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Formatted Output

Applications as well as Zephyr itself requires infrastructure to format values for user consumption. The standard C99 library *printf() functionality fulfills this need for streaming output devices or memory buffers, but in an embedded system devices may not accept streamed data and memory may not be available to store the formatted output.

Internal Zephyr API traditionally provided this both for printk() and for Zephyr’s internal minimal libc, but with separate internal interfaces. Logging, tracing, shell, and other applications made use of either these APIs or standard libc routines based on build options.

The cbprintf() public APIs convert C99 format strings and arguments, providing output produced one character at a time through a callback mechanism, replacing the original internal functions and providing support for almost all C99 format specifications. Existing use of s*printf() C libraries in Zephyr can be converted to snprintfcb() to avoid pulling in libc implementations.

Several Kconfig options control the set of features that are enabled, allowing some control over features and memory usage:

CONFIG_CBPRINTF_LIBC_SUBSTS can be used to provide functions that behave like standard libc functions but use the selected cbprintf formatter rather than pulling in another formatter from libc.

In addition CONFIG_CBPRINTF_NANO can be used to revert back to the very space-optimized but limited formatter used for printk() before this capability was added.

Cbprintf Packaging

Typically, strings are formatted synchronously when a function from printf family is called. However, there are cases when it is beneficial that formatting is deferred. In that case, a state (format string and arguments) must be captured. Such state forms a self-contained package which contains format string and arguments. Additionally, package may contain copies of strings which are part of a format string (format string or any %s argument). Package primary content resembles va_list stack frame thus standard formatting functions are used to process a package. Since package contains data which is processed as va_list frame, strict alignment must be maintained. Due to required padding, size of the package depends on alignment. When package is copied, it should be copied to a memory block with the same alignment as origin.

Package can have following variants:

Self-contained - non read-only strings appended to the package. String can be formatted from such package as long as there is access to read-only string locations. Package may contain information where read-only strings are located within the package. That information can be used to convert packet to fully self-contained package.
Fully self-contained - all strings are appended to the package. String can be formatted from such package without any external data.
Transient- only arguments are stored. Package contain information where pointers to non read-only strings are located within the package. Optionally, it may contain read-only string location information. String can be formatted from such package as long as non read-only strings are still valid and read-only strings are accessible. Alternatively, package can be converted to self-contained package or fully self-contained if information about read-only string locations is present in the package.

Package can be created using two methods:

runtime - using cbprintf_package() or cbvprintf_package(). This method scans format string and based on detected format specifiers builds the package.
static - types of arguments are detected at compile time by the preprocessor and package is created as simple assignments to a provided memory. This method is significantly faster than runtime (more than 15 times) but has following limitations: requires _Generic keyword (C11 feature) to be supported by the compiler and cannot distinguish between %p and %s if char pointer is used. It treats all (unsigned) char pointers as %s thus it will attempt to append string to a package. It can be handled correctly during conversion from transient package to self-contained package using CBPRINTF_PACKAGE_CONVERT_PTR_CHECK flag. However, it requires access to the format string and it is not always possible thus it is recommended to cast char pointers used for %p to void *. There is a logging warning generated by cbprintf_package_convert() called with CBPRINTF_PACKAGE_CONVERT_PTR_CHECK flag when char pointer is used with %p.

Several Kconfig options control behavior of the packaging:

Cbprintf package conversion

It is possible to convert package to a variant which contains more information, e.g transient package can be converted to self-contained. Conversion to fully self-contained package is possible if CBPRINTF_PACKAGE_ADD_RO_STR_POS flag was used when package was created.

cbprintf_package_copy() is used to calculate space needed for the new package and to copy and convert a package.

Cbprintf package format

Format of the package contains paddings which are platform specific. Package consists of header which contains size of package (excluding appended strings) and number of appended strings. It is followed by the arguments which contains alignment paddings and resembles va_list stack frame. It is followed by data associated with character pointer arguments used by the string which are not appended to the string (but may be appended later by cbprinf_package_convert()). Finally, package, optionally, contains appended strings. Each string contains 1 byte header which contains index of the location where address argument is stored. During packaging address is set to null and before string formatting it is updated to point to the current string location within the package. Updating address argument must happen just before string formatting since address changes whenever package is copied.

