LSEEK(2) — System Interface Manual — System Calls

NAME

lseek − move read/write pointer

SYNOPSIS

#define L_SET0/∗ set the seek pointer ∗/
#define L_FROMHERE1 /∗ reposition the seek pointer ∗/
#define L_FROMEOF2/∗ change the file size ∗/
pos = lseek(fildes, offset, whence)
int pos;
int d, offset, whence;

DESCRIPTION

The descriptor fildes refers to a file or device open for reading and/or writing. Lseek sets the file pointer of filedes as follows:

If whence is L_SET, the pointer is set to offset bytes.

If whence is L_FROMHERE, the pointer is set to its current location plus offset. offset can be negative in which case the pointer is set to its current location minus offset.

If whence is L_FROMEOF, the pointer is set to the size of the file plus offset. offset can be negative in which case the pointer is set to the size of the file minus offset.

Upon successful completion, the resulting pointer location as measured in bytes from beginning of the file is returned. Some devices are incapable of seeking. The value of the pointer associated with such a device is undefined.

Seeking far beyond the end of a file, then writing, creates a gap or “hole”, which occupies no physical space and reads as zeros.

RETURN VALUE

Upon successful completion, a non-negative integer indicating the file pointer value is returned. Otherwise, a value of −1 is returned and errno is set to indicate the error.

ERRORS

Lseek will fail and the file pointer will remain unchanged if:

[EBADF] Fildes is not an open file descriptor.

[ESPIPE] Fildes is associated with a pipe or a socket.

[EINVAL] Whence is not a proper value (also gives a SIGSYS signal).

[EINVAL] The resulting file pointer would be negative.

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