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c++posixlseek

Why do we need lseek when pread accepts offset value?

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?

Solution

  • 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.