fseek versus lseek

[dickiebird]

Hi
I'm having problems fseeking on a binary file.
I tried lseek with similar problems.
Fseek works for about 200 records then I get :
16846 Memory fault(coredump)
Lseek works for 1 record and gives same problem
Relevant code is :

long pos,ddd;
char buff[40];
FILE *fd = fopen("fileix20","rb"
while(fgets(buff,40,stdin))
{
pos = strtoul(buff,NULL,10) - 10;
printf("Trace A\n"
ddd=lseek(fd,pos,0);
printf("Trace B ddd=%d\n",ddd);
}

With fseek:
Trace A and B show 200 times
Trace B always prints a value ( = offset position )
but doesn't print after failure

With lseek
Trace A shows 2 times
Trace B has a value = -1
then doesn't show after failure

Apart from my memory fault, what is the difference between
lseek & fseek anyway. I've read the manuals, but can't appreciate any difference.
Any ideas would be welcome
TIA ;-) Dickie Bird
Honi soit qui mal y pense

 

[xwb]

fseek is ANSI, lseek isn't. lseek exists in lots of the older libraries before ANSI fseek became the standard.

 

[hnd]

fseek works with file pointers and lseek with file descriptors.

hnd
hasso55@yahoo.com

 

[jamisar]

Fseek() need a FILE *fd;
Lseek() need an int fd;
---------------------------
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#define MAX 40

main()
{
long pos,ddd;
char buff[MAX+1];

/***REM FILE *fd = fopen(&quot;fileix20&quot;,&quot;rb&quot; ***/

int fd = open(&quot;fileix20&quot;,O_RDONLY);

if(0 >fd) exit(1);

while(fgets(buff,MAX,stdin))
{
pos = strtoul(buff,(char *)NULL,10) - 10;
if(0 >pos ) break;
printf(&quot;Trace A %d\n&quot;,pos);
ddd = lseek(fd,pos,SEEK_SET);
printf(&quot;Trace B ddd=%d\n&quot;,ddd);
}
close(fd);
exit(0);
}
-----------
when they don't ask you anymore, where they are come from, and they don't tell you anymore, where they go ... you'r getting older !

 

[dickiebird]

Hmmmmm
I changed the FIle *fd to int fd;
and opened as suggested above, but now, when I try
getc(fd) - my compiler objects :
/*FILE *fd = fopen(&quot;fileix20&quot;,&quot;rb&quot;*/
int fd = open(&quot;fileix20&quot;,O_RDONLY);
while(fgets(buff,23,stdin))
{
pos = strtoul(buff,NULL,10) - 10;
if(0 >pos ) break;
ddd=lseek(fd,pos,0);
if(0 >ddd ) break;
memset(buff2,'\0',21);
for(nnn=0;nnn<20;nnn++) buff2[nnn]=getc(fd);
printf(buff2);
}

&quot;utget_write.c&quot;, line 64.42: 1506-122 (S) Expecting pointer to struct or union.
Can't I use getc now ?
Dickie Bird
Honi soit qui mal y pense

 

[jamisar]

getc() needs a FILE *fd
-----------
when they don't ask you anymore, where they are come from, and they don't tell you anymore, where they go ... you'r getting older !

 

[jamisar]

i prefer 'read()' -----------
when they don't ask you anymore, where they are come from, and they don't tell you anymore, where they go ... you'r getting older !

 

[dickiebird]

Ok - The story :
I need to examine and recreate data from a file with binary content. I can locate strings within the file and work out a position to start from, to load up a buffer with various strings, floats, ints etc. I will eventually use the output as the basis for a replacement for the original file.
All works well for the 1st 200 records, then I get a memory fault.
I can't post the data, its got 80% binary info, which will not show too well. There are null bytes too.
Here's the whole code :
#include <stdio.h>
#include <stdlib.h>
#include &quot;/usr/cg-index/include/ciutil.h&quot;
#include <sys/types.h>
#include <unistd.h>

static double to_power_of(double val, int power)
{
double num;
int c;

num = 1;
for (c=0; c < power; c++)
{
num = num * val;
}

return(num);
}


double cvt_bytes_to_num(char *value)
{
double num;
int loop;
int power = 5;
num = 0.0;

for (loop = 0; loop != 6; loop++)
num += (double)(value[loop] * to_power_of(2, 8 * (power--)));
return (num);
}