Header sizeof(void *)	1 byte: Argument list size including header and fmt (in 32 bit words)
	1 byte: Number of strings appended to the package
	1 byte: Number of read-only string argument locations
	1 byte: Number of transient string argument locations
	platform specific padding to sizeof(void *)
Arguments	Pointer to fmt (or null if fmt is appended to the package)
	(optional padding for platform specific alignment)
	argument 0
	(optional padding for platform specific alignment)
	argument 1
	…
String location information (optional)	Indexes of words within the package where read-only strings are located
String location information (optional)	Pairs of argument index and argument location index where transient strings are located
Appended strings (optional)	1 byte: Index within the package to the location of associated argument
	Null terminated string
	…

Warning

If CONFIG_MINIMAL_LIBC is selected in combination with CONFIG_CBPRINTF_NANO formatting with C standard library functions like printf or snprintf is limited. Among other things the %n specifier, most format flags, precision control, and floating point are not supported.

Limitations and recommendations

C11 _Generic support is required by the compiler to use static (fast) packaging.
It is recommended to cast any character pointer used with %p format specifier to other pointer type (e.g. void *). If format string is not accessible then only static packaging is possible and it will append all detected strings. Character pointer used for %p will be considered as string pointer. Copying from unexpected location can have serious consequences (e.g., memory fault or security violation).

API Reference

group cbprintf_apis

Defines

CBPRINTF_PACKAGE_ALIGNMENT: Required alignment of the buffer used for packaging.

CBPRINTF_MUST_RUNTIME_PACKAGE(flags, ...)

Determine if string must be packaged in run time.

Static packaging can be applied if size of the package can be determined at compile time. In general, package size can be determined at compile time if there are no string arguments which might be copied into package body if they are considered transient.

Note

By default any char pointers are considered to be pointing at transient strings. This can be narrowed down to non const pointers by using CBPRINTF_PACKAGE_CONST_CHAR_RO.

Parameters:

... – String with arguments.
flags – option flags. See Package flags.

Return values:

1 – if string must be packaged in run time.
0 – string can be statically packaged.

CBPRINTF_STATIC_PACKAGE(packaged, inlen, outlen, align_offset, flags, ...)

Statically package string.

Build string package from formatted string. It assumes that formatted string is in the read only memory.

If _Generic is not supported then runtime packaging is performed.

Parameters:

packaged – pointer to where the packaged data can be stored. Pass a null pointer to skip packaging but still calculate the total space required. The data stored here is relocatable, that is it can be moved to another contiguous block of memory. It must be aligned to the size of the longest argument. It is recommended to use CBPRINTF_PACKAGE_ALIGNMENT for alignment.
inlen – set to the number of bytes available at packaged. If packaged is NULL the value is ignored.
outlen – variable updated to the number of bytes required to completely store the packed information. If input buffer was too small it is set to -ENOSPC.
align_offset – input buffer alignment offset in bytes. Where offset 0 means that buffer is aligned to CBPRINTF_PACKAGE_ALIGNMENT. Xtensa requires that packaged is aligned to CBPRINTF_PACKAGE_ALIGNMENT so it must be multiply of CBPRINTF_PACKAGE_ALIGNMENT or 0.
flags – option flags. See Package flags.
... – formatted string with arguments. Format string must be constant.

Typedefs

typedef int (*cbprintf_cb)()

Signature for a cbprintf callback function.

This function expects two parameters:

c a character to output. The output behavior should be as if this was cast to an unsigned char.
ctx a pointer to an object that provides context for the output operation.

The declaration does not specify the parameter types. This allows a function like fputc to be used without requiring all context pointers to be to a FILE object.

Return:: the value of c cast to an unsigned char then back to int, or a negative error code that will be returned from cbprintf().

typedef int (*cbprintf_cb_local)(int c, void *ctx)

typedef int (*cbprintf_convert_cb)(const void *buf, size_t len, void *ctx)

Signature for a cbprintf multibyte callback function.

return Amount of copied data or negative error code.

Param buf:: data.
Param len:: data length.
Param ctx:: a pointer to an object that provides context for the operation.

typedef int (*cbvprintf_external_formatter_func)(cbprintf_cb out, void *ctx, const char *fmt, va_list ap)

Signature for a external formatter function identical to cbvprintf.

