Delete data at end of file

[The Doctor]

Hi,

Is it possible to delete data at the end of a file?

I am using read(), write(), open() and so, but I didn't find a function
for deleteing data at the end of the file. Or is it enough to write the
EOF at the end of the data?

Grtz,
Leon

 

[utab]

should be

 

[The Doctor]

Since you are using low level function, you are reading binary files,
How may bytes do you have to delete from the end of the file?

sth like: Read till a point you know that the delete point is reached
and write it to a new file

ifstream file(filename, ios::in | ios::binary); if(file.is_open())
ifstream:os_type sz;
// get file size
sz = file.tellg()
...
at this point you have an idea of the file size, and I suppose you also
have an idea about the portion to be deleted. You can write the portion
of the file until the deleted byte to another file. Can you post some
code and a minimal file?

Best

I determine the filesize by seeking to the end of the file with lseek(fd,
0, SEEK_END). The lseek then returns the size of the file (the offset
measured from the beginning of the file until the end of it).

Example code:

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>

int main()
{
int fd = open("file.txt", O_CREAT | O_RDWR);

write(fd, "Hello, world!", strlen("Hello, world!")+1);

lseek(fd, 5, SEEK_SET);
// Delete from here, so the file becomes: "Hello"

close(fd);
}

 

[Default User]

The Doctor wrote:

Example code:

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>

This is all non-standard (non-standard C++ that is). For POSIX
questions, the best place to ask is comp.unix.programmer.




Brian

 

[Stefan Ram]

ifstream file(filename, ios::in | ios::binary);
if(file.is_open())
ifstream:os_type sz;
// get file size
sz = file.tellg()
...
at this point you have an idea of the file size

In ISO/IEC 9899:1990:

»A binary stream need not meaningfully support
fseek calls with a whence value of SEEK_END.«

In ISO/IEC 14882:2003(E), »seekg« and »tellg« seem to be
defined using »pubseekoff«, which seems to be based on
»seekoff«, which seems to be based on »::std::fseek«,
for which the above quotation from ISO/IEC 9899:1990 applies.

 

[Default User]

The Doctor wrote:




This is all non-standard (non-standard C++ that is). For POSIX
questions, the best place to ask is comp.unix.programmer.

Ah, not string.h, of course.





Brian