Many library functions do not make any system calls at all. String manipulation functions fall into this category. For example, strlen(), strcmp(), strcat(), and memcpy() all perform their work entirely within user space without ever touching the kernel.

/* These work purely in user space — no system calls */
size_t len = strlen("hello");   /* No kernel involvement */
int cmp = strcmp("abc", "abd"); /* Purely user-space computation */

Other library functions are implemented by making one or more system calls. The library function fopen(), for instance, internally uses the open() system call to open a file. Similarly, printf() eventually calls the write() system call to write formatted output to the file descriptor.

/* fopen() is a library function layered on the open() system call */
FILE *fp = fopen("data.txt", "r");
/* Internally calls: open("data.txt", O_RDONLY) system call */

/* printf() is a library function layered on the write() system call */
printf("Hello, world!\n");
/* Internally buffers output, then calls: write(1, buf, n) */

Library functions often provide a more caller-friendly interface than the underlying system call. A key example is memory allocation. The malloc() and free() library functions perform a range of bookkeeping tasks that make them a much easier way to allocate and free memory than the underlying brk() system call.

/* printf() provides formatting + buffering — write() has neither */
printf("Value: %d\n", 42);    /* Library function: formatted + buffered */
write(1, "Value: 42\n", 10);  /* System call: raw bytes, no formatting */

/* malloc() provides bookkeeping — brk() requires manual management */
void *ptr = malloc(1024);     /* Library function: tracked allocation */
/* brk() would require calculating new program break address manually */

The glibc shared library file can be executed directly from the shell. When run as an executable, it displays version information and other details.

$ /lib/libc.so.6
GNU C Library stable release version 2.10.1, by Roland McGrath et al.
Copyright (C) 2009 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.
There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A
PARTICULAR PURPOSE.
Compiled by GNU CC version 4.4.0 20090506 (Red Hat 4.4.0-4).
Compiled on a Linux >>2.6.18-128.4.1.el5<< system on 2009-08-19.
Available extensions:
  The C stubs add-on version 2.1.2.
  crypt add-on version 2.1 by Michael Glad and others
  GNU Libidn by Simon Josefsson
  Native POSIX Threads Library by Ulrich Drepper et al
  BIND-8.2.3-T5B
  RT using linux kernel aio

Some Linux distributions place glibc at a pathname other than /lib/libc.so.6. Use the ldd (list dynamic dependencies) command against any dynamically linked executable to find the library’s actual location.

$ ldd myprog | grep libc
  libc.so.6 => /lib/tls/libc.so.6 (0x4004b000)

From glibc version 2.0 onward, two constants are defined for compile-time version testing. On a system with glibc 2.12, these constants have the values 2 and 12 respectively.

#include <stdio.h>

int main(void) {
#ifdef __GLIBC__
    printf("glibc major version: %d\n", __GLIBC__);
    printf("glibc minor version: %d\n", __GLIBC_MINOR__);
#else
    printf("Not using glibc\n");
#endif
    return 0;
}

/* Limitation: These constants reflect the compile-time glibc version,
   not the run-time version if the program runs on a different system */

To determine the version of glibc available at run time (rather than compile time), call gnu_get_libc_version(). This function returns a pointer to a version string such as "2.12".

#include <gnu/libc-version.h>

const char *gnu_get_libc_version(void);
/* Returns pointer to null-terminated, statically allocated string
   containing the GNU C library version number */

/* Usage example */
#include <stdio.h>
#include <gnu/libc-version.h>

int main(void) {
    printf("glibc runtime version: %s\n", gnu_get_libc_version());
    return 0;
}

Version information can also be obtained using the confstr() function with the glibc-specific configuration variable _CS_GNU_LIBC_VERSION. This call returns a string such as "glibc 2.12".

#include <unistd.h>
#include <stdio.h>

int main(void) {
    char buf[100];
    confstr(_CS_GNU_LIBC_VERSION, buf, sizeof(buf));
    printf("glibc version string: %s\n", buf);
    /* Output example: glibc 2.12 */
    return 0;
}