1. Technology
Linux / Unix Command: lseek
Command Library

NAME

lseek - reposition read/write file offset  

SYNOPSIS

#include <sys/types.h>
#include <unistd.h>

off_t lseek(int fildes, off_t offset, int whence);  

DESCRIPTION

The lseek function repositions the offset of the file descriptor fildes to the argument offset according to the directive whence as follows:
SEEK_SET
The offset is set to offset bytes.
SEEK_CUR
The offset is set to its current location plus offset bytes.
SEEK_END
The offset is set to the size of the file plus offset bytes.

The lseek function allows the file offset to be set beyond the end of the existing end-of-file of the file. If data is later written at this point, subsequent reads of the data in the gap return bytes of zeros (until data is actually written into the gap).  

RETURN VALUE

Upon successful completion, lseek returns the resulting offset location as measured in bytes from the beginning of the file. Otherwise, a value of (off_t)-1 is returned and errno is set to indicate the error.  

ERRORS

EBADF
Fildes is not an open file descriptor.
ESPIPE
Fildes is associated with a pipe, socket, or FIFO.
EINVAL
Whence is not a proper value.
 

CONFORMING TO

SVr4, POSIX, BSD 4.3  

RESTRICTIONS

Some devices are incapable of seeking and POSIX does not specify which devices must support it.

Linux specific restrictions: using lseek on a tty device returns ESPIPE. Other systems return the number of written characters, using SEEK_SET to set the counter. Some devices, e.g. /dev/null do not cause the error ESPIPE, but return a pointer which value is undefined.  

SEE ALSO

dup(2), fork(2), open(2), fseek(3)


Important: Use the man command (% man) to see how a command is used on your particular computer.

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