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 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 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 beyond the end of a file and then writing to the file creates a gap or hole which does not occupy physical space and reads as zeros. 

The system call tell returns the offset of the current byte relative to the beginning of the file associated with the file descriptor.

ENVIRONMENT

SYSTEM_FIVE

When you use the System V environment, note the following:

•If you compile a program in this 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. 

•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 fails and the file pointer remains unchanged if the following occurs:

[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. 

SEE ALSO

dup(2), open(2)

System Calls