VERY strange C anomaly

Bernard Liang · *nix forums beginner Joined: 21 Jun 2006 Posts: 4

Consider the following excerpt below, with the included relevant
declarations.

Firstly, the lines marked with **, ***, **** at the beginning are not
supposed to have those stars at the beginning; they are only used to
direct attention.

Suppose, first of all, that the lines with the **** are not present.
p_num is somehow getting changed from its previous value to 0 by the
sscanf command (***) that doesn't even involve p_num. I cannot
understand it, how I could possibly be doing anything that would modify
its value. The lines marked ** were added to determine that that was
indeed the line causing the problem. (the value was 1 before, and 0 after).

I then added the lines marked **** to remedy the problem, but being of
the philosophy that such "cheap fixes" are very poor programming style,
I wanted to see if anyone could shed some insight on this.

Code follows.

int main(int argc, char** argv) {
int pktsize, interval, numgums, p_pktsize, p_interval, p_num;
char trial, p_trial;
int wtf;
char buffer[200];
FILE* fp;
fp = fopen(argv[1], "r");
....

while (fgets(buffer, 160, fp) != NULL) {
**printf("how did p_numgums suddenly change to 0=%d?!?!\n", p_num);
****wtf = p_num;
*** if (sscanf(buffer, "%d %d %d %1s %*d %lf", &pktsize,
&interval, &numgums, &trial, &commrate) == 5) {
****p_num = wtf;
**printf("ok, now what? %d\n", p_num);
printf("from the string |%s", buffer);
printf(" cur data set: pktsize=%d interval=%d num=%d trial=%c cmr=%f\n",
pktsize, interval, numgums, trial, commrate);
printf("prev data set: pktsize=%d interval=%d num=%d trial=%c cmr=%f\n",
p_pktsize, p_interval, p_num, p_trial, p_commrate);

if (count == -1) {
count = 1; ++j;
min = max = runtot = commrate;
} else if ((p_pktsize == pktsize) && (p_interval ==
interval) && (p_num == numgums)) {
++count; ++j;
runtot += commrate;
if (fcompare(commrate, min) < 0) min = commrate;
if (fcompare(max, commrate) < 0) max = commrate;
} else {
printf("%d\t%d\t%d\t%c\t%d\t%f\t%f\t%f\n",
p_pktsize, p_interval, p_num, p_trial, count,
runtot / count, max, min);
count = 1; j = 1;
min = max = runtot = commrate;
}
p_pktsize = pktsize;
p_interval = interval;
printf("numgums was changed from %d", p_num);
p_num = numgums;
printf("to %d\n", p_num);
p_trial = trial;
p_commrate = commrate;
}
}
return 0;
}

The head (-n10) of the file I am feeding it via argv[1] looks likes this:
----
1000 10 1 a 10 91.041515 1500
1000 10 1 b 11 91.036757 1491
1000 10 1 c 12 87.535014 1500
1000 10 3 a 10 51.517332 859
1000 10 3 a 11 54.095827 910
1000 10 3 a 12 52.226982 890
1000 10 3 b 20 37.465007 629
1000 10 3 b 21 47.486709 786
1000 10 3 b 22 49.602106 829
1000 10 3 c 30 36.402570 629
----

Extremely confused,
-Bernard Liang

dcorbit@connx.com · *nix forums addict Joined: 13 Jun 2006 Posts: 92

If you post the actual source you are compiling (and not some fragments) you
can get a more reasonable answer.

For instance, commrate is not defined, but the type informationis crucial to
see if you are overwriting something via a bad sscanf() format specifier.

Ben Pfaff · *nix forums Guru Joined: 08 Apr 2005 Posts: 661

Bernard Liang <b_liang@berkeley.edu> writes:

Bernard Liang · *nix forums beginner Joined: 21 Jun 2006 Posts: 4

asdf, I thought I included everything. I guess not. Well here it is.
there might be some irrelevant decl's b/c I was modifying another script
I had written. but I don't think there's anything substantial that is in
here other than the commrate defn's.

