segmentation fault with gets()

[Peter Dragun]

I am generally new to programming under Unix, but know how to code under
Windows. I have to create a simple program, that takes the following
information from a file using redirection:

4 4 4 4 4 4 5 5 5 5 5 5 3 3 3 3 3 3 3 3 2 2 2 10 10 10 1 1
3 3 3 3 3 3 3 3 3 2 2 2 12 12 12 1 7 7 7 7 4
12 13 13 5 5 5 3 3 9 7 7 7 5 4 4 4 3 3 3 3

read each line, and "compress the information" to the following out print to
be printed on screen:
6 4
6 5
8 3
3 2
3 10
2 1
9 3
3 2
3 12
4 7
1 4
1 12
2 13
3 5
2 3
1 9
3 7
1 5
3 4
4 3

So the first column counts the occurences, and the second column in the
integer occurence.
I keep on getting a segmentation fault when I run my program, here is the
source code:

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

/* NUMBER, and COUNT are used in numbers array, indicating what is stored
*where
*/
#define NUMBER 0
#define COUNT 1

/* boolean definitions*/
#define TRUE 1
#define FALSE 0

/*flag to make sure first entry is processes properly*/
int first_run = TRUE;

/* next=store the next value;
* prev= stores the previous value from the stream;
* count=keep track how many times the same number is in sequence;
* incr_x = keeps track where to store the numbers in the numbers array;
* incr = used to display the numbers array, when encoding is complete.
*/
int next, prev, count, incr_x, incr;

/* Stream string to store input to be encoded*/
char stream[900] = "4 4 4 4 4 2 2 6 6 6 6 6 6 6 6 6 4 4 4 4 4 10 10 10 7 7 7
7 7 4 4";

/* Special delimiter to differient between values, this array is used with
* strtok function, the delimiter is set to whitespaces, so when a
whitespace
* is found, it indicates that the next element is a number.
*/
char delimiters[2] = " ";

/* Token pointer stores the outcome created by strtok*/
char *token;

/* The numbers array stores the numbers and count of them
* A number is stored using numbers[incr][NUMBERS]=n, and the count of that
* number found in the sequence i stored by numbers[incr][COUNT]=c
*/
int numbers[100][2];

int main() {
/* Standard input, sending data to the steam array for processing.*/

// This is where i get the fault, the program works, if I take out
the while, and just use the gets, but it only gets one line from the
// file.

while( gets( stream ) != NULL ) {
/* Get the first element that is delimited by whitespace. */
token = strtok( stream, delimiters );

do {
next = atoi( token );

/* If it is a first run, their is not previous value, so we
* assign prev with next.
*/
if( first_run ) {
prev = next;
first_run = FALSE;
}

/* If next is equal to the previous value, we increment the
count.*/
if( next == prev ) {
count++;

/* If the next value does not equal the previous,
that indicates a new
* number was found in the stream, so we insert the previous
value
* into the numbers array, and store how many times it was found
* sequentially, and then we reset the count, and increment
the numbers
* array for the next value.
*/
} else {
numbers[incr_x][NUMBER] = prev;
numbers[incr_x][COUNT] = count;
count = 1;
incr_x++;
}
prev = next;
/* We stop when the stream is finished.*/
} while( ( token = strtok( NULL,delimiters ) ) != NULL );

/* Since the strtok ends the do.while loop, before we can
process the
* last number found, we do it here.
*/
numbers[incr_x][NUMBER] = next;
numbers[incr_x][COUNT] = count;
incr_x ++;
}
/* Print out our results. */
for( incr=0; incr < incr_x; incr++ ) {
printf( "%d %d\n", numbers[incr][COUNT],
numbers[incr][NUMBER] );
}
return 0;
}

I wrote a comment where, I think the fault is occurring. Essentially, the
instructions told us we could only use scanf and printf for our I/O, but I
couldn't see how to use scanf to get what I want, so I decided to side line
one spec, and use gets(). With that in mind, I am limited to the use of
functions, and do not want to stray further from using gets or scanf.

Thanks

 

[Derk Gwen]

# /* Standard input, sending data to the steam array for processing.*/
#
# // This is where i get the fault, the program works, if I take out
# the while, and just use the gets, but it only gets one line from the
# // file.
#
# while( gets( stream ) != NULL ) {

Have you verified the input doesn't overflow the buffer? There's a reason
most people used fgets instead.

 

[Peter Dragun]

I have been reading upon, how dangerous gets() is, could you help me provide
an alternative using scanf or fgets(without reading a file, but still using
the stdin)?

 

[Irrwahn Grausewitz]

I have been reading upon, how dangerous gets() is, could you help me provide
an alternative using scanf or fgets(without reading a file, but still using
the stdin)?

<snip>

....
#define BUFLEN 900
....
char stream[BUFLEN];
....
while( fgets( stream, BUFLEN, stdin ) {
....

HTH

Regards

 

[Richard Heathfield]

I have been reading upon, how dangerous gets() is, could you help me
provide an alternative using scanf or fgets(without reading a file, but
still using the stdin)?

