 (even though this last version isn't widely
implemented yet), malloc() returns a void *, not a char * anymore.
It is very likely to work, since char* and void* are required to
have the same alignment etc., but probably a sufficiently deep
level of language lawyering would show that the behavior is
undefined by the International Standard.

Also, nowadays it is common to declare library functions with the
prototypes in the corresponding headers. For example, stdlib.h
contains a correct prototype for malloc(), and you can add it to
your code with the preprocessing directive
#include <stdlib.h>
which gets replaced with the contents of that file.
In C99 you *always* must declare functions, even if they return an
int, and doing so is a very good idea in C89, too.
--
Army1987 (Replace "NOSPAM" with "email")
No-one ever won a game by resigning. -- S. Tartakower

[Francine.Neary]

On Sep 1, 8:22 pm, Keith Thompson <[e-mail] rote:

Harald van D k <true...@gmail. comwrites:

Platonic Solid wrote:

char *r, *malloc(), *strcpy();

On the contrary, I've declared malloc correctly and store its return
value in a char *.

No, you haven't declared malloc correctly.

Specifically, malloc returns void*, not char*, and it takes an
argument of type size_t. Since you didn't declare the argument type,
something as simple as malloc(42) invokes undefined behavior, since
you're passing an argument of type int.

If you had declared it correctly as
void *malloc(size_t) ;
then a call like malloc(42) would implicitly convert the argument to
size_t -- and a call with a non-numeric argument would trigger an
error message.

Am I right in thinking that first standard promotions would apply, and
only then would the result be converted to size_t?

But the best way to provide the correct declaration is not to
manually re-write it yourself. It's to provide
#include <stdlib.h>
at the top of your source file. There's simply no good reason not to
do it that way.

The more correct information you can provide for the compiler, the
more it can help you. A lot of the facilities for this, particularly
function prototypes, were introduced with the ANSI C standard in 1989.

Prototypes were introduced into C 18 years ago. There are no longer
any significant compilers still being used that don't support them, so
there's no good reason not to use them, along with a number of other
features introduced by ANSI. (The latest ISO C standard, C99, hasn't
caught on as well, so there are still good reasons to avoid
C99-specific features.)

Your code is archaic. You can still write working archaic code if
you're careful enough, but you really should learn the language as it
is today. Reading K&R2 is probably the best way to do this.

--
Keith Thompson (The_Other_Keit h) [e-mail] <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <* <http://users.sdsc.edu/~kst>
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[Eric Sosman]

Platonic Solid wrote:

On 1 Sep 2007 at 16:42, CBFalconer wrote:

>Platonic Solid wrote:
... snip ...

>>OK, so that explains why strcpy produces a warning as it returns
char * and not int. But the compiler also complains about malloc
(I specifically supply the return type for that) and strlen, which
returns int so should be OK by Q 1.25.

You didn't give code, so we can't make suggestions.

On the contrary, I posted my code, but here it is again:

main(argc, argv)
int argc; char **argv;
{
char *strtrim(), *r=0;
puts(argc>1 && (r=strtrim(*++a rgv)) ? r : "Unspecifie d error");
free(r);
}

/* trims initial whitespace */
char *strtrim(s)
char *s;
{
char *r, *malloc(), *strcpy();
for( ; isspace(*s); s++);
r=malloc(strlen (s)+1);
if(r)
strcpy(r,s);
return r;
}

(I've made one fix, by putting in the correct return type for strcpy)

Yabbut, that's the wrong way to do it, and it still
doesn't fix the incorrect declaration of strlen(). The
right fix (hint: Have you learned about the #include
directive yet?) would take care of both problems, and
would enable the compiler to catch errors like strcpy(x)
or strlen(x, y), too.

>If you have problems with malloc you are probably a) casting the
return or b) failing to include <stdlib.hor c) capturing the return
value in other than a pointer or d) using a C++ compiler.

On the contrary, I've declared malloc correctly and store its return
value in a char *.

You have declared malloc *in*correctly. The cure is
similar to that for strlen() et al.

I'm not sure what to do about people's advice that some of the syntax in
the lecture notes is now old-fashioned - as it still seems to compile
just fine, I'm tempted to stick with what I know.