This function expects the following parameters:

Param out:: the function used to emit each generated character.
Param ctx:: a pointer to an object that provides context for the external formatter.
Param fmt:: a standard ISO C format string with characters and conversion specifications.
Param ap:: captured stack arguments corresponding to the conversion specifications found within fmt.
Return:: vprintf like return values: the number of characters printed, or a negative error value returned from external formatter.

Functions

int cbprintf_package(void *packaged, size_t len, uint32_t flags, const char *format, ...)

Capture state required to output formatted data later.

Like cbprintf() but instead of processing the arguments and emitting the formatted results immediately all arguments are captured so this can be done in a different context, e.g. when the output function can block.

In addition to the values extracted from arguments this will ensure that copies are made of the necessary portions of any string parameters that are not confirmed to be stored in read-only memory (hence assumed to be safe to refer to directly later).

Parameters:

packaged – pointer to where the packaged data can be stored. Pass a null pointer to store nothing but still calculate the total space required. The data stored here is relocatable, that is it can be moved to another contiguous block of memory. However, under condition that alignment is maintained. It must be aligned to at least the size of a pointer.
len – this must be set to the number of bytes available at packaged if it is not null. If packaged is null then it indicates hypothetical buffer alignment offset in bytes compared to CBPRINTF_PACKAGE_ALIGNMENT alignment. Buffer alignment offset impacts returned size of the package. Xtensa requires that buffer is always aligned to CBPRINTF_PACKAGE_ALIGNMENT so it must be multiply of CBPRINTF_PACKAGE_ALIGNMENT or 0 when packaged is null.
flags – option flags. See Package flags.
format – a standard ISO C format string with characters and conversion specifications.
... – arguments corresponding to the conversion specifications found within format.

Return values:

nonegative – the number of bytes successfully stored at packaged. This will not exceed len.
-EINVAL – if format is not acceptable
-EFAULT – if packaged alignment is not acceptable
-ENOSPC – if packaged was not null and the space required to store exceed len.

int cbvprintf_package(void *packaged, size_t len, uint32_t flags, const char *format, va_list ap)

Capture state required to output formatted data later.

Like cbprintf() but instead of processing the arguments and emitting the formatted results immediately all arguments are captured so this can be done in a different context, e.g. when the output function can block.

In addition to the values extracted from arguments this will ensure that copies are made of the necessary portions of any string parameters that are not confirmed to be stored in read-only memory (hence assumed to be safe to refer to directly later).

Parameters:

packaged – pointer to where the packaged data can be stored. Pass a null pointer to store nothing but still calculate the total space required. The data stored here is relocatable, that is it can be moved to another contiguous block of memory. The pointer must be aligned to a multiple of the largest element in the argument list.
len – this must be set to the number of bytes available at packaged. Ignored if packaged is NULL.
flags – option flags. See Package flags.
format – a standard ISO C format string with characters and conversion specifications.
ap – captured stack arguments corresponding to the conversion specifications found within format.

Return values:

nonegative – the number of bytes successfully stored at packaged. This will not exceed len.
-EINVAL – if format is not acceptable
-ENOSPC – if packaged was not null and the space required to store exceed len.

int cbprintf_package_convert(void *in_packaged, size_t in_len, cbprintf_convert_cb cb, void *ctx, uint32_t flags, uint16_t *strl, size_t strl_len)

Convert a package.

Converting may include appending strings used in the package to the package body. If input package was created with CBPRINTF_PACKAGE_ADD_RO_STR_POS or CBPRINTF_PACKAGE_ADD_RW_STR_POS, it contains information where strings are located within the package. This information can be used to copy strings during the conversion.

cb is called with portions of the output package. At the end of the conversion cb is called with null buffer.

Parameters:

in_packaged – Input package.
in_len – Input package length. If 0 package length will be retrieved from the in_packaged
cb – callback called with portions of the converted package. If null only length of the output package is calculated.
ctx – Context provided to the cb.
flags – Flags. See Package convert flags.
strl – [inout] if packaged is null, it is a pointer to the array where strl_len first string lengths will is stored. If packaged is not null, it contains lengths of first strl_len strings. It can be used to optimize copying so that string length is calculated only once (at length calculation phase when packaged is null.)
strl_len – Number of elements in strl array.

Return values:

Positive – output package size.
-ENOSPC – if packaged was not null and the space required to store exceed len.

static inline int cbprintf_package_copy(void *in_packaged, size_t in_len, void *packaged, size_t len, uint32_t flags, uint16_t *strl, size_t strl_len)

Copy package with optional appending of strings.

cbprintf_package_convert is used to convert and store converted package in the new location.

