LSEEK (2) | System calls | Unix Manual Pages | :man▋
lseek - reposition read/write file offset
.In unistd.h off_t lseek "int fildes" "off_t offset" "int whence"
The lseek system call repositions the offset of the file descriptor fildes to the argument offset according to the directive whence. The argument fildes must be an open file descriptor. The lseek system call repositions the file position pointer associated with the file descriptor fildes as follows:
| || If whence is SEEK_SET, the offset is set to offset bytes. |
| || If whence is SEEK_CUR, the offset is set to its current location plus offset bytes. |
| || If whence is SEEK_END, the offset is set to the size of the file plus offset bytes. |
The lseek system call 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).
Some devices are incapable of seeking. The value of the pointer associated with such a device is undefined.
Upon successful completion, lseek returns the resulting offset location as measured in bytes from the beginning of the file. Otherwise, a value of -1 is returned and errno is set to indicate the error.
The lseek system call will fail and the file position pointer will remain unchanged if:
| || The fildes argument is not an open file descriptor. |
| || The whence argument is not a proper value or the resulting file offset would be negative for a non-character special file. |
| || The resulting file offset would be a value which cannot be represented correctly in an object of type off_t. |
| || The fildes argument is associated with a pipe, socket, or FIFO. |
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