/* untested code - beware! */

#include <stdio.h>
#include <string.h>

int chop(char *s)
{
int chopped = 0;
char *p = strchr(s, '\n');
if(p != NULL)
{
*p = '\0';
chopped = 1;
}
return chopped;
}

#define SOME_SIZE_OR_OTHER 32

int main(void)
{
char buf[SOME_SIZE_OR_OTHER] = {0};

if(fgets(buf, sizeof buf, stdin) != NULL)
{
if(chop(buf))
{
printf("The string is [%s]\n", buf);
}
else
{
printf("The string was a tad long. Here's "
"some of it: [%s]\n", buf);
if(fgets(buf, sizeof buf, stdin) != NULL)
{
printf("Here's some more: %s\n", buf);
}
}
}
else
{
printf("EOF or error encountered.\n");
if(ferror(stdin))
{
printf("Error.\n");
}
else
{
printf("EOF.\n");
}
}
return 0;
}

 

[Richard Heathfield]

...
#define BUFLEN 900
...
char stream[BUFLEN];
...
while( fgets( stream, BUFLEN, stdin ) {

Better than BUFLEN: sizeof stream

 

[Irrwahn Grausewitz]

...
#define BUFLEN 900
...
char stream[BUFLEN];
...
while( fgets( stream, BUFLEN, stdin ) {

Better than BUFLEN: sizeof stream

Indeed.

 

[Irrwahn Grausewitz]

I am generally new to programming under Unix, but know how to code under
Windows. I have to create a simple program, that takes the following
information from a file using redirection:

read each line, and "compress the information" to the following out print to
be printed on screen:

<snip>

/* If this is homework, don't read any further!!!!
*
*
*
*
*
*
*
*
*
*
*
*
*
*/

#include <stdio.h>

int main( void )
{
int n = 0,
nn = 0,
cnt = 0,
run = 1;

while( run )
{
if ( scanf( "%d", &n ) != 1 )
run = 0;

if ( n != nn || !run )
{
if ( cnt )
printf( "%d %d\n", cnt, nn );
nn = n;
cnt = 1;
}
else
++cnt;
}
return 0;
}

Regards

 

[Joe Wright]

...
#define BUFLEN 900
...
char stream[BUFLEN];
...
while( fgets( stream, BUFLEN, stdin ) {

Better than BUFLEN: sizeof stream

Also better would be ...
while( fgets( stream, sizeof stream, stdin )) {}
so that the right paren doesn't get lonely.

 

[CBFalconer]

I am generally new to programming under Unix, but know how to
code under Windows. I have to create a simple program, that takes
the following information from a file using redirection:

4 4 4 4 4 4 5 5 5 5 5 5 3 3 3 3 3 3 3 3 2 2 2 10 10 10 1 1
3 3 3 3 3 3 3 3 3 2 2 2 12 12 12 1 7 7 7 7 4
12 13 13 5 5 5 3 3 9 7 7 7 5 4 4 4 3 3 3 3

read each line, and "compress the information" to the following
out print to be printed on screen:
6 4
6 5
8 3
3 2
3 10
2 1
8 3
.... snip ...

So the first column counts the occurences, and the second column
in the integer occurence.
I keep on getting a segmentation fault when I run my program,
here is the source code:
.... snip code using gets ...

I wrote a comment where, I think the fault is occurring.
Essentially, the instructions told us we could only use scanf
and printf for our I/O, but I couldn't see how to use scanf to
get what I want, so I decided to side line one spec, and use
gets(). With that in mind, I am limited to the use of functions,
and do not want to stray further from using gets or scanf.

Bad decision. Your instructor was leading you in the right
direction. Think about what scanf does ... it extracts a number
(or other things) from an incoming stream. It requires no extra
buffering of the incoming stream, and will skip all white space
while extracting numbers. It also tells you if it succeeded. So,
the phrase:

int anint;
....
if (1 == scanf(stdin, "%d", &anint)) {
/* whatever */
}

will extract a number and tell you if it succeeded. You can also
modify that into a while statement. Now think about how you can
tell if this is the same number as the previous one, and what to
do if it is, or if it isn't. Then think about how to get the
process started. You may well find you need a few extra variables
and initialization.

When you are finished you will be astounded at how simple it is.

 

[Chris Torek]

... I have to create a simple program, that takes the following
information from a file using redirection:

4 4 4 4 4 4 5 5 5 5 5 5 3 3 3 3 3 3 3 3 2 2 2 10 10 10 1 1
3 3 3 3 3 3 3 3 3 2 2 2 12 12 12 1 7 7 7 7 4
12 13 13 5 5 5 3 3 9 7 7 7 5 4 4 4 3 3 3 3

read each line, and "compress the information" ...

[in short, using a form of Run-Length Encoding; the output is
(N,value) pairs representing N instances of the given value].

Without answering questions about gets() (others have done that
already), I will note that the first thing I thought about when
I read the above is:

What if the input has the same integer value split across
several lines?

For instance, if the input is:

4
4
4 5
5 101 7

should the output be:

1 4
1 4
1 4
1 5
1 101
1 7

or should it be:

3 4
2 5
1 101
1 7

?

Later, you note:

the instructions told us we could only use scanf ...

which suggests (but does not say for certain) that the answer is
the latter. In particular, if you use scanf's "%d" format, the
scanf engine -- the code shared by scanf, fscanf, sscanf, and in
C99 the various vscanf routines -- will skip "white space", and
newlines are considered white space. Hence a loop of the form:

while (scanf("%d", &var) == 1)

will eat right through those newlines, producing the second kind
of output. (Note that scanf() will "jam" if it hits a non-numeric,
non-white-space character, so this kind of loop is certainly not
"robust" against malformed input. As a result you might actually
do better, in the end, using fgets(), unless of course you are Dan
Pop. )

(The reason I say "does not say for certain" is that if you use
scanf with "%c" formats, and "manual" conversion of integer values
one digit at a time, you will be able to "see" the newlines in the
input stream. This again gives you the choice of whether to consider
newlines significant. Note, however, that the first form of output
does *not* allow you to reconstruct the original set of newlines.
If the RLE-output says "N1 X" followed by "N2 X" -- i.e., if the
value being run-length encoded repeats -- then you know for certain
there was a newline between these, but if the value after N2 is
not X, you do not know whether there was a newline there. That
makes it hard for me to imagine why anyone would *want* the first
form of output.)