#include <stdio.h>
#include <math.h>
#include <float.h>
#include "gpslib.h"

int main(int argc, char** argv) {
int pktsize, interval, numgums, p_pktsize, p_interval, p_num;
char trial, p_trial;
int wtf;
double commrate, p_commrate;

int count = 1, bufpos = 0, num, i, j; // count = # of gums
recorded, bufpos = position in fgets buffer
double min = DBL_MAX, max = DBL_MIN, runtot = 0.0;
char buffer[200];
char* bufptr;

FILE* fp;

if ((argc != 3) || ((num = atoi(argv[2])) > 10)) {
printf("%s [stat_file] [num (<=10)]\n", argv[0]);
return 1;
}
fp = fopen(argv[1], "r");

count = -1;
j=0;
p_num = -1;

while (fgets(buffer, 160, fp) != NULL) {
printf("how the f*** did p_numgums suddenly change to 0=%d?!?!\n", p_num);
wtf = p_num;
sscanf(buffer, "%d %d %d %1s %*d %lf", &pktsize, &interval, &numgums,
&trial, &commrate);
p_num = wtf;
printf("ok, now what? %d\n", p_num);
printf("from the string |%s", buffer);
printf(" cur dataset: pktsize=%d interval=%d num=%d trial=%c cmr=%f\n",
pktsize, interval, numgums, trial, commrate);
printf("prev dataset: pktsize=%d interval=%d num=%d trial=%c cmr=%f\n",
p_pktsize, p_interval, p_num, p_trial, p_commrate);

if (count == -1) {
count = 1; ++j;
min = max = runtot = commrate;
} else if ((p_pktsize == pktsize) && (p_interval == interval) &&
(p_num == numgums) && (p_trial == trial)) {
++count; ++j;
runtot += commrate;
if (fcompare(commrate, min) < 0) min = commrate;
if (fcompare(max, commrate) < 0) max = commrate;
} else {
printf("%d\t%d\t%d\t%c\t%d\t%f\t%f\t%f\n",
p_pktsize, p_interval, p_num, p_trial, count, runtot / count, max,
min);
count = 1; j = 1;
min = max = runtot = commrate;
}
p_pktsize = pktsize;
p_interval = interval;
printf("numgums was changed from %d", p_num);
p_num = numgums;
printf("to %d\n", p_num);
p_trial = trial;
p_commrate = commrate;

}
printf("%d\t%d\t%d\t%c\t%d\t%f\t%f\t%f\n", p_pktsize, p_interval, p_num,
p_trial, count, runtot / count, max, min);

return 0;
}

Bernard Liang wrote:

Eric Sosman · *nix forums Guru Joined: 21 Feb 2005 Posts: 1246

Bernard Liang wrote On 07/20/06 17:32,:

Chris Torek · *nix forums Guru Joined: 26 Feb 2005 Posts: 481

In article <e9ospl$2h2c$1@geode.berkeley.edu>
Bernard Liang <b_liang@berkeley.edu> wrote:

dcorbit@connx.com · *nix forums addict Joined: 13 Jun 2006 Posts: 92

Using this file:

#include <stdio.h>
#include <math.h>
#include <float.h>
/* #include "gpslib.h" */

int main(int argc, char **argv)
{
int pktsize,
interval,
numgums,
p_pktsize,
p_interval,
p_num;
char trial,
p_trial;
int wtf;
double commrate,
p_commrate;

int count = 1,
bufpos = 0,
num,
i,
j;
double min = DBL_MAX,
max = DBL_MIN,
runtot = 0.0;
char buffer[200];
char *bufptr;

FILE *fp;

if ((argc != 3) || ((num = atoi(argv[2])) > 10)) {
printf("%s [stat_file] [num (<=10)]\n", argv[0]);
return 1;
}
fp = fopen(argv[1], "r");

count = -1;
j = 0;
p_num = -1;

while (fgets(buffer, 160, fp) != NULL) {
printf("how the f*** did p_numgums suddenly change to 0=%d?!?!\n",
p_num);
wtf = p_num;
sscanf(buffer, "%d %d %d %1s %*d %lf", &pktsize, &interval,
&numgums,
&trial, &commrate);
p_num = wtf;
printf("ok, now what? %d\n", p_num);
printf("from the string |%s", buffer);
printf(" cur dataset: pktsize=%d interval=%d num=%d trial=%c
cmr=%f\n",
pktsize, interval, numgums, trial, commrate);
printf("prev dataset: pktsize=%d interval=%d num=%d trial=%c
cmr=%f\n",
p_pktsize, p_interval, p_num, p_trial, p_commrate);

if (count == -1) {
count = 1;
++j;
min = max = runtot = commrate;
} else if ((p_pktsize == pktsize) && (p_interval == interval) &&
(p_num == numgums) && (p_trial == trial)) {
++count;
++j;
runtot += commrate;
if (fcompare(commrate, min) < 0)
min = commrate;
if (fcompare(max, commrate) < 0)
max = commrate;
} else {
printf("%d\t%d\t%d\t%c\t%d\t%f\t%f\t%f\n",
p_pktsize, p_interval, p_num, p_trial, count, runtot /
count, max,
min);
count = 1;
j = 1;
min = max = runtot = commrate;
}
p_pktsize = pktsize;
p_interval = interval;
printf("numgums was changed from %d", p_num);
p_num = numgums;
printf("to %d\n", p_num);
p_trial = trial;
p_commrate = commrate;

