Why do we need lseek when pread accepts offset value?

Question

I am having trouble to understand why lseek function is useful. Assuming I got a parameter like this given to me from the user:

off_t offset = 10;

And I wanted to read from the offset 100 bytes. I can use pread like this:

void * buf = malloc(100);
if (buf == NULL) { exit(1);}
int res = pread(file_id, buf, 100, offset);

On the other hand, I understand I can set the file with lseek like this:

off_t seek = lseek(file_id, offset, SEEK_SET);

So I believe I achieve reading using pread already. What did I miss regarding lseek in what it can do to help me read the file?

Neither lseek nor pread are part of Standard C++. Or of Standard C, for that matter. — , Jun 03 '17 at 14:35
@NeilButterworth If its going through a `C++` compiler then it is `C++` code. — Galik, Jun 03 '17 at 14:38
@Galik Because we don't want the C++ tag being swamped with questions specifically about the millions of non-standard APIs that are out there. — , Jun 03 '17 at 14:39
@NeilButterworth So why do we even have tags like `posix` and `lseek` and `mysql` etc...? — Galik, Jun 03 '17 at 14:40
@Galik That question makes no sense. We have those tags for questions about posix and linux. I've already tagged this as posix. — , Jun 03 '17 at 14:41
@NeilButterworth there are 500K questions tagged "c++", I don't think you are going to be able to avoid "swamping" — M.M, Jun 07 '17 at 05:09
The simple answer is that `lseek()` came first, by about 30 years. — user207421, Jun 07 '17 at 06:20

score 1 · Accepted Answer · answered Jun 07 '17 at 04:56

A function may have to read/write from/to a given file handle at a location that is not known to it (say current position), so you need to uncouple seeking from reading (or writing) because the caller may have a need to set the location.

More generally, many I/Os are just sequential so seeking is not necessary, while pread forces seeking.

Why do we need lseek when pread accepts offset value?

1 Answers1