Newbie needing some help with segmentation fault

H

Hendrik Maryns

Hi all,

I received a tiny program from a colleague. I want to use it in my Java
program. I invoke it with Java’s ProcessBuilder. That not being very
relevant, the problem is the following: it causes a segfault. Debugging
tells me the following line is the culprit:

while(fscanf(fp, "%s -> %s -> %s -> %s", name, left, right, symbol) == 4)

This is the stack:

Thread [0] (Suspended: Signal 'SIGSEGV' received. Description:
Segmentation fault.)
4 _IO_vfscanf() 0x00002ae4d5a0565f
3 fscanf() 0x00002ae4d5a11a58
2 loadTree() /home/hendrik/workspace/ivb/ivb.c:54 0x0000000000402cbc
1 main() /home/hendrik/workspace/ivb/ivb.c:23 0x0000000000402a95

The relevant function is below. (Since the problematic line is in the
beginning, you might not need all of it but I am really too new in C to
be able to shorten this. Pointers to a relevant site are welcome.)

What it does is it takes a file which contains a tree in some funny
format and constructs the tree in way MONA [1] can handle them.

Thank you very much.

[1]: http://www.brics.dk/mona/

mgTreeNode *loadTree(char *file)
{
hcreate(MAX_NODES);

char symbol[MAX_SYMBOL];
char name[MAX_NODENAME];
char left[MAX_NODENAME];
char right[MAX_NODENAME];
ENTRY node;
char* cache[MAX_NODES][3];
size_t nodeCounter=0;
size_t i;

FILE *fp = fopen(file, "r");
while(fscanf(fp, "%s -> %s -> %s -> %s", name, left, right, symbol) == 4)
{
// construct the node
mgTreeNode *t = tAlloc();

// convert the characters to numbers:
unsigned int len = strlen(symbol);
t->a = (char*)malloc(len*(sizeof(char))); // freed in freeTree
for(i = 0; i < len; ++i) {
t->a = symbol - '0';
}

t->left = NULL;
t->right = NULL;

// insert it into the hashtable
node.key = strdup(name); // this memory is freed after hdestroy
node.data = t;
hsearch(node, ENTER);

// insert it into an array
cache[nodeCounter][0] = node.key;
cache[nodeCounter][1] = strdup(left); // freed after hdestroy
cache[nodeCounter][2] = strdup(right); // freed after hdestroy

++nodeCounter;
}
fclose(fp);

// loop over the array and fill in the links
for(i=0; i < nodeCounter; ++i)
{
node.key = cache[0];
ENTRY *n = hsearch(node, FIND);

node.key = cache[1];
ENTRY *l = hsearch(node, FIND);

node.key = cache[2];
ENTRY *r = hsearch(node, FIND);

if(l != NULL) ((mgTreeNode *) n->data)->left = (l->data);
if(r != NULL) ((mgTreeNode *) n->data)->right = (r->data);
}

node.key = ROOT_NODE_NAME;
ENTRY *root = hsearch(node, FIND);

mgTreeNode *rootNode = NULL;
if(root != NULL) rootNode = root->data;

// free the hashtable
hdestroy();
// free the keys
for(i=0; i < nodeCounter; ++i)
{
int j;
for(j=0; j<3; ++j)
{
free(cache[j]);
}
}
return rootNode;
}
--
Hendrik Maryns
http://tcl.sfs.uni-tuebingen.de/~hendrik/
==================
http://aouw.org
Ask smart questions, get good answers:
http://www.catb.org/~esr/faqs/smart-questions.html


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W

Walter Roberson

the problem is the following: it causes a segfault. Debugging
tells me the following line is the culprit:
while(fscanf(fp, "%s -> %s -> %s -> %s", name, left, right, symbol) ==
4) [...]
char symbol[MAX_SYMBOL];
char name[MAX_NODENAME];
char left[MAX_NODENAME];
char right[MAX_NODENAME];
while(fscanf(fp, "%s -> %s -> %s -> %s", name, left, right, symbol) == 4)

That fscanf() can cause an access violation if it encounters
a line in which the length of any of the strings in the "name" or
"left" or "right" positions are more than MAX_NODENAME-1 characters long,
or if the string in the "symbol" position is more than MAX_SYMBOL-1
characters long. In particular watch out for cases in which the
string are -exactly- MAX_NODENAME or MAX_SYMBOL long: in that
case, fscanf() would need to store a trailing '\0' after the MAX_*
characters had been stored in the variable.
 
