What is the difference / relationship between FILE * and int fd value?
I noticed that in C, file I / O depends on the file descriptor. However, in some standard library functions, such as write(int fd, xxxx)
, a file descriptor is represented as an integer, while in some other functions, such as fseek(FILE *stream, xxxx)
, it is represented as a pointer.
Are int
fd and values the FILE *
same? Why does C leave such a difference?
(I tried to find a similar question but couldn't, although my question seems to be preliminary)
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The standard file type in C is FILE *
. Any functions you see with int fd
are an extension, not a C standard. These are usually POSIX extensions .
int
there are file descriptors because that's what the base kernel uses to describe files. On systems where file descriptors are used FILE *
, this is a wrapper around the file descriptor, adding buffering, etc. But not all systems use file descriptors, for example I'm pretty sure Windows doesn't, although I haven't confirmed it yet. In these systems FILE *
, something even more suitable is wrapped there.
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Are int fd and FILE * values the same?
Not. This is not true.
A file descriptor is int
, whereas FILE *
is a pointer to a file. The main difference is that the latter is buffered while the former is not.
The file pointer ( FILE*
) usually contains more information about the stream, such as the current location, end of the file marker, errors in the stream, etc. But a file descriptor is simply a positive integer representing a "file" (which can be a pipe, socket, or any other stream).
You can get a file descriptor from a file pointer with fileno()
:
int fd = fileno(fp);
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