C file position indicator

See Binary Streams. The following symbolic constants are defined for use as the whence argument to fseek. This is an integer constant which, when used as the whence argument to the fseek or fseeko functions, specifies that the offset provided is relative to the beginning of the file.

This is an integer constant which, when used as the whence argument to the fseek or fseeko functions, specifies that the offset provided is relative to the current file position. This is an integer constant which, when used as the whence argument to the fseek or fseeko functions, specifies that the offset provided is relative to the end of the file. The rewind function positions the stream stream at the beginning of the file. They are defined in two different header files: fcntl.

This value can be positive or negative. In the following code, fseek reads and outputs the last 10 characters of the file gettysburg.

The fseek function at Line 18 positions the file indicator to 10 bytes characters before the EOF. At Line 24, the fseek function is pinch-hitting for rewind. In effect, the fseek function in that specific format works the same as rewind. Given what you know about fseek , how would you concoct code to read the middle 10 bytes of a file?

The fseek function doesn't have a whence position for the file's center. So how could you determine the file's overall size, split that in two, and then position the file indicator to read the middle 10 bytes of a file? Yes, this is a Lesson, not an Exercise, so while you could rush off and attempt to figure out a solution on your own and thank you for trying , my solution is this:.

I have a file named "filename. I do not understand why the second time I try to use read , I get no bytes from the file. Also, why does the file position indicator not move when I read contents from the file using read? On the second fseek , advancing 4 bytes instead of 3 did also not work. Any suggestions? Use fseek and fread or lseek and read , but do not mix the two APIs, it won't work.

It is not guaranteed that the real file position indicator one that lseek is responsible for changes, and if it does, it is not known by how much.

You're not explicitly adding a null-terminator byte so your program might print garbage after the first 4 bytes of the buffer, but you've been lucky and your compiler has initialized the buffer to zeroes so you haven't noticed this problem. FILE structs contain a buffer and a couple of ints to support more efficient and convenient access to the file behind a file descriptor.

The low-level calls read , write , open , close , seek completely ignore the FILE struct. I ran strace on your program. The strace utility logs all system calls made by a process. I've omitted all the uninteresting stuff up to your open call. Here is where fdopen is happening. The brk calls are evidence of memory allocation, presumably for something like malloc sizeof FILE. Here is the first fseek, which has decided the easiest way to get to position 5 in the file is to just read in 5 bytes and ignore them.

Here is the third printf. Notice that there is no evidence of a ftell call. Here is your read call.