J

jacob navia

Hendrik said:
[1]: http://www.brics.dk/mona/

mgTreeNode *loadTree(char *file)
{
hcreate(MAX_NODES);

char symbol[MAX_SYMBOL];
char name[MAX_NODENAME];
char left[MAX_NODENAME];
char right[MAX_NODENAME];
ENTRY node;
char* cache[MAX_NODES][3];
size_t nodeCounter=0;
size_t i;

FILE *fp = fopen(file, "r");

WHAT HAPPENS IF fopen FAILS (the file doesn't exist, or has
another name, or you have no access to it etc etc???)

while(fscanf(fp, "%s -> %s -> %s -> %s", name, left, right, symbol) == 4)
{

This will trap if fp is NULL because fopen failed.
 
M

Martin Ambuhl

Hendrik said:
Hi all,

I received a tiny program from a colleague. I want to use it in my Java
program. I invoke it with Java’s ProcessBuilder. That not being very
relevant, the problem is the following: it causes a segfault. Debugging
tells me the following line is the culprit:

while(fscanf(fp, "%s -> %s -> %s -> %s", name, left, right, symbol) == 4)

It seems that one or more of name, left, right, symbolm is either non
allocated space of is allocated too little space for the input string or
fp was not properly assigned.

[implementation-specific, and non-informative, stack dump removed]
mgTreeNode *loadTree(char *file)
{
hcreate(MAX_NODES);

char symbol[MAX_SYMBOL];
char name[MAX_NODENAME];
char left[MAX_NODENAME];
char right[MAX_NODENAME];
ENTRY node;
char* cache[MAX_NODES][3];
size_t nodeCounter=0;
size_t i;

FILE *fp = fopen(file, "r");

Did this fopen succeed? If it did not (and you don't check) the
following fscanf dereferences a null pointer.
while(fscanf(fp, "%s -> %s -> %s -> %s", name, left, right, symbol) == 4)

If the fopen succeeded, one of MAX_SYMBOL or MAX_NODENAME is too small
for the input data. The data itself may be munged. This is not a safe
way to read these strings.
 
J

John Gordon

In said:
I received a tiny program from a colleague.
What it does is it takes a file which contains a tree in some funny
format and constructs the tree in way MONA [1] can handle them.
FILE *fp = fopen(file, "r");
while(fscanf(fp, "%s -> %s -> %s -> %s", name, left, right, symbol) == 4)

My first guess is that it's trying to open the tree file and failing,
because it can't find it. It then tries to read from the invalid file
pointer and dumps core.

Do you have a copy of the tree file? Is is named correctly? Is it located
in the proper subdirectory? Does it have appropriate permissions for
reading?
 
E

EventHelix.com

Hi all,

signature.asc
1KDownload

I received a tiny program from a colleague. I want to use it in my Java
program. I invoke it with Java's ProcessBuilder. That not being very
relevant, the problem is the following: it causes a segfault. Debugging
tells me the following line is the culprit:

while(fscanf(fp, "%s -> %s -> %s -> %s", name, left, right, symbol) == 4)

This is the stack:

Thread [0] (Suspended: Signal 'SIGSEGV' received. Description:
Segmentation fault.)
4 _IO_vfscanf() 0x00002ae4d5a0565f
3 fscanf() 0x00002ae4d5a11a58
2 loadTree() /home/hendrik/workspace/ivb/ivb.c:54 0x0000000000402cbc
1 main() /home/hendrik/workspace/ivb/ivb.c:23 0x0000000000402a95

The relevant function is below. (Since the problematic line is in the
beginning, you might not need all of it but I am really too new in C to
be able to shorten this. Pointers to a relevant site are welcome.)

