What is the role of libc(glibc) in our linux app?
libc implements both standard C functions like strcpy() and POSIX functions (which may be system calls) like getpid(). Note that not all standard C functions are in libc - most math functions are in libm.
You cannot directly make system calls in the same way that you call normal functions because calls to the kernel aren't normal function calls, so they can't be resolved by the linker. Instead, architecture-specific assembly language thunks are used to call into the kernel - you can of course write these directly in your own program too, but you don't need to because libc provides them for you.
Note that in Linux it is the combination of the kernel and libc that provides the POSIX API. libc adds a decent amount of value - not every POSIX function is necessarily a system call, and for the ones that are, the kernel behaviour isn't always POSIX conforming.
libc is a single library file (both .so and .a versions are available) and in most cases resides in /usr/lib. However, the glibc (GNU libc) project provides more than just libc - it also provides the libm mentioned earlier, and other core libraries like libpthread. So libc is just one of the libraries provided by glibc - and there are other alternate implementations of libc other than glibc.
With regard to the first two, glibc is both the C standard library (e.g, "standard C functions") and a wrapper for system calls. You cannot issue system calls directly because the compiler doesn't know how -- glibc contains the "glue" which is necessary to issue system calls, which is written in assembly. (It is possible to reimplement this yourself, but it's far more trouble than it's worth.)
(The C++ standard library is a separate thing; it's called libstdc++.)
glibc isn't a single .so (dynamic library) file -- there are a bunch, but libc and libm are the most commonly-used two. All of the static and dynamic libraries are stored in /lib.
libc is a generic term used to refer to all C standard libraries -- there are several. glibc is the most commonly used one; others include eglibc, uclibc, and dietlibc.
It's the "standard library". It's exactly like "MSVCRTL" in the Windows world.
The Gnu standard library ("glibc") is the implementation of libc most commonly (almost universally?) found on Linux systems. Here are the relevant files on an old SusE Linux system:
ls -l /lib =>-rwxr-xr-x 1 root root 1383527 2005-06-14 08:36 libc.so.6ls -l /usr/lib =>-rw-r--r-- 1 root root 2580354 2005-06-14 08:20 libc.a-rw-r--r-- 1 root root 204 2005-06-14 08:20 libc.soThis link should answer any additional questions you might have (including references to the full and complete GLibc source code):