why doesn't this code work? :)

[buda]

All error checking is removed for clarity. This turns into a infinite loop
that writes at the end of a file, and makes it grow until a "forced brake".
Thanks for the help (btw, both stdio.h and stdlib.h are included)

int zamijeni (char *ime_datoteke) {
FILE *fp;
int t, flag = 0;

fp = fopen( ime_datoteke, "r+b" );
while ( fread( (void *)&t, (long)sizeof( t ), 1, fp ) == 1 ) {
if ( t < 0 ) {
t = -t;
++flag;
fseek( fp, -1L * (long)sizeof( t ), SEEK_CUR );
fwrite( (void *)&t, (long)sizeof( t ), 1, fp );
}
}
return flag > 0;
}

 

[Luis Diego]

char *lineptr[MAXLINE];

With this declaration you have a pointer to an array and each field of the
array is an "char *"

The problem is that each field in lineptr array must be allocated (because
they are pointers).
So you need to change your code:

char *lineptr[MAXLINE];

...
...
...

for (i=0; fgets(line, LINESIZE, fp) != NULL ; i++) {
printf("%s",line);
/* here i would like to do something */
/* like */
/* strcpy(lineptr, line); */
/* or */
/* lineptr = line; */
}

char lineptr[MAXLINE][LINESIZE];

for (i=0; fgets(line, LINESIZE, fp) != NULL ; i++)
{
printf("%s",line);
strcpy(lineptr, line);

/*The following commente line can't be used, would be a big mistake*/
/* lineptr = line; */
}

I don't promise it would works AS IS.

I strongly recommend "The C Programming Language" book from BW Kernighan and
DM Ritchie, great book to start with C.


Ing. Luis Diego Bolanos Quiros
NPY & PE Dept.
(e-mail address removed)
Inet 8-256-6536
Office(506) 298-8939
Cell (506) 817-6566

 

[Luis Diego]

sorry I make a mistake, this is for another Post

: D

char *lineptr[MAXLINE];

With this declaration you have a pointer to an array and each field of the
array is an "char *"

The problem is that each field in lineptr array must be allocated (because
they are pointers).
So you need to change your code:

char *lineptr[MAXLINE];

...
...
...

for (i=0; fgets(line, LINESIZE, fp) != NULL ; i++) {
printf("%s",line);
/* here i would like to do something */
/* like */
/* strcpy(lineptr, line); */
/* or */
/* lineptr = line; */
}

char lineptr[MAXLINE][LINESIZE];

for (i=0; fgets(line, LINESIZE, fp) != NULL ; i++)
{
printf("%s",line);
strcpy(lineptr, line);

/*The following commente line can't be used, would be a big mistake*/
/* lineptr = line; */
}

I don't promise it would works AS IS.

I strongly recommend "The C Programming Language" book from BW Kernighan and
DM Ritchie, great book to start with C.


Ing. Luis Diego Bolanos Quiros
NPY & PE Dept.
(e-mail address removed)
Inet 8-256-6536
Office(506) 298-8939
Cell (506) 817-6566

All error checking is removed for clarity. This turns into a infinite loop
that writes at the end of a file, and makes it grow until a "forced brake".
Thanks for the help (btw, both stdio.h and stdlib.h are included)

int zamijeni (char *ime_datoteke) {
FILE *fp;
int t, flag = 0;

fp = fopen( ime_datoteke, "r+b" );
while ( fread( (void *)&t, (long)sizeof( t ), 1, fp ) == 1 ) {
if ( t < 0 ) {
t = -t;
++flag;
fseek( fp, -1L * (long)sizeof( t ), SEEK_CUR );
fwrite( (void *)&t, (long)sizeof( t ), 1, fp );
}
}
return flag > 0;
}

 

[Francois Grieu]

All error checking is removed for clarity. This turns into a infinite loop
that writes at the end of a file, and makes it grow until a "forced brake".
Thanks for the help (btw, both stdio.h and stdlib.h are included)

int zamijeni (char *ime_datoteke) {
FILE *fp;
int t, flag = 0;

fp = fopen( ime_datoteke, "r+b" );
while ( fread( (void *)&t, (long)sizeof( t ), 1, fp ) == 1 ) {
if ( t < 0 ) {
t = -t;
++flag;
fseek( fp, -1L * (long)sizeof( t ), SEEK_CUR );
fwrite( (void *)&t, (long)sizeof( t ), 1, fp );
}
}
return flag > 0;
}

Could the fwrite fail ? e.g. file is locked, on read-only media..

Also: with a C compiler, you can remove (void *) and (long);
and there is a risk that flag overflows to 0.