What it does is it takes a file which contains a tree in some funny
format and constructs the tree in way MONA [1] can handle them.

Thank you very much.

[1]:http://www.brics.dk/mona/

mgTreeNode *loadTree(char *file)
{
hcreate(MAX_NODES);

char symbol[MAX_SYMBOL];
char name[MAX_NODENAME];
char left[MAX_NODENAME];
char right[MAX_NODENAME];
ENTRY node;
char* cache[MAX_NODES][3];
size_t nodeCounter=0;
size_t i;

FILE *fp = fopen(file, "r");
while(fscanf(fp, "%s -> %s -> %s -> %s", name, left, right, symbol) == 4)
{
// construct the node
mgTreeNode *t = tAlloc();

// convert the characters to numbers:
unsigned int len = strlen(symbol);
t->a = (char*)malloc(len*(sizeof(char))); // freed in freeTree
for(i = 0; i < len; ++i) {
t->a = symbol - '0';
}

t->left = NULL;
t->right = NULL;

// insert it into the hashtable
node.key = strdup(name); // this memory is freed after hdestroy
node.data = t;
hsearch(node, ENTER);

// insert it into an array
cache[nodeCounter][0] = node.key;
cache[nodeCounter][1] = strdup(left); // freed after hdestroy
cache[nodeCounter][2] = strdup(right); // freed after hdestroy

++nodeCounter;
}
fclose(fp);

// loop over the array and fill in the links
for(i=0; i < nodeCounter; ++i)
{
node.key = cache[0];
ENTRY *n = hsearch(node, FIND);

node.key = cache[1];
ENTRY *l = hsearch(node, FIND);

node.key = cache[2];
ENTRY *r = hsearch(node, FIND);

if(l != NULL) ((mgTreeNode *) n->data)->left = (l->data);
if(r != NULL) ((mgTreeNode *) n->data)->right = (r->data);
}

node.key = ROOT_NODE_NAME;
ENTRY *root = hsearch(node, FIND);

mgTreeNode *rootNode = NULL;
if(root != NULL) rootNode = root->data;

// free the hashtable
hdestroy();
// free the keys
for(i=0; i < nodeCounter; ++i)
{
int j;
for(j=0; j<3; ++j)
{
free(cache[j]);
}
}
return rootNode;}


The following links might help in the investigation:

http://www.eventhelix.com/RealtimeMantra/Basics/debugging_software_crashes.htm

http://www.eventhelix.com/RealtimeMantra/Basics/debugging_software_crashes_2.htm

--
EventStudio - http://www.Eventhelix.com/Eventstudio/
Sequence diagram based systems engineering tool
 
H

Hendrik Maryns

Walter Roberson schreef:
the problem is the following: it causes a segfault. Debugging
tells me the following line is the culprit:
while(fscanf(fp, "%s -> %s -> %s -> %s", name, left, right, symbol) ==
4) [...]
char symbol[MAX_SYMBOL];
char name[MAX_NODENAME];
char left[MAX_NODENAME];
char right[MAX_NODENAME];
while(fscanf(fp, "%s -> %s -> %s -> %s", name, left, right, symbol) == 4)

That fscanf() can cause an access violation if it encounters
a line in which the length of any of the strings in the "name" or
"left" or "right" positions are more than MAX_NODENAME-1 characters long,
or if the string in the "symbol" position is more than MAX_SYMBOL-1
characters long. In particular watch out for cases in which the
string are -exactly- MAX_NODENAME or MAX_SYMBOL long: in that
case, fscanf() would need to store a trailing '\0' after the MAX_*
characters had been stored in the variable.

I suppose that is the reason why one shouldn’t use scanf. However, the
problem was due to the file not being properly opened.

