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

EMLINK: 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