Using Picolibc in Embedded Systems
Picolibc is designed to be used in deeply embedded systems, those running either no operating system or a small RTOS. It is designed to be linked statically along with any operating system and application code.
Compiling with Picolibc
To compile source code to use Picolibc, you can use the GCC .specs file delivered with Picolibc. This will set the system header file path and the linker library path to point at Picolibc. For example, to compile a single file and generate a .o file:
$ gcc --specs=picolibc.specs -c foo.c
When installed directly into the system, picolibc.specs is placed in the gcc directory so that it will be found using just the base name of the file. If installed in a separate location, you will need to provide an absolute pathname to the picolibc.specs file:
$ gcc --specs=/usr/local/picolibc/picolibc.specs -c foo.c
When building for an embedded system, you'll probably need to use a
longer name for the compiler, something like
Initializing an embedded system usually requires a combination of hardware, run-time, library and application setup. You may want to perform all of these in your own code, or you be able to use the Picolibc startup code. Either works fine, using the Picolibc code means writing less of your own.
Picocrt — Picolibc startup code
Picocrt is the crt0.o code provided by Picolibc. This is enabled by default when using -specs=picolibc.specs:
$ gcc -specs=picolibc.specs -o foo.elf foo.c
Picocrt goes through a sequence of initialization steps, many of which you can plug your own code into:
Architecture-specific runtime initialization. For instance, RISC-V needs the
gpregister initialized for quick access to global variables while ARM with hardware floating point needs to have the FPSCR register set up to match C semantics for rounding.
Data initialization. Here's the code inside Picocrt:
memcpy(__data_start, __data_source, (uintptr_t) __data_size);
For this to work, the linker script must assign correct values to each of these symbols:
- __data_start points to the RAM location where the .data segment starts.
- __data_source points to the Flash location where the .data segment initialization values are stored.
- __data_size is an absolute symbol noting the size of the initialized data segment
BSS initialization. Here's the code inside Picocrt:
memset(__bss_start, '\0', (uintptr_t) __bss_size);
Assign these symbols in the linker script as follows:
- __bss_start points to the RAM location where the .bss segment starts.
- __bss_size is an absolute symbol noting the size of the cleared data segment
Optionally call constructors:
The default and hosted crt0 variants call initializers/constructors using
The minimal crt0 variant doesn't call any constructors
After main returns:
The default and minimal crt0 variants run an infinite loop if main returns.
The hosted crt0 variant calls
exit, passing it the value returned from main.
Picolibc provides two sample linker scripts:
picolibc.ld for C
picolibcpp.ld for C++ applications. These are
designed to be useful for fairly simple applications running on
embedded hardware which need read-only code and data stored in flash
and read-write data allocated in RAM and initialized by picolibc at
boot time. You can read more about this on the linking
“Semihosting” is a mechanism used when the application is run under a debugger or emulation environment that provides access to some functionality of the OS running the debugger or emulator. Picolibc has semihosting support for console I/O and the exit() call.
One common use for semihosting is to generate debug messages before any of the target hardware has been set up.
Picolibc distributes the semihosting implementation as a separate
libsemihost.a. Because it provides interfaces that are used
by libc itself, it must be included in the linker command line after
libc. You can do this by using the GCC --oslib=semihost
command line flag defined by picolibc.specs:
$ gcc --specs=picolibc.specs --oslib=semihost -o program.elf program.o
You can also list libc and libsemihost in the correct order explicitly:
$ gcc --specs=picolibc.specs -o program.elf program.o -lc -lsemihost
crt0 version provided by Picolibc calls any constructors
before it calls main and then goes into an infinite loop after main
returns to avoid requiring an
In an environment which provides a useful
_exit implementation, applications
may want to use the
crt0-hosted variant that calls
exit when main
returns, resulting in a clean return to the hosting environment (this
conforms to a hosted execution environment as per the C
specification). Select this using the
$ gcc --specs=picolibc.specs --crt0=hosted -o program.elf program.o
In a smaller environment which is not using any constructors (or any
Picolibc code which requires constructors) may want to use the
crt0-minimal variant that removes the call to
__libc_init_array(). This variant also runs an infinite loop in case
main returns. Select this using the
$ gcc --specs=picolibc.specs --crt0=minimal -o program.elf program.o