Thanks, H.
--
Hendrik Maryns
http://tcl.sfs.uni-tuebingen.de/~hendrik/
==================
http://aouw.org
Ask smart questions, get good answers:
http://www.catb.org/~esr/faqs/smart-questions.html


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H

Hendrik Maryns

Martin Ambuhl schreef:
Hendrik said:
Hi all,

I received a tiny program from a colleague. I want to use it in my Java
program. I invoke it with Java’s ProcessBuilder. That not being very
relevant, the problem is the following: it causes a segfault. Debugging
tells me the following line is the culprit:

while(fscanf(fp, "%s -> %s -> %s -> %s", name, left, right, symbol)
== 4)

It seems that one or more of name, left, right, symbolm is either non
allocated space of is allocated too little space for the input string or
fp was not properly assigned.

[implementation-specific, and non-informative, stack dump removed]

Please have some patience here. I am a regular poster to c.l.java.*,
and now how annoying it can be to answer newbie questions. That’s why I
stressed that twice.
mgTreeNode *loadTree(char *file)
{
hcreate(MAX_NODES);

char symbol[MAX_SYMBOL];
char name[MAX_NODENAME];
char left[MAX_NODENAME];
char right[MAX_NODENAME];
ENTRY node;
char* cache[MAX_NODES][3];
size_t nodeCounter=0;
size_t i;

FILE *fp = fopen(file, "r");

Did this fopen succeed? If it did not (and you don't check) the
following fscanf dereferences a null pointer.

This indeed was the problem: I gave the wrong arguments to the debug
program.

How do I properly check whether the open succeeded? Is
if (fp == 0) exit(-2);
enough?
If the fopen succeeded, one of MAX_SYMBOL or MAX_NODENAME is too small
for the input data. The data itself may be munged. This is not a safe
way to read these strings.

I suppose. Once again: I didn’t write this code, I only want to use it,
for now; until I get time to write it properly.

Thanks, H.
--
Hendrik Maryns
http://tcl.sfs.uni-tuebingen.de/~hendrik/
==================
http://aouw.org
Ask smart questions, get good answers:
http://www.catb.org/~esr/faqs/smart-questions.html


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S

santosh

Hendrik said:
Martin Ambuhl schreef:


This indeed was the problem: I gave the wrong arguments to the debug
program.

How do I properly check whether the open succeeded? Is
if (fp == 0) exit(-2);
enough?

fopen() returns a NULL pointer if it fails. So you check like:

FILE *f = fopen("file.txt" "r");
if (f == NULL) { puts("fopen() failed."); /* whatever */ }
else { /* proceed */ }

<snip>
 
H

Hendrik Maryns

santosh schreef:
fopen() returns a NULL pointer if it fails. So you check like:

FILE *f = fopen("file.txt" "r");
if (f == NULL) { puts("fopen() failed."); /* whatever */ }
else { /* proceed */ }

Thanks. I’m getting the hang of this, I think. I prefer to write such
stuff to stderr though, so I did the following:

FILE *fp = fopen(file, "r");
if (fp == NULL) {
fputs("Opening of the tree file failed. Did you enter it correctly
and do you have read access?\n", stderr);
exit(-2);
}

Cheers, H.
--
Hendrik Maryns
http://tcl.sfs.uni-tuebingen.de/~hendrik/
==================
http://aouw.org
Ask smart questions, get good answers:
http://www.catb.org/~esr/faqs/smart-questions.html


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S

santosh

Hendrik said:
santosh schreef:

Thanks. I?m getting the hang of this, I think. I prefer to write
such stuff to stderr though, so I did the following:

FILE *fp = fopen(file, "r");
if (fp == NULL) {
fputs("Opening of the tree file failed. Did you enter it
correctly
and do you have read access?\n", stderr);
exit(-2);
}

One nit. The only portable exit status values that the C Standard
describes are 0, EXIT_SUCCESS and EXIT_FAILURE. The latter two are
defined in stdlib.h header. The value -2 may have meaning on some
systems, but may cause undefined behaviour on others.
 
J

James Kuyper

Hendrik said:
Martin Ambuhl schreef:
Hendrik Maryns wrote: ....
mgTreeNode *loadTree(char *file)
{
hcreate(MAX_NODES);

char symbol[MAX_SYMBOL];
char name[MAX_NODENAME];
char left[MAX_NODENAME];
char right[MAX_NODENAME];
ENTRY node;
char* cache[MAX_NODES][3];
size_t nodeCounter=0;
size_t i;

FILE *fp = fopen(file, "r");
Did this fopen succeed? If it did not (and you don't check) the
following fscanf dereferences a null pointer.

