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segmentation fault

can anyone tell me why the following piece of code causes a segmentation fault please ??
char *getFNFromIden( char *ident)
{
int i = 0 ;
printf("identif ier %s\n", allMenus[i].identifier) ;
while(allMenus[i].identifier[0] != '\0')
{
printf("Hello World") ;
i++ ;
}
}

As far as i can tell the seg fault is happening at while(bla) as the first printf is working, printing out what is in allmenus[i].identifier.
Thanks for any help
Matt
Nov 13 '05 #1
15 7274
On Tue, 29 Jul 2003 12:21:26 +0100
Matthew Jakeman <ma************ @yahoo.co.uk> wrote:
can anyone tell me why the following piece of code causes a segmentation fault
please ??
char *getFNFromIden( char *ident)
{
int i = 0 ;
printf("identif ier %s\n", allMenus[i].identifier) ; What's an allMenus? What if allMenus[0] doesn't exist? Is it's identifier field
defined? IS it even a string? We don't know that!
while(allMenus[i].identifier[0] != '\0') what if allMenus[i] is out of bounds? There's no check for that.. And what is
identifier itself is null? Are you certain that the'identifier' of the last
element is empty? What if it isn't? There's no bounds checking whatsoever.
{
printf("Hello World") ;
i++ ;
}
}

Why is the ident argument not used? Where is the code that actually matters?

In other words, there's not enough information here to distill an answer that is
useful for you, unless you found my remarks useful. Give us more.

--
main(int c,char*k,char*s ){c>0?main(0,"a dceoX$_k6][^hn","-7\
0#05&'40$.6'+). 3+1%30"),puts(" "):*s?c=!c?-*s:(putchar(45) ,c
),putchar(main( c,k+=*s-c*-1,s+1)):(s=0);r eturn!s?10:10+* k;}
Nov 13 '05 #2
On Tue, 29 Jul 2003 12:48:14 +0200
Pieter Droogendijk <gi*@binky.home unix.org> wrote:
On Tue, 29 Jul 2003 12:21:26 +0100
Matthew Jakeman <ma************ @yahoo.co.uk> wrote:
can anyone tell me why the following piece of code causes a segmentation fault
please ??
char *getFNFromIden( char *ident)
{
int i = 0 ;
printf("identif ier %s\n", allMenus[i].identifier) ; What's an allMenus? What if allMenus[0] doesn't exist? Is it's identifier field
defined? IS it even a string? We don't know that!


allMenus is a sstruct and identifier is a char *.
while(allMenus[i].identifier[0] != '\0')

what if allMenus[i] is out of bounds? There's no check for that.. And what is
identifier itself is null? Are you certain that the'identifier' of the last
element is empty? What if it isn't? There's no bounds checking whatsoever.


allMenus is not out of bounds and identifier is not null because as i said the printf displays its contents, i haven't done any bounds checking yet as i know exactly what is contained in all of the structs and i just want to get the main body of the code working first.
{
printf("Hello World") ;
i++ ;
}
}

Why is the ident argument not used? Where is the code that actually matters?


Argument isnt used as i want to fix this problem before i write the rest of the function, there is no other code in, this is it.
In other words, there's not enough information here to distill an answer that is
useful for you, unless you found my remarks useful. Give us more.

--
main(int c,char*k,char*s ){c>0?main(0,"a dceoX$_k6][^hn","-7\
0#05&'40$.6'+). 3+1%30"),puts(" "):*s?c=!c?-*s:(putchar(45) ,c
),putchar(main( c,k+=*s-c*-1,s+1)):(s=0);r eturn!s?10:10+* k;}

Thanks again.
Matt
Nov 13 '05 #3
Matthew Jakeman wrote:
can anyone tell me why the following piece of code causes a segmentation
fault please ??
There's probably a bad pointer in your data.
char *getFNFromIden( char *ident)
{
int i = 0 ;
printf("identif ier %s\n", allMenus[i].identifier) ;
We don't know what the declaration of allMenus is, nor what
its contents are, because YOU HAVEN'T SHOWN US. We're reduced
to guessing.
while(allMenus[i].identifier[0] != '\0')
How many elements does allMenus have? What value of `i` does
the loop die on? Can you prove that allMenus[i] has a valid
non-null pointer at this value of `i`?
{
printf("Hello World") ;
i++ ;
}
}


What is your test code? How will you know if this function works?

