What is the role of libc(glibc) in our linux app?

When we debug a program using gdb, we usually see functions with strange names defined in libc(glibc?). My questions are:

1. Is libc/glibc the standard implementation of some standard C/C++ functions like strcpy,strlen,malloc?
2. Or, is it not only of the first usage as described above, but also an wrapper of Unix/Linux system calls like open,close,fctl? If so, why can't we issue syscalls directly, without libc?
3. Does libc only consist of one lib (.a or .so) file, or many lib files (in this case, libc is the general name of this set of libs)? Where do these lib file(s) reside?
4. What is the difference between libc and glibc?

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.6

ls -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.so

This link should answer any additional questions you might have (including references to the full and complete GLibc source code):

• http://sourceware.org/glibc/wiki/HomePage

You can check the detailed information about "libc" and "glibc" from the man pages on your linux sytem by typing "man libc" on the shell, copied as below;

LIBC(7)      Linux Programmer's Manual      LIBC(7)   

NAME
       libc - overview of standard C libraries on Linux

DESCRIPTION
       The term "libc" is commonly used as a shorthand for the "standard C library", a library of standard functions that can be
       used by all C programs (and sometimes by programs in other languages).  Because of some history (see below), use  of  the
       term "libc" to refer to the standard C library is somewhat ambiguous on Linux.

   glibc
       By  far  the most widely used C library on Linux is the GNU C Library ⟨http://www.gnu.org/software/libc/⟩, often referred
       to as glibc.  This is the C library that is nowadays used in all major Linux distributions.  It is  also  the  C  library
       whose details are documented in the relevant pages of the man-pages project (primarily in Section 3 of the manual).  Doc‐
       umentation of glibc is also available in the glibc manual, available via the command info libc.  Release 1.0 of glibc was
       made in September 1992.  (There were earlier 0.x releases.)  The next major release of glibc was 2.0, at the beginning of
       1997.

       The pathname /lib/libc.so.6 (or something similar) is normally a symbolic link that points to the location of  the  glibc
       library,  and executing this pathname will cause glibc to display various information about the version installed on your
       system.

   Linux libc
       In the early to mid 1990s, there was for a while Linux libc, a fork of glibc 1.x created by  Linux  developers  who  felt
       that  glibc  development  at  the  time  was  not  sufficing for the needs of Linux.  Often, this library was referred to
       (ambiguously) as just "libc".  Linux libc released major versions 2, 3, 4, and 5 (as well as many minor versions of those
       releases).  For a while, Linux libc was the standard C library in many Linux distributions.

       However, notwithstanding the original motivations of the Linux libc effort, by the time glibc 2.0 was released (in 1997),
       it was clearly superior to Linux libc, and all major Linux distributions that had been using  Linux  libc  soon  switched
       back to glibc.  Since this switch occurred long ago, man-pages no longer takes care to document Linux libc details.  Nev‐
       ertheless, the history is visible in vestiges of information about Linux libc that remain in some manual pages,  in  par‐
       ticular, references to libc4 and libc5.