Feb 19, 2012 · Use lseek when using low-level POSIX file descriptor I/O. The difference between the various seek functions is just the kind of file/stream objects on which they operate. On Linux, seekg and fseek are probably implemented in terms of lseek.

Apr 3, 2017 · Since read() and lseek() are system calls, they shall correspond to changes in updated file if kernel/OS buffer regularly syncs with file in Hard disk. But that is not the case. The changes in file are not reflected from read(), instead it continues to print the initial content.

Nov 12, 2018 · The character l in the name lseek means "long integer". Before the introduction of the off_t data type, the offset argument and the return value were long integers. lseek was introduced with Version 7 when long integers were added to C. (Similar functionality was provided in Version 6 by the functions seek and tell.)

Oct 27, 2018 · Generally, the best place to look for answers to questions like this is the man page. On most Linux/BSD/etc. systems, you can find them by just typing man lseek (or whatever function you're interested in). In this case, the man page will explain that offset is …

Jan 6, 2017 · The lseek() function allows the file offset to be set beyond the end of the file (but this does not change the size of the file). If data is later written at this point, subsequent reads of the data in the gap (a "hole") return null bytes ('\0') until data is actually written into the gap. (From linux lseek(2) man page) –

When you open a file with O_APPEND, all data gets written to the end Not all data. Per POSIX, the pwrite function (ssize_t pwrite(int fildes, const void *buf, size_t nbyte, off_t offset);) can write to any offset: "The pwrite() function shall be equivalent to write(), except that it writes into a given position and does not change the file offset (regardless of whether O_APPEND is set)."

Oct 18, 2010 · Assuming the file is a text file, then you will have to pad the lines to a constant width. Then use lseek(), read(), and write() to modify the lines. Having a constant length for each line is crucial: it allows you to compute the starting position of each line.

The lseek call you do right after open is not required because when you first open a file the current seek offset is zero. After each successful read or write operation, the seek offset moves forward by the number of bytes read/written.

Nov 5, 2013 · Yes, you can use lseek(): off_t lseek(int fd, off_t offset, int whence); The lseek() function repositions the offset of the open file associated with the file descriptor fd to the argument offset according to the directive whence as follows: SEEK_SET. The offset is set to offset bytes. SEEK_CUR. The offset is set to its current location plus ...

Jan 27, 2021 · Via lseek: 9223372036854775807 via telldir: 9223372036854775807 Quoting from the telldir(3) man page: In early filesystems, the value returned by telldir() was a simple file offset within a directory. Modern filesystems use tree or hash structures, rather than flat tables, to represent directories.