Parameters:

in_packaged – Input package.
in_len – Input package length. If 0 package length will be retrieved from the in_packaged
packaged – [out] Output package. If null only length of the output package is calculated.
len – Available space in the location pointed by packaged. Not used when packaged is null.
flags – Flags. See Package convert flags.
strl – [inout] if packaged is null, it is a pointer to the array where strl_len first string lengths will is stored. If packaged is not null, it contains lengths of first strl_len strings. It can be used to optimize copying so that string length is calculated only once (at length calculation phase when packaged is null.)
strl_len – Number of elements in strl array.

Return values:

Positive – Output package size.
-ENOSPC – if packaged was not null and the space required to store exceed len.

static inline int cbprintf_fsc_package(void *in_packaged, size_t in_len, void *packaged, size_t len)

Convert package to fully self-contained (fsc) package.

Package may not be self contain since strings by default are stored by address. Package may be partially self-contained when transient (not read only) strings are appended to the package. Such package can be decoded only when there is an access to read-only strings.

Fully self-contained has (fsc) contains all strings used in the package. A package can be converted to fsc package if it was create with CBPRINTF_PACKAGE_ADD_RO_STR_POS flag. Such package will contain necessary data to find read only strings in the package and copy them into the package body.

Parameters:

in_packaged – pointer to original package created with CBPRINTF_PACKAGE_ADD_RO_STR_POS.
in_len – in_packaged length.
packaged – pointer to location where fully self-contained version of the input package will be written. Pass a null pointer to calculate space required.
len – must be set to the number of bytes available at packaged. Not used if packaged is null.

Return values:

nonegative – the number of bytes successfully stored at packaged. This will not exceed len. If packaged is null, calculated length.
-ENOSPC – if packaged was not null and the space required to store exceed len.
-EINVAL – if in_packaged is null.

int cbpprintf_external(cbprintf_cb out, cbvprintf_external_formatter_func formatter, void *ctx, void *packaged)

Generate the output for a previously captured format operation using an external formatter.

Note

Memory indicated by packaged will be modified in a non-destructive way, meaning that it could still be reused with this function again.

Parameters:

out – the function used to emit each generated character.
formatter – external formatter function.
ctx – a pointer to an object that provides context for the external formatter.
packaged – the data required to generate the formatted output, as captured by cbprintf_package() or cbvprintf_package(). The alignment requirement on this data is the same as when it was initially created.

Returns:

printf like return values: the number of characters printed, or a negative error value returned from external formatter.

int cbprintf(cbprintf_cb out, void *ctx, const char *format, ...)

*printf-like output through a callback.

This is essentially printf() except the output is generated character-by-character using the provided out function. This allows formatting text of unbounded length without incurring the cost of a temporary buffer.

All formatting specifiers of C99 are recognized, and most are supported if the functionality is enabled.

Note

The functionality of this function is significantly reduced when CONFIG_CBPRINTF_NANO is selected.

Parameters:

out – the function used to emit each generated character.
ctx – context provided when invoking out
format – a standard ISO C format string with characters and conversion specifications.
... – arguments corresponding to the conversion specifications found within format.

Returns:

the number of characters printed, or a negative error value returned from invoking out.

static inline int cbvprintf(cbprintf_cb out, void *ctx, const char *format, va_list ap)

varargs-aware *printf-like output through a callback.

This is essentially vsprintf() except the output is generated character-by-character using the provided out function. This allows formatting text of unbounded length without incurring the cost of a temporary buffer.

Note

This function is available only when CONFIG_CBPRINTF_LIBC_SUBSTS is selected.

Note

The functionality of this function is significantly reduced when CONFIG_CBPRINTF_NANO is selected.

Parameters:

out – the function used to emit each generated character.
ctx – context provided when invoking out
format – a standard ISO C format string with characters and conversion specifications.
ap – a reference to the values to be converted.

Returns:

the number of characters generated, or a negative error value returned from invoking out.

static inline int cbvprintf_tagged_args(cbprintf_cb out, void *ctx, const char *format, va_list ap)

varargs-aware *printf-like output through a callback with tagged arguments.

This is essentially vsprintf() except the output is generated character-by-character using the provided out function. This allows formatting text of unbounded length without incurring the cost of a temporary buffer.