--
Chris "electric hedgehog" Dollin
C FAQs at: http://www.faqs.org/faqs/by-newsgrou...mp.lang.c.html
C welcome: http://www.angelfire.com/ms3/bchambl...me_to_clc.html
Nov 13 '05 #4
On Tue, 29 Jul 2003 13:31:40 +0100, Matthew Jakeman wrote:
On Tue, 29 Jul 2003 12:48:14 +0200
Pieter Droogendijk <gi*@binky.home unix.org> wrote:
What's an allMenus? What if allMenus[0] doesn't exist? Is it's
identifier field defined? IS it even a string? We don't know that!


allMenus is a sstruct and identifier is a char *.


Argument isnt used as i want to fix this problem before i write the rest
of the function, there is no other code in, this is it.


Show us the calling code and the initialization of allMenus.

regards
NPV
Nov 13 '05 #5
On Tue, 29 Jul 2003 13:31:40 +0100
Matthew Jakeman <ma************ @yahoo.co.uk> wrote:
On Tue, 29 Jul 2003 12:48:14 +0200
Pieter Droogendijk <gi*@binky.home unix.org> wrote:
On Tue, 29 Jul 2003 12:21:26 +0100
What's an allMenus? What if allMenus[0] doesn't exist? Is it's identifier
field defined? IS it even a string? We don't know that!


allMenus is a sstruct and identifier is a char *.


How is it declared? Is it static? Dynamic? How is it defined? What does it
contain?
what if allMenus[i] is out of bounds? There's no check for that.. And what
is identifier itself is null? Are you certain that the'identifier' of the
last element is empty? What if it isn't? There's no bounds checking
whatsoever.


allMenus is not out of bounds and identifier is not null because as i said the
printf displays its contents, i haven't done any bounds checking yet as i know
exactly what is contained in all of the structs and i just want to get the
main body of the code working first.


the printf only uses allMenus[0]. And how many are there? I'd assume more, but
still I've seen nothing of the definition of allMenus. For all I know,
allMenus[1].identifier, or even allMenus[1] could be undefined.
Why is the ident argument not used? Where is the code that actually matters?


Argument isnt used as i want to fix this problem before i write the rest of
the function, there is no other code in, this is it.


There is other code. This alone would never compile, would it? I don't even know
what the main datatype here, allMenus, looks like, or what it contains.
In order to get help, you'll have to post the declaration and definition of
allMenus, including everything it contains (or as little as possible to make the
program crash), and a call to getFNFromIden. In other words, the rest of the
code. Like I said.
--
main(int c,char*k,char*s ){c>0?main(0,"a dceoX$_k6][^hn","-7\
0#05&'40$.6'+). 3+1%30"),puts(" "):*s?c=!c?-*s:(putchar(45) ,c
),putchar(main( c,k+=*s-c*-1,s+1)):(s=0);r eturn!s?10:10+* k;}
Nov 13 '05 #6
On Tue, 29 Jul 2003 11:58:16 GMT
"Nils Petter Vaskinn" <no@spam.for.me .invalid> wrote:
On Tue, 29 Jul 2003 13:31:40 +0100, Matthew Jakeman wrote:
On Tue, 29 Jul 2003 12:48:14 +0200
Pieter Droogendijk <gi*@binky.home unix.org> wrote:


What's an allMenus? What if allMenus[0] doesn't exist? Is it's
identifier field defined? IS it even a string? We don't know that!


allMenus is a sstruct and identifier is a char *.


Argument isnt used as i want to fix this problem before i write the rest
of the function, there is no other code in, this is it.


Show us the calling code and the initialization of allMenus.

regards
NPV


Here's the struct :

struct menuDetail
{
char *identifier ;
char *name ;
char *ansfile ;
char *ascfile ;
char *keys[] ;
};
struct menuDetail allMenus[128] ;

Here's where it is initialised :

while(fLines[i] != NULL)
{
lToken = strtok(fLines[i], " ") ;
if(strcmp(lToke n, "MENU") == 0)
{
lToken = strtok(NULL, "\r\n") ;
allMenus[noMenus].identifier = lToken ;
noMenus++ ;
//Need a better way to store the menu information or a way to map
//the menu number to the menu name
printf("noMenus : %i\nMenu Identifier : %s\n", noMenus , allMenus[noMenus].identifier) ;
i++ ;
The loop then goes on to do a load of other stuff that i think is irrelelevent..

And here's the code that i am having a problem with :

char *getFNFromIden( char *ident)
{
int i = 0 ;
printf("identif ier %s\n", allMenus[i].identifier) ;
while(allMenus[i].identifier[0] != '\0')
{
printf("going through the loop again") ;
i++ ;
}
}

Thanks
Matt
Nov 13 '05 #7
On Tue, 29 Jul 2003 14:13:07 +0100
Matthew Jakeman <ma************ @yahoo.co.uk> wrote:
On Tue, 29 Jul 2003 11:58:16 GMT
"Nils Petter Vaskinn" <no@spam.for.me .invalid> wrote:
On Tue, 29 Jul 2003 13:31:40 +0100, Matthew Jakeman wrote:
On Tue, 29 Jul 2003 12:48:14 +0200
Pieter Droogendijk <gi*@binky.home unix.org> wrote:


> What's an allMenus? What if allMenus[0] doesn't exist? Is it's
> identifier field defined? IS it even a string? We don't know that!

allMenus is a sstruct and identifier is a char *.


