seg fault

[Bill Cunningham]

I have put together this program that seems to do what I want without
error checking added yet. If I just run the program it produces a
segmentation fault. If I add the proper value say 43.56 it's fine. Does
anyone see what's wrong ? I have a c99 compiler. Thanks.

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

int main ( int argc, char *argv[] ) {
FILE*fp;
double x;
x=strtod(argv[1],NULL);
if (argc !=2) {
fprintf(stderr, "usage error\n");
exit(EXIT_FAILU RE);
}
fp=fopen( "data", "a");
ftell(fp);
fseek(fp,sizeof (double),SEEK_C UR);
fprintf(fp,"%.2 f\n",x);
fclose(fp);
}

[Malcolm McLean]

"Bill Cunningham" <no****@nspam.c omwrote in message news:

I have put together this program that seems to do what I want without
error checking added yet. If I just run the program it produces a
segmentation fault. If I add the proper value say 43.56 it's fine. Does
anyone see what's wrong ? I have a c99 compiler. Thanks.

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

int main ( int argc, char *argv[] ) {
FILE*fp;
double x;
x=strtod(argv[1],NULL);

If argc is 1 this passes a null pointer to strtod, which could well cause a
segfault.

>
if (argc !=2) {
fprintf(stderr, "usage error\n");
exit(EXIT_FAILU RE);
}
fp=fopen( "data", "a");

You also need to check fp for nullness here.

>
ftell(fp);
fseek(fp,sizeof (double),SEEK_C UR);
fprintf(fp,"%.2 f\n",x);
fclose(fp);
}

--
Free games and programming goodies.
http://www.personal.leeds.ac.uk/~bgy1mm

[Bill Cunningham]

"Malcolm McLean" <re*******@btin ternet.comwrote in message
news:7u******** *************** *******@bt.com. ..

"Bill Cunningham" <no****@nspam.c omwrote in message news:

> I have put together this program that seems to do what I want without
error checking added yet. If I just run the program it produces a
segmentation fault. If I add the proper value say 43.56 it's fine. Does
anyone see what's wrong ? I have a c99 compiler. Thanks.

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

int main ( int argc, char *argv[] ) {
FILE*fp;
double x;
x=strtod(argv[1],NULL);

If argc is 1 this passes a null pointer to strtod, which could well cause
a segfault.

This may be where it is. Just running the program argv[0] would be
execution, giving no arguments like I didn't would be NULL.

>>
if (argc !=2) {
fprintf(stderr, "usage error\n");
exit(EXIT_FAILU RE);
}
fp=fopen( "data", "a");

You also need to check fp for nullness here.

>>
ftell(fp);
fseek(fp,sizeof (double),SEEK_C UR);
fprintf(fp,"%.2 f\n",x);
fclose(fp);
}

[santosh]

Bill Cunningham wrote:

I have put together this program that seems to do what I want
without
error checking added yet. If I just run the program it produces a
segmentation fault. If I add the proper value say 43.56 it's fine.
Does anyone see what's wrong ? I have a c99 compiler. Thanks.

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

int main ( int argc, char *argv[] ) {
FILE*fp;
double x;
x=strtod(argv[1],NULL);
if (argc !=2) {
fprintf(stderr, "usage error\n");
exit(EXIT_FAILU RE);
}

This test should come before the conversion with strtod. Otherwise
strtod is passed a null pointer, which is what causes the segmentation
fault.

fp=fopen( "data", "a");
ftell(fp);

What is the purpose of calling ftell and discarding the return?

fseek(fp,sizeof (double),SEEK_C UR);

When you open a file in the append mode the file position indicator is
already positioned after any contents the file may have. You are now
trying to seek sizeof(double) bytes beyond the end of the file, which
will return an error. Exactly what do you hope to accomplish by doing
so?

fprintf(fp,"%.2 f\n",x);
fclose(fp);
}

[Johannes Bauer]

Bill Cunningham schrieb:

This may be where it is. Just running the program argv[0] would be
execution, giving no arguments like I didn't would be NULL.

"may be"? Say do you think debugging C-programs is like guessing where
the fault could occur? Why don't you use a helper tool, say, uhmm, a
debugger?

With a program that short however you could well have inserted some
fprintf(stderr, "xyz\n") statements after every line and found it yourself.

Regards,
Johannes

--
"Wer etwas kritisiert muss es noch lange nicht selber besser können. Es
reicht zu wissen, daß andere es besser können und andere es auch
besser machen um einen Vergleich zu bringen." - Wolfgang Gerber
in de.sci.electron ics <47************ ***********@new s.freenet.de>

[Bill Cunningham]

"santosh" <sa*********@gm ail.comwrote in message
news:g3******** **@registered.m otzarella.org.. .

Bill Cunningham wrote:

> I have put together this program that seems to do what I want
without
error checking added yet. If I just run the program it produces a
segmentation fault. If I add the proper value say 43.56 it's fine.
Does anyone see what's wrong ? I have a c99 compiler. Thanks.

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

int main ( int argc, char *argv[] ) {
FILE*fp;
double x;
x=strtod(argv[1],NULL);
if (argc !=2) {
fprintf(stderr, "usage error\n");
exit(EXIT_FAILU RE);
}

This test should come before the conversion with strtod. Otherwise
strtod is passed a null pointer, which is what causes the segmentation
fault.

> fp=fopen( "data", "a");
ftell(fp);

What is the purpose of calling ftell and discarding the return?

> fseek(fp,sizeof (double),SEEK_C UR);

When you open a file in the append mode the file position indicator is
already positioned after any contents the file may have. You are now
trying to seek sizeof(double) bytes beyond the end of the file, which
will return an error. Exactly what do you hope to accomplish by doing
so?

