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Library Call

457 words, 3 min read
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Xinyang YU

Abstract

  • We can’t directly make System Call (系统调用) in the same way that we call functions in a program, different Instruction Set Architecture (ISA) has a different implementation of System Calls. So we have Library call that is built on top of system calls to abstract this complexity away from the application programmers

Abstraction

Make it possible to make System Call (系统调用) from user program written in languages like Java and C via an unified interface like POSIX without considering the underlying ISA.

For example, when the C program makes a malloc() library call, the malloc() library call uses corresponding Assembly language to complete the memory allocation on the Heap Segment.

Tip

Library calls can be traced with lstrace.

Attention

Library call always runs in User Mode, unless it needs the involvement of Kernel to complete its task.

Library Procedure

libc

Attention

Not all standard C functions are in libc - most math functions are in libm.

Tip

If you run a C binary and get an error like -bash: ./<c binary>: No such file or directory. It probably means you are missing the required libc, it can be glibc or musl libc etc. In Ubuntu, if the system is missing musl libc, we can install it using sudo apt-get install musl.

glibc

  • glibc (GNU libc) provides more than just libc, it also provides the libm, and other core libraries like libpthread in separate files
  • For ubuntu, you can find all the glibc files under /lib/x86_64-linux-gnu and /usr/lib/x86_64-linux-gnu

musl libc

  • A lightweight implementation of libc with libpthread, libm, librt, etc united into one single file libc.so
  • For ubuntu, you can find all the musl libc files under /lib/x86_64-linux-musl and /usr/lib/x86_64-linux-musl

Quote

musl’s efficiency is unparalleled in Linux libc implementations. Designed from the ground up for static linking, musl carefully avoids pulling in large amounts of code or data that the application will not use. Dynamic linking is also efficient; by integrating the entire standard library implementation, including threads, math, and even the dynamic linker itself into a single shared object, most of the startup time and memory overhead of dynamic linking have been eliminated.

Info

musleabi - targets systems using the standard ARM EABI (Embedded Application Binary Interface).

musleabihf - targets systems using the ARM EABI with hard-float support (hardware-accelerated floating-point operations).

References

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