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Picolibc and Operating Systems

Picolibc is designed to be operating-system independent, so it doesn't embed any operating system support into libc. Some portions of Picolibc need system support, like I/O and task termination.

System Interfaces used by Picolibc

Here's the full list of system functions used by Picolibc, split into sections based on what libc support they enable

stdin/stdout/stderr

Picolibc stdio splits support for simple console input/output and more complex file operations so that a minimal system can easily support the former with only a few functions.

To get stdin/stdout/stderr working, the application needs to define the stdin, stdout and stderr globals, which contain pointers to FILE objects. The pointers may reside in read-only memory, but the FILE objects may not. A single FILE object may be used for all three pointers, and linker aliases may be used to make all three pointers be stored in the same location. The FILE object contains function pointers for putc, getc, which might be defined as follows:

static int
sample_putc(char c, FILE *file)
{
	(void) file;		/* Not used in this function
	__uart_putc(c);		/* Defined by underlying system */
	return c;
}

static int
sample_getc(FILE *file)
{
	unsigned char c;
	(void) file;		/* Not used in this function */
	c = __uart_getc();	/* Defined by underlying system */
	return c;
}

It also contains a pointer to an optional flush function, which, if provide, will be called to flush pending output to the file, e.g., when the fflush() function is called:

static int
sample_flush(FILE *file)
{
	/* This function doesn't need to do anything */
	(void) file;		/* Not used in this function */
	return 0;
}

These functions are used to initialize a FILE structure:

static FILE __stdio = FDEV_SETUP_STREAM(sample_putc,
					sample_getc,
					NULL,
					_FDEV_SETUP_RW);

This defines a FILE which can read and write characters using the putc and getc functions described above, but not using any flush function. The final paramter, which specifies the operations supported, can be one of the following:

Mode Operations Required Functions
_FDEV_SETUP_READ Read getc
_FDEV_SETUP_WRITE Write putc
_FDEV_SETUP_RW Read/Write putc, getc

Finally, the FILE is used to initialize the stdin, stdout and stderr values, the latter two of which are simply aliases to stdin:

FILE *const stdin = &__stdio; __strong_reference(stdin, stdout); __strong_reference(stdin, stderr);

fopen, fdopen

Support for these requires malloc/free along with a handful of POSIX-compatible functions:

int open (const char *, int, ...);
int close (int fd);
ssize_t read (int fd, void *buf, size_t nbyte);
ssize_t write (int fd, const void *buf, size_t nbyte);
off_t lseek (int fd, off_t offset, int whence);

The code needed for this is built into Picolibc by default, but can be disabled by specifying -Dposix-io=false in the meson command line.

exit

Exit is just a wrapper around _exit that also calls destructors and callbacks registered with atexit. To make it work, you'll need to implement the _exit function:

void	_exit (int status) _ATTRIBUTE ((__noreturn__));

malloc and free

Both versions of malloc in picolibc require sbrk to be supported. The smaller version, enabled (by default) with -Dnewlib-nano-malloc=true, can handle sbrk returning dis-continuous memory while the larger version (enabled with -Dnewlib-nano-malloc=false) requires sbrk return contiguous chunks of memory.

sbrk

Picolibc includes a simple version of sbrk that can return chunks of memory from a pre-defined contiguous heap. To use this function, your application linking process needs to define two symbols:

  • __heap_start — points at the start of the heap available for sbrk.
  • __heap_end — points at the end of the heap available for sbrk.

The sample linker script provided with picolibc defines these two symbols to enclose all RAM which is not otherwise used by the application.

Linking with System Library

To get Picolibc to use a system library, that library needs to be specified after libc on the linker command line. The picolibc.specs file provides a way to specify a library after libc using the --oslib= parameter:

$ gcc -o program.elf program.o --oslib=myos

This will include -lmyos after -lc so that the linker can resolve functions used by picolibc from libmyos.a. You can, alternatively, include the functions in object files with the rest of your application, which avoids the problem with libraries. Note that this mechanism requires the definition of _exit in the myos library.

Semihosting support

For RISC-V and ARM processors, Picolibc provides an implementation of all of the above functions along with a couple more POSIX APIs used by the Picolibc test suite. This implementation relies on semihosting support from the execution environment, which is available when running under qemu or when using openocd and gdb. Link this into your application using --oslib=semihost in your link command line. Please note that this will replace the default crt0 with a variant calling exit upon return from main. The default is to enter an infinite loop, and the change ensure a clean return to the execution environment.

POSIX console support

As a build-time option, Picolibc can be configured to use POSIX read and write APIs to support stdin, stdout and stderr. Add -Dposix-console=true to enable this. This is incompatible with semihosting support above.

Building picolibc on native POSIX systems

To allow for testing of picolibc and applications using picolibc, you can actually build picolibc on a full POSIX system. In this configuration, picolibc provides the non-POSIX libc APIs while the underlying system C library is used for the POSIX functions described above. To build in this mode, you'll need to override a few default picolibc configuration parameters:

$ meson \
	-Dtls-model=global-dynamic \
	-Dmultilib=false \
	-Dpicolib=false \
	-Dpicocrt=false \
	-Dposix-console=true \
	-Dnewlib-global-atexit=true \
	-Dincludedir=lib/picolibc/include \
	-Dlibdir=lib/picolibc/lib \
	-Dspecsdir=none

  • -Dtls-model=global-dynamic makes picolibc use the default TLS model for GCC.

  • -Dmultilib-false makes picolibc build only a single library for the default GCC configuration.

  • -Dpicolib=false disables building the TLS and sbrk support built-in to picolibc so that the underlying system support is used instead.

  • -Dpicocrt=false disables building the C startup code as that is provided by the underlying system.

  • -Dposix-console=true uses POSIX I/O read/write APIs for stdin, stdout and stderr.

  • -Dnewlib-global-atexit=true disables the per-thread atexit behavior so that picolibc acts like a regular C library.

  • -Dincludedir and -Dlibdir specify install locations for the headers and library

  • -Dspecsdir=none disables installing picolibc.specs as that file is not useful in this environment

Once built, you can install and use picolibc on the host:

$ cc -I/usr/local/lib/picolibc/include hello-world.c \
	/usr/local/lib/picolibc/lib/libc.a