}
printf("%d\t%d\t%d\t%c\t%d\t%f\t%f\t%f\n", p_pktsize, p_interval, p_num,
p_trial, count, runtot / count, max, min);

return 0;
}

Lint said:
--- Module: tt.c
_
if ((argc != 3) || ((num = atoi(argv[2])) > 10)) {
tt.c(33) : Error 1055: atoi undeclared, assumed to return int
tt.c(33) : Info 746: call to atoi() not made in the presence of a prototype
_
while (fgets(buffer, 160, fp) != NULL) {
tt.c(43) : Warning 668: Possibly passing a null pointer to function
fgets(char
*, int, struct _iobuf *), arg. no. 3
tt.c(43) : Warning 668: Possibly passing a null pointer to function
fgets(char
*, int, struct _iobuf *), arg. no. 3
_
p_pktsize, p_interval, p_num, p_trial, p_commrate);
tt.c(54) : Warning 530: p_pktsize (line 11) not initialized
tt.c(54) : Warning 530: p_interval (line 12) not initialized
tt.c(54) : Warning 530: p_trial (line 15) not initialized
tt.c(54) : Warning 530: p_commrate (line 1 Cool

not initialized
_
if (fcompare(commrate, min) < 0)
tt.c(65) : Error 1055: fcompare undeclared, assumed to return int
tt.c(65) : Info 746: call to fcompare() not made in the presence of a
prototype
_
printf("%d\t%d\t%d\t%c\t%d\t%f\t%f\t%f\n", p_pktsize, p_interval, p_num,
tt.c(86) : Warning 668: Possibly passing a null pointer to function
fgets(char
*, int, struct _iobuf *), arg. no. 3
_
}
tt.c(90) : Warning 550: num (line 22) not accessed
tt.c(90) : Warning 529: i (line 23) not subsequently referenced
tt.c(90) : Warning 550: j (line 24) not accessed
tt.c(90) : Warning 529: bufpos (line 21) not subsequently referenced
tt.c(90) : Warning 529: bufptr (line 29) not subsequently referenced

--- Wrap-up for Module: tt.c

Info 766: Header file 'C:\lang\VC98\include\math.h' not used in module
'tt.c'
Error 305: Unable to open module: tt.c0

And Splint said:
Splint 3.0.1.6 --- 11 Feb 2002

tt.c: (in function main)
tt.c(43,31): Possibly null storage fp passed as non-null param: fgets (...,
fp)
A possibly null pointer is passed as a parameter corresponding to a formal
parameter with no /*@null@*/ annotation. If NULL may be used for this
parameter, add a /*@null@*/ annotation to the function parameter
declaration.
(Use -nullpass to inhibit warning)
tt.c(37,10): Storage fp may become null
tt.c(46,9): Return value (type int) ignored: sscanf(buffer, "...
Result returned by function call is not used. If this is intended, can
cast
result to (void) to eliminate message. (Use -retvalint to inhibit warning)
tt.c(54,16): Variable p_pktsize used before definition
An rvalue is used that may not be initialized to a value on some execution
path. (Use -usedef to inhibit warning)
tt.c(54,27): Variable p_interval used before definition
tt.c(54,46): Variable p_trial used before definition
tt.c(54,55): Variable p_commrate used before definition
tt.c(65,17): Unrecognized identifier: fcompare
Identifier used in code has not been declared. (Use -unrecog to inhibit
warning)
tt.c(86,4 Cool

: Variable p_pktsize used before definition
tt.c(86,59): Variable p_interval used before definition
tt.c(87,12): Variable p_trial used before definition
tt.c(21,21): Variable bufpos declared but not used
A variable is declared but never used. Use /*@unused@*/ in front of
declaration to suppress message. (Use -varuse to inhibit warning)
tt.c(23,21): Variable i declared but not used
tt.c(29,21): Variable bufptr declared but not used

Finished checking --- 13 code warnings

Ben Pfaff · *nix forums Guru Joined: 08 Apr 2005 Posts: 661

Eric Sosman <Eric.Sosman@sun.com> writes:

Barry Schwarz · *nix forums Guru Joined: 09 Apr 2005 Posts: 390

On Thu, 20 Jul 2006 14:48:18 -0700, Bernard Liang
<b_liang@berkeley.edu> wrote:

Mark McIntyre · *nix forums Guru Joined: 16 Feb 2005 Posts: 1564

On Thu, 20 Jul 2006 14:48:18 -0700, in comp.lang.c , Bernard Liang
<b_liang@berkeley.edu> wrote:

Google