Note that the argument list ap are tagged.

Note

This function is available only when CONFIG_CBPRINTF_LIBC_SUBSTS is selected.

Note

The functionality of this function is significantly reduced when CONFIG_CBPRINTF_NANO is selected.

Parameters:

out – the function used to emit each generated character.
ctx – context provided when invoking out
format – a standard ISO C format string with characters and conversion specifications.
ap – a reference to the values to be converted.

Returns:

the number of characters generated, or a negative error value returned from invoking out.

static inline int cbpprintf(cbprintf_cb out, void *ctx, void *packaged)

Generate the output for a previously captured format operation.

Note

Memory indicated by packaged will be modified in a non-destructive way, meaning that it could still be reused with this function again.

Parameters:

out – the function used to emit each generated character.
ctx – context provided when invoking out
packaged – the data required to generate the formatted output, as captured by cbprintf_package() or cbvprintf_package(). The alignment requirement on this data is the same as when it was initially created.

Returns:

the number of characters printed, or a negative error value returned from invoking out.

int fprintfcb(FILE *stream, const char *format, ...)

fprintf using Zephyrs cbprintf infrastructure.

return The number of characters printed.

Note

This function is available only when CONFIG_CBPRINTF_LIBC_SUBSTS is selected.

Note

The functionality of this function is significantly reduced when CONFIG_CBPRINTF_NANO is selected.

Parameters:

stream – the stream to which the output should be written.
format – a standard ISO C format string with characters and conversion specifications.
... – arguments corresponding to the conversion specifications found within format.

int vfprintfcb(FILE *stream, const char *format, va_list ap)

vfprintf using Zephyrs cbprintf infrastructure.

Note

This function is available only when CONFIG_CBPRINTF_LIBC_SUBSTS is selected.

Note

The functionality of this function is significantly reduced when CONFIG_CBPRINTF_NANO is selected.

Parameters:

stream – the stream to which the output should be written.
format – a standard ISO C format string with characters and conversion specifications.
ap – a reference to the values to be converted.

Returns:

The number of characters printed.

int printfcb(const char *format, ...)

printf using Zephyrs cbprintf infrastructure.

Note

This function is available only when CONFIG_CBPRINTF_LIBC_SUBSTS is selected.

Note

The functionality of this function is significantly reduced when CONFIG_CBPRINTF_NANO is selected.

Parameters:

format – a standard ISO C format string with characters and conversion specifications.
... – arguments corresponding to the conversion specifications found within format.

Returns:

The number of characters printed.

int vprintfcb(const char *format, va_list ap)

vprintf using Zephyrs cbprintf infrastructure.

Note

This function is available only when CONFIG_CBPRINTF_LIBC_SUBSTS is selected.

Note

The functionality of this function is significantly reduced when CONFIG_CBPRINTF_NANO is selected.

Parameters:

format – a standard ISO C format string with characters and conversion specifications.
ap – a reference to the values to be converted.

Returns:

The number of characters printed.

int snprintfcb(char *str, size_t size, const char *format, ...)

snprintf using Zephyrs cbprintf infrastructure.

Note

This function is available only when CONFIG_CBPRINTF_LIBC_SUBSTS is selected.

Note

The functionality of this function is significantly reduced when CONFIG_CBPRINTF_NANO is selected.

Parameters:

str – where the formatted content should be written
size – maximum number of chaacters for the formatted output, including the terminating null byte.
format – a standard ISO C format string with characters and conversion specifications.
... – arguments corresponding to the conversion specifications found within format.

Returns:

The number of characters that would have been written to str, excluding the terminating null byte. This is greater than the number actually written if size is too small.

int vsnprintfcb(char *str, size_t size, const char *format, va_list ap)

vsnprintf using Zephyrs cbprintf infrastructure.

Note

This function is available only when CONFIG_CBPRINTF_LIBC_SUBSTS is selected.

Note

The functionality of this function is significantly reduced when CONFIG_CBPRINTF_NANO is selected.

Parameters:

str – where the formatted content should be written
size – maximum number of chaacters for the formatted output, including the terminating null byte.
format – a standard ISO C format string with characters and conversion specifications.
ap – a reference to the values to be converted.

Returns:

The number of characters that would have been written to str, excluding the terminating null byte. This is greater than the number actually written if size is too small.