I bet your 1955 Studebaker runs just fine without those
pesky seat belts, air bags, anti-lock brakes, and emission-
control devices, too.

--
Eric Sosman
es*****@ieee-dot-org.invalid

[Peter J. Holzer]

On 2007-09-01 18:35, Platonic Solid <pl*****@mailin ator.comwrote:

main(argc, argv)
int argc; char **argv;
{
char *strtrim(), *r=0;
puts(argc>1 && (r=strtrim(*++a rgv)) ? r : "Unspecifie d error");
free(r);
}

/* trims initial whitespace */
char *strtrim(s)
char *s;
{
char *r, *malloc(), *strcpy();
for( ; isspace(*s); s++);
r=malloc(strlen (s)+1);
if(r)
strcpy(r,s);
return r;
}

[...]

On the contrary, I've declared malloc correctly

No, you haven't.

and store its return value in a char *.

I'm not sure what to do about people's advice that some of the syntax in
the lecture notes is now old-fashioned - as it still seems to compile
just fine, I'm tempted to stick with what I know.

It *seems* to compile fine. It also *seems* to execute fine on your
system. But it's still wrong, and works only by chance. It may not work
on a different system.

For example, you haven't defined strlen, so the compiler has to assume
it returns an int. But it returns a size_t and a size_t may be returned
differently than an int (for example, on the 8086, in some memory
models, a size_t was returned in a pair of registers, but an int in only
one register. Declaring strlen as an int instead of a size_t would yield
wrong results if the string was longer than 32767 characters).

Then you pass the int which is the result of strlen(s)+1 to malloc.
Since no prototype is in scope, the compiler has to assume that malloc
expects an int, when really it expects a size_t. Using the example of
the 8086 compiler again, malloc would take 4 bytes off the stack, but
strtrim put only 2 bytes there, so the other two bytes contain some
random garbage (maybe the uninitialized value of r, or a part of the
value of s from the call to isspace). So malloc may try to allocate some
ridiculous amount of memory (basically rand() * 65536 + strlen(s)+1) and
fail. So strtrim also fails.

(This is harder to demonstrate with 64 bit processors: The calling
convention for x86_64 is to pass the first 6 parameters in registers, so
an int/size_t mismatch matters only if it happens in 7th or later
parameter. But assuming you can be sloppy if you only have functions
with 6 or less parameters sounds like an extremely bad idea to me)

As an erstwhile regular in this group wrote: "If you lie to the compiler
it will get its revenge!"

hp

--
_ | Peter J. Holzer | I know I'd be respectful of a pirate
|_|_) | Sysadmin WSR | with an emu on his shoulder.
| | | hj*@hjp.at |
__/ | http://www.hjp.at/ | -- Sam in "Freefall"

[Harald van =?UTF-8?B?RMSzaw==?=]

Fr************@ googlemail.com wrote:

On Sep 1, 8:22 pm, Keith Thompson <[e-mail] rote:

>Harald van D k <true...@gmail. comwrites:

Platonic Solid wrote:
char *r, *malloc(), *strcpy();

>On the contrary, I've declared malloc correctly and store its return
value in a char *.

No, you haven't declared malloc correctly.

Specifically , malloc returns void*, not char*, and it takes an
argument of type size_t. Since you didn't declare the argument type,
something as simple as malloc(42) invokes undefined behavior, since
you're passing an argument of type int.

If you had declared it correctly as
void *malloc(size_t) ;
then a call like malloc(42) would implicitly convert the argument to
size_t -- and a call with a non-numeric argument would trigger an
error message.

Am I right in thinking that first standard promotions would apply, and
only then would the result be converted to size_t?

Compare these functions:

unsigned short f1 (x)
unsigned short x;
{
return x;
}

unsigned short f2 (unsigned short x)
{
return x;
}

int main(void) {
unsigned short x = 3;
return f1(x) - f2(x);
}

The call to f1 results in x being promoted to int (or unsigned int,
depending on the system), and then converted back to unsigned short. The
call to f2 does not cause x to undergo any promotion to int. The given
declaration of malloc falls in the same category as f2, not f1. However, I
would be very surprised to find an implementation for which the end result
is any different for integer types.