lseek(2)
Name
lseek, tell − move read or write pointer
Syntax
#include <sys/types.h>
#include <unistd.h>
pos = lseek(d, offset, whence)
off_t pos;
int d, whence;
off_t offset;
pos = tell(d)
off_t pos;
int d;
Description
The system call lseek 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 or writing. The lseek system call sets the file pointer of d as follows:
•If whence is SEEK_SET, the pointer is set to offset bytes.
•If whence is SEEK_CUR the pointer is set to its current location plus offset.
•If whence is SEEK_END, the pointer is set to the size of the file plus offset.
Seeking beyond the end of a file and then writing to the file creates a gap or hole that does not occupy 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
System Five
If you compile a program in the System Five environment, an invalid whence argument causes SIGSYS to be sent. This complies with the behavior described in the System V Interface Definition (SVID), Issue 1.
Return Values
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 fails and the file pointer remains unchanged under the following conditions:
[EBADF] The fildes is not an open file descriptor.
[EINVAL] The whence is not a proper value.
[ESPIPE] The fildes is associated with a pipe or a socket.