Argument isnt used as i want to fix this problem before i write the rest
of the function, there is no other code in, this is it.


Show us the calling code and the initialization of allMenus.

regards
NPV


Here's the struct :

struct menuDetail
{
char *identifier ;
char *name ;
char *ansfile ;
char *ascfile ;
char *keys[] ;
};
struct menuDetail allMenus[128] ;

Here's where it is initialised :

while(fLines[i] != NULL)
{
lToken = strtok(fLines[i], " ") ;
if(strcmp(lToke n, "MENU") == 0)
{
lToken = strtok(NULL, "\r\n") ;
allMenus[noMenus].identifier = lToken ;
noMenus++ ;
//Need a better way to store the menu information or a way to map
//the menu number to the menu name
printf("noMenus : %i\nMenu Identifier : %s\n", noMenus , allMenus[noMenus].identifier) ;
i++ ;
The loop then goes on to do a load of other stuff that i think is irrelelevent..

And here's the code that i am having a problem with :

char *getFNFromIden( char *ident)
{
int i = 0 ;
printf("identif ier %s\n", allMenus[i].identifier) ;
while(allMenus[i].identifier[0] != '\0')
{
printf("going through the loop again") ;
i++ ;
}
}

Thanks
Matt


I might not have mentioned that the printf inside the loop never displays anything, so it must exit the first time the while loop statement is evaluated!!
Nov 13 '05 #8
In <20************ *************** *********@yahoo .co.uk> Matthew Jakeman <ma************ @yahoo.co.uk> writes:
can anyone tell me why the following piece of code causes a segmentation fault please ??
char *getFNFromIden( char *ident)
{
int i = 0 ;
printf("identif ier %s\n", allMenus[i].identifier) ;
while(allMenus[i].identifier[0] != '\0')
{
printf("Hello World") ;
i++ ;
}
}

As far as i can tell the seg fault is happening at while(bla) as the first printf is working, printing out what is in allmenus[i].identifier.


Something as simple and "insignific ant" as appending a newline character
to the the "Hello World" string will give you an idea about the value of i
by the time the program crashes.

An even better idea is to print the value of i inside the loop, instead of
"Hello World", again followed by a newline character. This information,
combined with a perusal of the code that initialises the array of
structs, should point you to the reason of the segmentation fault.

Writing C code is easy. Figuring out why it doesn't work as intended is
a bit more difficult, but, without this skill, no one can become a C
programmer.

Dan
--
Dan Pop
DESY Zeuthen, RZ group
Email: Da*****@ifh.de
Nov 13 '05 #9
On Tue, 29 Jul 2003 14:13:07 +0100, Matthew Jakeman wrote:
Here's the struct :

struct menuDetail
{
char *identifier ;
char *name ;
char *ansfile ;
char *ascfile ;
char *keys[] ;
};
struct menuDetail allMenus[128] ;

Here's where it is initialised :

while(fLines[i] != NULL)
{
lToken = strtok(fLines[i], " ") ;
if(strcmp(lToke n, "MENU") == 0)
{
lToken = strtok(NULL, "\r\n") ;
This may be where it all goes wrong, strtok might return NULL
allMenus[noMenus].identifier = lToken ;
Which is then stored into the identifier
noMenus++ ;
//Need a better way to store the menu information or a way to map
//the menu number to the menu name
printf("noMenus : %i\nMenu Identifier : %s\n", noMenus , allMenus[noMenus].identifier) ;
i++ ;
The loop then goes on to do a load of other stuff that i think is irrelelevent..

And here's the code that i am having a problem with :

char *getFNFromIden( char *ident)
{
int i = 0 ;
printf("identif ier %s\n", allMenus[i].identifier) ;
while(allMenus[i].identifier[0] != '\0')
And then you dereference that null pointer.
{
printf("going through the loop again") ;
i++ ;
}
}

Thanks
Matt


Try to insert:

assert(lToken != NULL);

where my first comment was to see if this is the case.

or you could replace the final check with:

while (allMenus[i].identifier && allMenus[i].identifier[0] != '\0')

You don't tell where flines comes from, but if you're reading from a file
the last line isn't guaranteed to contain a newline. (depending on
who/what created the file)

hth
NPV

Nov 13 '05 #10

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