lseek(2)
NAME
lseek, tell − move read/write pointer
SYNTAX
#include <sys/file.h> /*Include definitions of L_SET, L_INCR, L_XTND*/
pos = lseek(d, offset, whence)
int pos;
int d, whence;
off_t offset;
pos = tell(d)
int pos;
int d;
DESCRIPTION
The lseek system call moves the file pointer associated with a file or device open for reading or writing.
The descriptor d refers to a file or device open for reading and/or writing. The lseek system call sets the file pointer of d as follows:
If whence is L_SET, the pointer is set to offset bytes.
If whence is L_INCR, the pointer is set to its current location plus offset.
If whence is L_XTND, the pointer is set to the size of the file plus offset.
Seeking far beyond the end of a file, then writing, creates a gap or “hole”, which occupies no physical space and reads as zeros.
The tell system call returns the offset of the current byte relative to the beginning of the file associated with the file descriptor.
ENVIRONMENT
When your program is compiled using the System V environment, an invalid whence argument causes SIGSYS to be sent. Note that this behavior is as specified by SVID Issue 1.
Also, when using the System V environment, the return value of lseek and the parameter offset are of type long.
RETURN VALUE
Upon successful completion, a long integer, the current file pointer value, is returned. This pointer is measured in bytes from the beginning of the file, where the first byte is byte 0. (Note that some devices are incapable of seeking. The value of the pointer associated with such a device is undefined.) If a value of −1 is returned, errno is set to indicate the error.
DIAGNOSTICS
The lseek system call will fail and the file pointer will remain unchanged if:
[EBADF] The fildes is not an open file descriptor.
[ESPIPE] The fildes is associated with a pipe or a socket.
[EINVAL] The whence is not a proper value.
[ESPIPE] The resulting file pointer would be negative.