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Minimal libc

The most basic C library, named “minimal libc”, is part of the Zephyr codebase and provides the minimal subset of the standard C library required to meet the needs of Zephyr and its subsystems, primarily in the areas of string manipulation and display.

It is very low footprint and is suitable for projects that do not rely on less frequently used portions of the ISO C standard library. It can also be used with a number of different toolchains.

The minimal libc implementation can be found in lib/libc/minimal in the main Zephyr tree.

Formatted Output

The minimal libc does not implement its own formatted output processor; instead, it maps the C standard formatted output functions such as printf and sprintf to the cbprintf() function, which is Zephyr’s own C99-compatible formatted output implementation.

For more details, refer to the Formatted Output OS service documentation.

Dynamic Memory Management

Dynamic memory management in the minimal libc can be enabled by selecting the CONFIG_MINIMAL_LIBC_MALLOC in the application configuration file.

The minimal libc internally uses the kernel memory heap API to manage the memory heap used by the standard dynamic memory management interface functions such as malloc() and free().

The internal memory heap is normally located in the .bss section. When userspace is enabled, however, it is placed in a dedicated memory partition called z_malloc_partition, which can be accessed from the user mode threads. The size of the internal memory heap is specified by the CONFIG_MINIMAL_LIBC_MALLOC_ARENA_SIZE.

The standard dynamic memory management interface functions implemented by the minimal libc are thread safe and may be simultaneously called by multiple threads. These functions are implemented in lib/libc/minimal/source/stdlib/malloc.c.

Error numbers

Error numbers are used throughout Zephyr APIs to signal error conditions as return values from functions. They are typically returned as the negative value of the integer literals defined in this section, and are defined in the errno.h header file.

A subset of the error numbers defined in the POSIX errno.h specification and other de-facto standard sources have been added to the minimal libc.

A conscious effort is made in Zephyr to keep the values of the minimal libc error numbers consistent with the different implementations of the C standard libraries supported by Zephyr. The minimal libc errno.h is checked against that of the Newlib to ensure that the error numbers are kept aligned.

Below is a list of the error number definitions. For the actual numeric values please refer to errno.h.

group system_errno

System error numbers Error codes returned by functions. Includes a list of those defined by IEEE Std 1003.1-2017.

Defines

errno
EPERM

Not owner

ENOENT

No such file or directory

ESRCH

No such context

EINTR

Interrupted system call

EIO

I/O error

ENXIO

No such device or address

E2BIG

Arg list too long

ENOEXEC

Exec format error

EBADF

Bad file number

ECHILD

No children

EAGAIN

No more contexts

ENOMEM

Not enough core

EACCES

Permission denied

EFAULT

Bad address

ENOTBLK

Block device required

EBUSY

Mount device busy

EEXIST

File exists

EXDEV

Cross-device link

ENODEV

No such device

ENOTDIR

Not a directory

EISDIR

Is a directory

EINVAL

Invalid argument

ENFILE

File table overflow

EMFILE

Too many open files

ENOTTY

Not a typewriter

ETXTBSY

Text file busy

EFBIG

File too large

ENOSPC

No space left on device

ESPIPE

Illegal seek

EROFS

Read-only file system

Too many links

EPIPE

Broken pipe

EDOM

Argument too large

ERANGE

Result too large

ENOMSG

Unexpected message type

EDEADLK

Resource deadlock avoided

ENOLCK

No locks available

ENOSTR

STREAMS device required

ENODATA

Missing expected message data

ETIME

STREAMS timeout occurred

ENOSR

Insufficient memory

EPROTO

Generic STREAMS error

EBADMSG

Invalid STREAMS message

ENOSYS

Function not implemented

ENOTEMPTY

Directory not empty

ENAMETOOLONG

File name too long

ELOOP

Too many levels of symbolic links

EOPNOTSUPP

Operation not supported on socket

EPFNOSUPPORT

Protocol family not supported

ECONNRESET

Connection reset by peer

ENOBUFS

No buffer space available

EAFNOSUPPORT

Addr family not supported

EPROTOTYPE

Protocol wrong type for socket

ENOTSOCK

Socket operation on non-socket

ENOPROTOOPT

Protocol not available

ESHUTDOWN

Can’t send after socket shutdown

ECONNREFUSED

Connection refused

EADDRINUSE

Address already in use

ECONNABORTED

Software caused connection abort

ENETUNREACH

Network is unreachable

ENETDOWN

Network is down

ETIMEDOUT

Connection timed out

EHOSTDOWN

Host is down

EHOSTUNREACH

No route to host

EINPROGRESS

Operation now in progress

EALREADY

Operation already in progress

EDESTADDRREQ

Destination address required

EMSGSIZE

Message size

EPROTONOSUPPORT

Protocol not supported

ESOCKTNOSUPPORT

Socket type not supported

EADDRNOTAVAIL

Can’t assign requested address

ENETRESET

Network dropped connection on reset

EISCONN

Socket is already connected

ENOTCONN

Socket is not connected

ETOOMANYREFS

Too many references: can’t splice

ENOTSUP

Unsupported value

EILSEQ

Illegal byte sequence

EOVERFLOW

Value overflow

ECANCELED

Operation canceled

EWOULDBLOCK

Operation would block