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Hi all,
I'm aware that this might not be a "C-specific" question, if so,
please let me know which group is the most appropriate for this kind of
question.

This is a question on unix filehandling in C, all I have to do is
rewind to the beginning of the file immediately after "write"ing some
bytes. My code looks something like this -


/* create a file and write something into it */
int fd = open("test.txt", O_CREAT|O_WRONLY, S_IRUSR|S_IWUSR);
char b[100] = "this is some stupid text!\n";
write(fd, b, sizeof(b));

p = lseek(fd, 0, SEEK_SET);
printf("lseek returned = %d\n",p); /* returns 0 */

/* char block[BUFSIZ]; */
/* char tmp[] = "/tmp/fileXXXXXX"; */

newfd = mkstemp(tmp);

//copy contents of fd to newfd
while((nread = read(fd, block, sizeof(block))) > 0) /* READ FAILS
HERE <<< */
write(newfd, block, nread);



I am clueless as to why after a write to a new file, and an lseek to
position zero, a subsequent read fails. Even if I do an fsync() after
the write, it fails. I'm on RHEL3.


Thanks in advance.
Raj

Sorry folks, fixed it - a simple case of changing O_WRONLY to O_RDWR ..
duh
Rajorshi Biswas wrote:
> Hi all,
> I'm aware that this might not be a "C-specific" question, if so,
> please let me know which group is the most appropriate for this kind of
> question.
>
> This is a question on unix filehandling in C, all I have to do is
> rewind to the beginning of the file immediately after "write"ing some
> bytes. My code looks something like this -
>
>
> /* create a file and write something into it */
> int fd = open("test.txt", O_CREAT|O_WRONLY, S_IRUSR|S_IWUSR);
> char b[100] = "this is some stupid text!\n";
> write(fd, b, sizeof(b));
>
> p = lseek(fd, 0, SEEK_SET);
> printf("lseek returned = %d\n",p); /* returns 0 */
>
> /* char block[BUFSIZ]; */
> /* char tmp[] = "/tmp/fileXXXXXX"; */
>
> newfd = mkstemp(tmp);
>
> //copy contents of fd to newfd
> while((nread = read(fd, block, sizeof(block))) > 0) /* READ FAILS
> HERE <<< */
> write(newfd, block, nread);
>
>
>
> I am clueless as to why after a write to a new file, and an lseek to
> position zero, a subsequent read fails. Even if I do an fsync() after
> the write, it fails. I'm on RHEL3.
>
>
> Thanks in advance.
> Raj

Rajorshi Biswas wrote:
> Hi all,
> I'm aware that this might not be a "C-specific" question, if so,
> please let me know which group is the most appropriate for this kind of
> question.
>
> This is a question on unix filehandling in C, all I have to do is
> rewind to the beginning of the file immediately after "write"ing some
> bytes. My code looks something like this -
>
> /* create a file and write something into it */
> int fd = open("test.txt", O_CREAT|O_WRONLY, S_IRUSR|S_IWUSR);

open and friends are not part of standard C so you would do better
asking on comp.unix.programmer, however I would point out that if
O_WRONLY has the obvious meaning of opening the file for writing only
you will not be able to read from it.

<snip>

> while((nread = read(fd, block, sizeof(block))) > 0) /* READ FAILS
> HERE <<< */

<snip>

If you want further information on using open and friends, please ask in
comp.unix.programmer
--
Flash Gordon, living in interesting times.
Web site - http://home.flash-gordon.me.uk/
comp.lang.c posting guidelines and intro:
http://clc-wiki.net/wiki/Intro_to_clc

Thanks for pointing me to the appropriate group, Gordon!

~Rajorshi

Flash Gordon wrote:
> Rajorshi Biswas wrote:
> > Hi all,
> > I'm aware that this might not be a "C-specific" question, if so,
> > please let me know which group is the most appropriate for this kind of
> > question.
> >
> > This is a question on unix filehandling in C, all I have to do is
> > rewind to the beginning of the file immediately after "write"ing some
> > bytes. My code looks something like this -
> >
> > /* create a file and write something into it */
> > int fd = open("test.txt", O_CREAT|O_WRONLY, S_IRUSR|S_IWUSR);
>
> open and friends are not part of standard C so you would do better
> asking on comp.unix.programmer, however I would point out that if
> O_WRONLY has the obvious meaning of opening the file for writing only
> you will not be able to read from it.
>
> <snip>
>
> > while((nread = read(fd, block, sizeof(block))) > 0) /* READ FAILS
> > HERE <<< */
>
> <snip>
>
> If you want further information on using open and friends, please ask in
> comp.unix.programmer
> --
> Flash Gordon, living in interesting times.
> Web site - http://home.flash-gordon.me.uk/
> comp.lang.c posting guidelines and intro:
> http://clc-wiki.net/wiki/Intro_to_clc