The C library is not a single library file. It is composed of four main parts that together implement the POSIX functions API:
libc
: The main C library that contains the well-known POSIX functions such as printf
, open
, close
, read
, write
, and so onlibm
: Maths functions such as cos
, exp
, and log
libpthread
: All the POSIX thread functions with names beginning with pthread_
librt
: The real-time extensions to POSIX, including shared memory and asynchronous I/OThe first one, libc
, is always linked in but the others have to be explicitly linked with the -l
option. The parameter to -l
is the library name with lib
stripped off. So, for example, a program that calculates a sine function by calling sin()
would be linked with libm
using -lm
:
arm-cortex_a8-linux-gnueabihf-gcc myprog.c -o myprog -lm
You can verify which libraries have been linked in this or any other program by using the readelf
command:
$ arm-cortex_a8-linux-gnueabihf-readelf -a myprog | grep "Shared library" 0x00000001 (NEEDED) Shared library: [libm.so.6] 0x00000001 (NEEDED) Shared library: [libc.so.6]
Shared libraries need a run-time linker, which you can expose using:
$ arm-cortex_a8-linux-gnueabihf-readelf -a myprog | grep "program interpreter" [Requesting program interpreter: /lib/ld-linux-armhf.so.3]
This is so useful that I have a script file with these commands into a shell script:
#!/bin/sh ${CROSS_COMPILE}readelf -a $1 | grep "program interpreter" ${CROSS_COMPILE}readelf -a $1 | grep "Shared library"