This indeed was the problem: I gave the wrong arguments to the debug
program.

How do I properly check whether the open succeeded? Is
if (fp == 0) exit(-2);
enough?

There are only three things you can portably pass to exit():
EXIT_SUCCESS, EXIT_FAILURE (both #defined in <stdlib.h> or 0. Note: not
all programs need to be portable, but you should write non-portable code
only by reason of a deliberate decision, not as a matter of habit.

There's no guarantee that EXIT_SUCCESS is 0, nor is there any guarantee
that EXIT_SUCCESS is not 0. It can be arguably be inferred that
EXIT_FAILURE must be different from both 0 and EXIT_SUCCESS.

I normally would add a perror(file) call before the exit() call, for
diagnostic purposes, but only if it's acceptable for your program to
send something to stderr.

I would also recommend avoiding direct calls to exit(). In most cases a
subroutine should report a problem to it's caller, allowing the caller
to perform any cleanup that may be needed.
 
W

William Pursell

santosh schreef:



Thanks. I'm getting the hang of this, I think. I prefer to write such
stuff to stderr though, so I did the following:

FILE *fp = fopen(file, "r");
if (fp == NULL) {
fputs("Opening of the tree file failed. Did you enter it correctly
and do you have read access?\n", stderr);
exit(-2);

}

Much better is:

if( fp == NULL ) {
perror( file );
exit( EXIT_FAILURE );
}

Note that not all implementations will properly set errno
when fopen fails, so you might prefer:

if( fp == NULL ) {
fprintf( stderr, "Error opening %s: %s\n", file,
strerror( errno ));
exit( EXIT_FAILURE );
}

This will give you nicer messages explaining the reason. Eg:

$ ./a.out bar
bar: No such file or directory
$ ./a.out foo
foo: Permission denied

It's always a good idea to use the error message that have
been kindly provided by the implementation rather than
trying to role your own.
 
K

Keith Thompson

jacob navia said:
Hendrik Maryns wrote: [...]
mgTreeNode *loadTree(char *file)
{ [...]
FILE *fp = fopen(file, "r");

WHAT HAPPENS IF fopen FAILS (the file doesn't exist, or has
another name, or you have no access to it etc etc???)

while(fscanf(fp, "%s -> %s -> %s -> %s", name, left, right, symbol) == 4)
{

This will trap if fp is NULL because fopen failed.

It *might* trap. It *will* invoke undefined behavior.

(It may well be the case that it will "trap", whatever that happens to
mean, on most or all existing implementations, but that's not
guaranteed.)
 
H

Hendrik Maryns

William Pursell schreef:
Much better is:

if( fp == NULL ) {
perror( file );
exit( EXIT_FAILURE );
}

What is ‘file’ here? Should I replace it by stderr? (I am doing no
logging to a file, it is really a tiny app.)
Note that not all implementations will properly set errno
when fopen fails, so you might prefer:

if( fp == NULL ) {
fprintf( stderr, "Error opening %s: %s\n", file,
strerror( errno ));
exit( EXIT_FAILURE );
}

I get the following compiler error here:

error: ‘errno’ undeclared (first use in this function)

It is a bit unclear which part of the code you provide can be copied
over, and which parts I have to replace by meaningful variables. Or do
I simply have to declare errno somewhere?
This will give you nicer messages explaining the reason. Eg:

$ ./a.out bar
bar: No such file or directory
$ ./a.out foo
foo: Permission denied

It's always a good idea to use the error message that have
been kindly provided by the implementation rather than
trying to role your own.

Of course. Only that I didn’t know about them. I think I really have
to go and read some tutorials now, before trying to enhance other
people’s code in a language I am not familiar with.

Thank you all for your kind help.