int main(int argc, char **argv)
{
int nnn;
ulong pos;
ulong rpos;
ulong chqno;
double dchqno;
int range;
int curiss;
char buff[24];
char fuff[21];
char nuff[21];
char fluff[30];
FILE *fd = fopen(&quot;lrlaix20&quot;,&quot;rb&quot;
/*int fd = open(&quot;lrlaix20&quot;,O_RDONLY);*/
FILE *fout = fopen(&quot;lrlamn20&quot;,&quot;w&quot;
while(fgets(buff,23,stdin))
{
pos = strtoul(buff,NULL,10) - 10;
fseek(fd,pos,0);
memset(fuff,'\0',21);
for(nnn=0;nnn<20;nnn++) fuff[nnn]=getc(fd);
if(fuff[0]=='ÿ') memcpy(nuff,fuff+1,20); else memcpy(nuff,fuff,20);
if((nuff[0]==0)&&(nuff[1]==0)) memcpy(fuff,nuff+1,20); else memcpy(fuff,nuff,20);
rpos=c_to_l(fuff+16,0);
curiss=c_to_s(fuff,0);
range=c_to_s(fuff+8,0);
if(curiss%10!=6)
{
chqno=c_to_l(fuff+4,0);
sprintf(fluff,&quot;%3d%c%c%09ld%04d%c%c%c%c%c%c%5d\n&quot;,curiss,fuff[2],fuff[3],chqno,range,fuff[
10],fuff[11],fuff[12],fuff[13],fuff[14],fuff[15],rpos);
}
else
{
dchqno=cvt_bytes_to_num(fuff+2);
sprintf(fluff,&quot;%3d|%013.0f|%04d|%c%c%c%c%c%c|%5d\n&quot;,curiss,dchqno,range,fuff[10],fuff[11],
fuff[12],fuff[13],fuff[14],fuff[15],rpos);
}

printf(fluff);
/*fwrite(fout,21,1,fluff);*/
}
fclose(fd);
fclose(fout);
}
here's the last lines of my output :

fuff=|0|110|73|84|2|103|65|126|0|9|83|48|51|52|32|32|0|0|41|10
F
G
H
110IT0403214060009S034 10506
A
C
D
E
fuff=|0|116|5|8|5|225|129|148|0|99|52|48|51|50|49|32|0|0|41|0
F
G
H
116|5532016542100|0099|40321 |10496
A
ksh: 17214 Memory fault(coredump)

Shows last 2 records processed OK, then start on the next (where I display A) and it falls over.
Now I'm stuck !!!!
;-)
Dickie Bird
Honi soit qui mal y pense

 

[jamisar]

you are PROBABLY blowing up your double vars.
use dc|bc
i posted in this forum: pipe to dc (using c)
i will read your code this weekend.

-----------
when they don't ask you anymore, where they are come from, and they don't tell you anymore, where they go ... you'r getting older !

 

[jamisar]

a first look:


for(nnn=0;nnn<20;nnn++) fuff[nnn]=getc(fd);
if(fuff[0]=='ÿ') memcpy(nuff,fuff+1, 20 );

should not be

for(nnn=0;nnn<20;nnn++) fuff[nnn]=getc(fd);
if(fuff[0]=='ÿ') memcpy(nuff,fuff+1, 19 );

??
post also the c_to_[ls]() functions.
-----------
when they don't ask you anymore, where they are come from, and they don't tell you anymore, where they go ... you'r getting older !

 

[jamisar]

i don't understand, this code should crash on the fist

sprintf(fluff,&quot;%3d| %013.0f |%04d|%c%c%c%c%c%c|%5d\n&quot;,curiss, dchqno ,range,fuff[10],....

dchqno is a double !!! not a float -----------
when they don't ask you anymore, where they are come from, and they don't tell you anymore, where they go ... you'r getting older !

 

[dickiebird]

Thanks for your efforts, Jamisar
Here's the c_to_l/s code

Name:
long c_to_l( rec, cindex )
Parameters:
char *rec - char array to be converted into long.
int cindex - If true, long stored in char arrayis in cindex
format
Purpose:
Converts a char array into a long. If the IEEE format is
defined the top bit ( i.e. the bit that indicates the sign )
is toggled. This results in a byte by byte comparison of two
converted longs (one negative and one positive ) to return
the same result as if the two longs were compared.
Returns:
Long.
--------------------------------------------------------------------*/
long c_to_l( rec, cindex )
unsigned char *rec;
int cindex;
{
static char c[sizeof(long)];
static unsigned long l;
unsigned char t;
register int ii;

memcpy( c, rec, sizeof(long) );

if ( cindex )
{
/* a long integer is 4 bytes when stored in the file */
#ifdef IEEE

c[0] = ( c[0] & 0x80 ) ? (c[0] & 0x7F) : (c[0] | 0x80) ;

