CODEWITHSUNDEEP
clinuxposix
devgirl05
devgirl05

Reputation: 363

Why do both fstat(2) and fstat(3) exist and which one should I use?

I was trying to read the man page for fstat, and I got fstat(2), fstat(3) and fstat(3P).

Having in the past used a system (2) command, I know the difference is that I have to write a prototype myself to announce the function (which is reflected in the fstat(2) man page).

But never have I seen that a function is both a C function (3) and a system function (2) at the same time. What would be the benefit of using one over the other? How would C even differentiate whether I am using (2) or (3).

Also, I understand that sys/stat.h is platform specific, so which of (2) and (3) would be safer to use for cross-platform? Since I don't see a prototype line in the Windows page example, I assume it is a (3)

Please explain this to be because I cannot wrap my head around why both would exist.

Upvotes: 3

Views: 140

Answers (1)

rici
rici

Reputation: 241901

Web-based manpages are always a second-best option, since they can only show you a generic view of the interfaces. If you use the man command on your own system (at least for Unix-like systems), the result should reflect your actual installation, assuming you've correctly installed the documentation for your distribution.

Section 2 of the manual is intended for the description of system calls rather than standard library APIs. However, at least in the case of Linux documentation, section 2 is also used to describe the library wrappers around the system calls, which effectively documents the particular features implemented by the standard C library, normally glibc. In such cases, section 3P is used to document the to contain documentation extracted or adapted from the Posix standard. So if you want to write portable code, use only the features documented in section 3P. If you are happy to use all the extensions available to you on your system, use section 2.

man7.org and linux.die.net use different manpage repositories; on the latter, section 3P doesn't exist and the Posix programming manual is found in section 3. So https://www.man7.org/linux/man-pages/man3/fstat.3p.html and https://linux.die.net/man/3/fstat contain the same information. If you use a linux distribution, you'll probably find that man 3 fstat actually gives you the page from section 3p.

In any event, there is only one interface in the C library, which should conform to the Posix standard, but may extend it.

Upvotes: 3

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