H.
--
Hendrik Maryns
http://tcl.sfs.uni-tuebingen.de/~hendrik/
==================
http://aouw.org
Ask smart questions, get good answers:
http://www.catb.org/~esr/faqs/smart-questions.html


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H

Hendrik Maryns

James Kuyper schreef:
Hendrik said:
Martin Ambuhl schreef:
Hendrik Maryns wrote: ...
mgTreeNode *loadTree(char *file)
{
hcreate(MAX_NODES);

char symbol[MAX_SYMBOL];
char name[MAX_NODENAME];
char left[MAX_NODENAME];
char right[MAX_NODENAME];
ENTRY node;
char* cache[MAX_NODES][3];
size_t nodeCounter=0;
size_t i;

FILE *fp = fopen(file, "r");
Did this fopen succeed? If it did not (and you don't check) the
following fscanf dereferences a null pointer.

This indeed was the problem: I gave the wrong arguments to the debug
program.

How do I properly check whether the open succeeded? Is
if (fp == 0) exit(-2);
enough?

There are only three things you can portably pass to exit():
EXIT_SUCCESS, EXIT_FAILURE (both #defined in <stdlib.h> or 0. Note: not
all programs need to be portable, but you should write non-portable code
only by reason of a deliberate decision, not as a matter of habit.

I totally agree. However, I use the exit value in the calling program
(in Java). And there, 1 has the special meaning: ACCEPT. So I would
not want to return 1, to which EXIT_FAILURE is defined. I guess I will
make it a little bit more non-portable for now, until I read up on JNI
and can avoid using the exit value by calling the relevant functions
directly from my Java program. But I’ll definitely remember this.
There's no guarantee that EXIT_SUCCESS is 0, nor is there any guarantee
that EXIT_SUCCESS is not 0. It can be arguably be inferred that
EXIT_FAILURE must be different from both 0 and EXIT_SUCCESS.

I normally would add a perror(file) call before the exit() call, for
diagnostic purposes, but only if it's acceptable for your program to
send something to stderr.

I would also recommend avoiding direct calls to exit(). In most cases a
subroutine should report a problem to it's caller, allowing the caller
to perform any cleanup that may be needed.

Good point. I’m so used to Java’s automatic garbage collection that I
forgot about this. But even there it is better to throw an exception or
something.
However, if the process is totally dead, won’t the OS reclaim the memory?

Thanks, H.
--
Hendrik Maryns
http://tcl.sfs.uni-tuebingen.de/~hendrik/
==================
http://aouw.org
Ask smart questions, get good answers:
http://www.catb.org/~esr/faqs/smart-questions.html


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J

James Kuyper

Hendrik said:
William Pursell schreef:

What is ‘file’ here? Should I replace it by stderr? (I am doing no
logging to a file, it is really a tiny app.)

The variable named 'file' is the same one here that it was in the call
to fopen().
I get the following compiler error here:

error: ‘errno’ undeclared (first use in this function)

You need to add

#include <errno.h>

before using either perror() or errno.
 
J

James Kuyper

Hendrik said:
James Kuyper schreef: ....

Good point. I’m so used to Java’s automatic garbage collection that I
forgot about this. But even there it is better to throw an exception or
something.
However, if the process is totally dead, won’t the OS reclaim the memory?

Yes, but relying upon that fact is a sign of sloppy programming. There's
often something that can be done about a failure condition that is more
useful than simply calling exit(). Avoiding the use of exit() helps make
it easier for you to identify those situations.
 
C

CBFalconer

Hendrik said:
.... snip ...

I get the following compiler error here:

error: ‘errno’ undeclared (first use in this function)

Just #include <errno.h>. All perfectly standard.
 
C

CBFalconer

Hendrik said:
James Kuyper schreef:
.... snip ...


I totally agree. However, I use the exit value in the calling
program (in Java). And there, 1 has the special meaning: ACCEPT.
So I would not want to return 1, to which EXIT_FAILURE is defined.
I guess I will make it a little bit more non-portable for now,
until I read up on JNI and can avoid using the exit value by
calling the relevant functions directly from my Java program.
But I’ll definitely remember this.

Sounds like IBM hardware. In general the exit status value
returned (in C) is transformed by the run-time into something
suitable for the actual system. Stick with the C standards.
 

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