> fprintf(fp,"%.2 f\n",x);
fclose(fp);
}

Well I've added error checking to ftell, fseek, and fopen and the
program works. It will still segfault if the program is run and nothing
entered. I've used these return values.
For ftell and fseek -1. For fopen NULL. I believe those are the values
to test against.

Bill

[Kenny McCormack]

In article <g3**********@r egistered.motza rella.org>,
Richard <rg****@gmail.c omwrote:

>"Bill Cunningham" <no****@nspam.c omwrites:

>"Richard" <rg****@gmail.c omwrote in message
news:g3******* ***@registered. motzarella.org. ..

>>Try using a debugger. How many more times?

RIGHT.

Because I actually think you are too stuck in your ways to bother to
learn how to use one and you seem incapable of using Google I provide
you this link: (I think you mentioned you use gcc)

Let me put in a word for the other side.

The problem with debuggers (at least all the ones I've used, including
gdb), is that they are very unfriendly, and mostly undocumented. I have
no doubt that when fully understood, they are quite powerful, but I have
never been able to wrap my head around one (of the type that we are
discussing here). man pages are no help. There's no built-in help.
There's no built-in useful error messages.

To continue this rant, I'm sure if I downloaded and studied (for a few
days and/or weeks) the voluminous documentation (in some weird GNU
format), I could probably become an expert in it, but it just ain't
worth the trouble.

[Richard]

"Bill Cunningham" <no****@nspam.c omwrites:

"Richard" <rg****@gmail.c omwrote in message
news:g3******** **@registered.m otzarella.org.. .

>"Bill Cunningham" <no****@nspam.c omwrites:

>>"Richard" <rg****@gmail.c omwrote in message
news:g3****** ****@registered .motzarella.org ...
Try using a debugger. How many more times?

RIGHT.

Because I actually think you are too stuck in your ways to bother to
learn how to use one and you seem incapable of using Google I provide
you this link: (I think you mentioned you use gcc)

http://dirac.org/linux/gdb/

Read the "What is a debugger" part thoroughly.

http://dirac.org/linux/gdb/01-Introd...hatisadebugger

Many people suggest using printf's. This is, IMO, amateurish at best.

From this web page:

[snip]

I use fprintfs to stderr so there's an error only if there's an error. I've
tried learning gdb. I don't read debugging symbols and gdb looks very
overwhelming atleast to me. I have it in my system and looked over the docs.
I thought about putting breakpoints into my program and gave up because
things were too complicated. With the trouble I'm having with C and I want
to stay with it I think my plate's pretty full right now. But I'll look at
your sites.

Bill

It is one site. And it is there to help people. It could not be
easier. If you can not follow the instructions there then you should
really consider giving up - you will never be a programmer.

[Richard]

ga*****@xmissio n.xmission.com (Kenny McCormack) writes:

In article <g3**********@r egistered.motza rella.org>,
Richard <rg****@gmail.c omwrote:

>>"Bill Cunningham" <no****@nspam.c omwrites:

>>"Richard" <rg****@gmail.c omwrote in message
news:g3****** ****@registered .motzarella.org ...
Try using a debugger. How many more times?

RIGHT.

Because I actually think you are too stuck in your ways to bother to
learn how to use one and you seem incapable of using Google I provide
you this link: (I think you mentioned you use gcc)

Let me put in a word for the other side.

The problem with debuggers (at least all the ones I've used, including
gdb), is that they are very unfriendly, and mostly undocumented. I have
no doubt that when fully understood, they are quite powerful, but I have
never been able to wrap my head around one (of the type that we are
discussing here). man pages are no help. There's no built-in help.
There's no built-in useful error messages.

To continue this rant, I'm sure if I downloaded and studied (for a few
days and/or weeks) the voluminous documentation (in some weird GNU
format), I could probably become an expert in it, but it just ain't
worth the trouble.

Or alternatively you could just read the tutorial I linked to ....

Using a debugger is a lot easier than sprinkling prints around,
recompiling etc.

And with Bill's attention span god knows that he will mess up when he
keeps re-editing the code to add and remove printfs.

[Kenny McCormack]

In article <g3**********@r egistered.motza rella.org>,
Richard <rg****@gmail.c omwrote:

>ga*****@xmissi on.xmission.com (Kenny McCormack) writes:

>In article <g3**********@r egistered.motza rella.org>,
Richard <rg****@gmail.c omwrote:

>>>"Bill Cunningham" <no****@nspam.c omwrites:

"Richard" <rg****@gmail.c omwrote in message
news:g3***** *****@registere d.motzarella.or g...
Try using a debugger. How many more times?

RIGHT.

Because I actually think you are too stuck in your ways to bother to
learn how to use one and you seem incapable of using Google I provide
you this link: (I think you mentioned you use gcc)

Let me put in a word for the other side.

The problem with debuggers (at least all the ones I've used, including
gdb), is that they are very unfriendly, and mostly undocumented. I have
no doubt that when fully understood, they are quite powerful, but I have
never been able to wrap my head around one (of the type that we are
discussing here). man pages are no help. There's no built-in help.
There's no built-in useful error messages.

To continue this rant, I'm sure if I downloaded and studied (for a few
days and/or weeks) the voluminous documentation (in some weird GNU
format), I could probably become an expert in it, but it just ain't
worth the trouble.

Or alternatively you could just read the tutorial I linked to ....

Using a debugger is a lot easier than sprinkling prints around,
recompiling etc.

And with Bill's attention span god knows that he will mess up when he
keeps re-editing the code to add and remove printfs.

Like I said, I've tried. I've tried all those so-called tutorials.

There seems to be a cult associated with C debuggers, much like the cult
associated with this newsgroup. The "It was hard for me; it should be
hard for you" cult.