#ifdef RIEEE
t = c[0]; c[0] = c[3]; c[3] = t;
t = c[1]; c[1] = c[2]; c[2] = t;
#endif

memcpy( &l, c, 4);
#else
/*
Generalized number for which we do not know the
machine bit pattern.
*/

l = ( c[0] & 0x80 ) ? (c[0] & 0x7F) : -(c[0] & 0x7F);

for (ii = 1; ii < 4; )
{
l = l * 0x100 + c[ii++];
}
#endif
}
else
{
l = c[0];

for (ii = 1; ii < sizeof(long); ++ii )
{
l = l * 0x100 + c[ii];
}
}

return( (long)l );
}
/*--------------------------------------------------------------------
Name:
short c_to_s( c, cindex )
Parameters:
char *c - char array to be converted into long.
int cindex - If true, short stored in char array is in cindex
format
Purpose:
Converts a char array into a short. If the IEEE format is
defined the top bit ( i.e. the bit that indicates the sign )
is toggled. This results in a byte by byte comparison of two
converted shorts (one negative and one positive ) to return
the same result as if the two shorts were compared.
Returns:
Short.
--------------------------------------------------------------------*/
short c_to_s( rec, cindex )
unsigned char *rec;
int cindex;
{
char c[sizeof(short)];
static unsigned short s;
unsigned char t;
register int ii;


memcpy( c, rec, sizeof(short) );

if ( cindex )
{
/* a short integer is 2 bytes when stored in the file */
#ifdef IEEE
c[0] = ( c[0] & 0x80 ) ? c[0] & 0x7F : c[0] | 0x80;

#ifdef RIEEE
t = c[0]; c[0] = c[1]; c[1] = t;
#endif

memcpy( &s, c, 2 );
#else
/*
Generalized number for which we do not know the
machine bit pattern.
*/

s = ( c[0] & 0x80 ) ? (c[0] & 0x7F) : -(c[0] & 0x7F);
s = s * 0x100 + c[1];
#endif
}
else
{
s = c[0];

for (ii = 1; ii < sizeof(short); ++ii )
{
s = s * 0x100 + c[ii];
}
}

return( (short)s );
}

Though, as my trace seems to show, its the fseek that appears to be failing.
I'm losing it with this project ! ;-( Dickie Bird
Honi soit qui mal y pense

 

[jamisar]

forget last remarque. -----------
when they don't ask you anymore, where they are come from, and they don't tell you anymore, where they go ... you'r getting older !

 

[jamisar]

xwb: about ANSI or not ANSI, lock at the fseek-code
included below )

dbird: i compiled your code, it's ok, questions:
how big is the input 'irlaix20'
exec a ls -l && wc on it
can you post lines 190-210 of the inputfile 'stdin'
for testing: can you e-mail the first 2-3000 chars
of the 'irlaix20'
my be you have to split it
to: priam@freenet.de
xwb -------------- fseek.c
/*LINTLIBRARY*/
/*
* Seek for standard library. Coordinates with buffering.
*/
#include <stdio.h>

extern long lseek();
/* aaaaaaannnnnnnssssssiiiii JAMISAR dixit*/
extern int fflush();

int
fseek(iop, offset, ptrname)
register FILE *iop;
long offset;
int ptrname;
{
register int resync, c;
long p = -1; /* can't happen? */

iop->_flag &= ~_IOEOF;
if(iop->_flag & _IOREAD) {
if(ptrname < 2 && iop->_base && !(iop->_flag&_IONBF)) {
c = iop->_cnt;
p = offset;
if(ptrname == 0) {
long curpos = lseek(fileno(iop), 0L, 1);
/* aaaaaaannnnnnnssssssiiiii JAMISAR dixit*/
if (curpos == -1)
return (-1);
p += c - curpos;
resync = offset&01;
} else {
offset -= (long)c;
resync = 0;
}
if(!(iop->_flag&_IORW) && c > 0 && p <= c &&
p >= iop->_base - iop->_ptr) {
iop->_ptr += (int)p;
iop->_cnt -= (int)p;
return(0);
}
} else
resync = 0;
if(iop->_flag & _IORW) {
iop->_ptr = iop->_base;
iop->_flag &= ~_IOREAD;
resync = 0;
}
p = lseek(fileno(iop), offset-resync, ptrname);
/* aaaaaaannnnnnnssssssiiiii JAMISAR dixit*/
iop->_cnt = 0;
if (resync && p != -1)
if (getc(iop) == EOF)
p = -1;
} else if(iop->_flag & (_IOWRT | _IORW)) {
p = fflush(iop);
iop->_cnt = 0;
if(iop->_flag & _IORW) {
iop->_flag &= ~_IOWRT;
iop->_ptr = iop->_base;
}
return(lseek(fileno(iop), offset, ptrname) == -1 || p == EOF ?
/* aaaaaaannnnnnnssssssiiiii JAMISAR dixit*/
-1 : 0);
}
return((p == -1)? -1: 0);
}

/* aaaaaaannnnnnnssssssiiiii JAMISAR dixit ))))*/
-----------
when they don't ask you anymore, where they are come from, and they don't tell you anymore, where they go ... you'r getting older !

 

[dickiebird]

Jamisar
Found the error - char fluff[30] wasn't big enough for the sprintf !!!!!!!!!!!
Gross mistake on my part !!
Sorry to waste so much of your time.
Regards Dickie Bird
Honi soit qui mal y pense