/* return -1 if error, 1 if negatives found and fixed, else 0 */
int zamijeni (char *ime_datoteke) {
FILE *fp;
int t, flag = 0;

if ((fp = fopen( ime_datoteke, "r+b"))!=NULL)
return -1;
while ( fread(&t, sizeof t, 1, fp )==1) {
if ( t < 0 ) {
t = -t;
flag = 1;
if (fseek( fp, -sizeof t, SEEK_CUR )!=0
|| fwrite(&t, sizeof t, 1, fp )!=1)
return -1;
}
if (fclose(fp)!=0)
return -1;
}
return flag;
}


François Grieu

 

[Francois Grieu]

All error checking is removed for clarity. This turns into a infinite loop
that writes at the end of a file, and makes it grow until a "forced brake".
Thanks for the help (btw, both stdio.h and stdlib.h are included)

int zamijeni (char *ime_datoteke) {
FILE *fp;
int t, flag = 0;

fp = fopen( ime_datoteke, "r+b" );
while ( fread( (void *)&t, (long)sizeof( t ), 1, fp ) == 1 ) {
if ( t < 0 ) {
t = -t;
++flag;
fseek( fp, -1L * (long)sizeof( t ), SEEK_CUR );
fwrite( (void *)&t, (long)sizeof( t ), 1, fp );
}
}
return flag > 0;
}

Could the fwrite fail ? e.g. file is locked, on read-only media..

Also: with a C compiler, you can remove (void *) and (long);
and there is a risk that flag overflows to 0.


/* return -1 if error, 1 if negatives found and fixed, else 0 */
int zamijeni (char *ime_datoteke) {
FILE *fp;
int t, flag = 0;

if ((fp = fopen( ime_datoteke, "r+b"))==NULL)
return -1;
while ( fread(&t, sizeof t, 1, fp )==1) {
if ( t < 0 ) {
t = -t;
flag = 1;
if (fseek( fp, -sizeof t, SEEK_CUR )!=0
|| fwrite(&t, sizeof t, 1, fp )!=1)
return -1;
}
if (fclose(fp)!=0)
return -1;
}
return flag;
}


François Grieu

 

[buda]

Could the fwrite fail ? e.g. file is locked, on read-only media..

Also: with a C compiler, you can remove (void *) and (long);
and there is a risk that flag overflows to 0.


/* return -1 if error, 1 if negatives found and fixed, else 0 */
int zamijeni (char *ime_datoteke) {
FILE *fp;
int t, flag = 0;

if ((fp = fopen( ime_datoteke, "r+b"))==NULL)
return -1;
while ( fread(&t, sizeof t, 1, fp )==1) {
if ( t < 0 ) {
t = -t;
flag = 1;
if (fseek( fp, -sizeof t, SEEK_CUR )!=0
|| fwrite(&t, sizeof t, 1, fp )!=1)
return -1;
}
if (fclose(fp)!=0)
return -1;
}
return flag;
}


François Grieu

No, none of the file functions ever fails (the running code has a bunch of
error checking, but it has no effect; also, i ran it trough the debugger and
saw for myself that the fread and fwrite both return 1 on each pass, and
fseek returns 0 which is also OK).

Btw, I know about the casts, but I guess it's a habbit from back when I
didn't know you didn't need them (or better yet, when I used them as a
matter of style in this sort of situations

 

[Martin Dickopp]

All error checking is removed for clarity. This turns into a infinite loop
that writes at the end of a file, and makes it grow until a "forced brake".
Thanks for the help (btw, both stdio.h and stdlib.h are included)

int zamijeni (char *ime_datoteke) {
FILE *fp;
int t, flag = 0;

fp = fopen( ime_datoteke, "r+b" );
while ( fread( (void *)&t, (long)sizeof( t ), 1, fp ) == 1 ) {
if ( t < 0 ) {
t = -t;
++flag;
fseek( fp, -1L * (long)sizeof( t ), SEEK_CUR );
fwrite( (void *)&t, (long)sizeof( t ), 1, fp );
}
}
return flag > 0;
}

A write operation to a stream may not be directly followed by a read
operation from the same stream. Between them, the stream must be
flushed or the file position must be changed. (The file position must
also be changed if writing follows reading, and that's what you do with
the `fseek' call.)

Insert `fflush(fp);' after the `fwrite' call.

Martin

 

[buda]

Thank you very much.

A write operation to a stream may not be directly followed by a read
operation from the same stream. Between them, the stream must be
flushed or the file position must be changed. (The file position must
also be changed if writing follows reading, and that's what you do with
the `fseek' call.)

Insert `fflush(fp);' after the `fwrite' call.

Martin


--
,--. Martin Dickopp, Dresden, Germany ,= ,-_-. =.
/ ,- ) http://www.zero-based.org/ ((_/)o o(\_))
\ `-' `-'(. .)`-'
`-. Debian, a variant of the GNU operating system. \_/

 

[Barry Schwarz]

Could the fwrite fail ? e.g. file is locked, on read-only media..

Also: with a C compiler, you can remove (void *) and (long);
and there is a risk that flag overflows to 0.

Since flag is a signed int, any overflow leads to undefined behavior.

/* return -1 if error, 1 if negatives found and fixed, else 0 */
int zamijeni (char *ime_datoteke) {
FILE *fp;
int t, flag = 0;

if ((fp = fopen( ime_datoteke, "r+b"))==NULL)
return -1;
while ( fread(&t, sizeof t, 1, fp )==1) {
if ( t < 0 ) {
t = -t;
flag = 1;
if (fseek( fp, -sizeof t, SEEK_CUR )!=0
|| fwrite(&t, sizeof t, 1, fp )!=1)
return -1;
}
if (fclose(fp)!=0)
return -1;
}
return flag;
}


François Grieu

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