I remember in my early days of learning C that knowing what the function abbreviation stand for helped me a lot. So this is an attempt to demystify the abbreviations.

This post focuses on the abbreviation, not on the meaning. Thus some important points need to be raised:

some abbreviations do not make make sense just by themselves. Either because some words are implicit, or because the order of the letters is weird when expanding the abbreviation. For instance iswprint() is an abbreviation for "is wide printable". The meaning is "wide character is a printing character"
some abbreviation are misleading. E.g. the FPE in SIGFPE stands for "Floating Point Exception", but the category is for arithmetic exceptions. E.g. qsort() stands for "quick sort" but the standard doesn't require this algorithm and it is usually implemented as a polymorphic sort.

Standard Library

Structure follows https://en.cppreference.com/w/c

Type Support

Symbol	Meaning
`size_t`	size type
`ptrdiff_t`	pointer difference type
`NULL`	null
`max_align_t`	maximum align type
`offsetof`	offset of
`alignas`	align as
`alignof`	align of
`noreturn`	no return

Program support utilities

Program termination

Symbol	Meaning
`abort`	abort
`exit`	exit
`quick_exit`	quick exit
`_Exit`	exit
`atexit`	at exit
`at_quick_exit`	at quick exit

Communicating with the environment

Symbol	Meaning
`system`	system
`getenv`	get environment (variables)
`getenv_s`	get environment (variables) safe

Signals

Symbol	Meaning
`signal`	signal
`raise`	raise
`sig_atomic_t`	signal atomic type
`SIG_DFL`	signal default
`SIG_IGN`	signal ignore
`SIG_ERR`	signal error

Signal Types

Symbol	Meaning
`SIGTERM`	signal termination
`SIGSEGV`	signal segmentation violation
`SIGINT`	signal interrupt
`SIGILL`	signal illegal instruction
`SIGABRT`	signal abnormal termination
`SIGFPE`	signal floating point exception

Non-local jumps

Symbol	Meaning
`setjmp`	set jump
`longjmp`	long jump

Types

Symbol	Meaning
`jmp_buf`	jump buffer

Variadic function

Symbol	Meaning
`va_start`	variable arguments start
`va_end`	variable arguments end
`va_copy`	variable arguments copy
`va_arg`	variable arguments' argument
`va_list`	variable arguments list

Memory management

Symbol	Meaning
`malloc`	memory allocation
`calloc`	clean allocation
`aligned_alloc`	aligned allocation
`realloc`	reallocate
`free`	free

Date and time utilities

TODO

String Library

Null-terminated byte strings

Character classification

Symbol	Meaning
`isalnum`	is alphanumeric
`isalpha`	is alphabetic
`islower`	is lowercase
`isupper`	is uppercase
`isdigit`	is digit
`isxdigit`	is hexadecimal digit
`iscntrl`	is control
`isgraph`	is graphic
`isspace`	is space
`isblank`	is blank
`isprint`	is printable
`ispunct`	is punctuation

Character manipulation

Symbol	Meaning
`tolower`	to lowercase
`toupper`	to uppercase

Conversions to numeric formats

Symbol	Meaning
`atof`	ASCII to floating-point
`atoi`	ASCII to `int`
`atol`	ASCII to `long`
`atoll`	ASCII to `long long`
`strtol`	string to `long`
`strtoll`	string to `long long`
`strtoul`	string to `unsigned long`
`strtoull`	string to `unsigned long long`
`strtof`	string to `float`
`strtod`	string to `double`
`strtold`	string to `long double`
`strtoimax`	string to `intmax_t`
`strtoumax`	string to `uintmax_t`

String manipulation

Symbol	Meaning
`strcpy`	string copy
`strcpy_s`	string copy safe
`strncpy`	string n copy
`strncpy_s`	string n copy safe
`strcat`	string concatenation
`strcat_s`	string concatenation safe
`strncat`	string n concatenation
`strncat_s`	string n concatenation safe
`strxfrm`	string transformation

String examination

Symbol	Meaning
`strlen`	string length
`strlen_s`	string length safe
`strcmp`	string comparison
`strncmp`	string n comparison
`strcoll`	string collation
`strchr`	string (find) character
`strrchr`	string reverse (find) character
`strspn`	string span
`strcspn`	string complementary span
`strpbrk`	string pointer break
`strstr`	string (find) string
`strtok`	string tokenization
`strtok_s`	string tokenization safe

Character array manipulation

Symbol	Meaning
`memchr`	memory (search) character
`memcmp`	memory comparison
`memset`	memory set
`memset_s`	memory set safe
`memcpy`	memory copy
`memcpy_s`	memory copy safe
`memmove`	memory move
`memmove_s`	memory move safe

Miscellaneous

Symbol	Meaning
`strerror`	string error
`strerror_s`	string error safe
`strerrorlen_s`	string error length safe

Null-terminated multibyte strings

Multibyte/wide character conversions

Symbol	Meaning
`mblen`	multibyte length
`mbtowc`	multibyte to wide character
`wctomb`	wide character to multibyte
`wctomb_s`	wide character to multibyte safe
`mbstowcs`	multibyte string to wide character string
`mbstowcs_s`	multibyte string to wide character string safe
`wcstombs`	wide character string to multibyte string
`wcstombs_s`	wide character string to multibyte string safe
`mbsinit`	`mbstate_t` initialization
`btowc`	byte to wide character
`wctob`	wide character to byte
`mbrlen`	multibyte reentrant length
`mbrtowc`	multibyte reentrant to wide character
`wcrtomb`	wide character reentrant to multibyte
`wcrtomb_s`	wide character reentrant to multibyte safe
`mbsrtowcs`	multibyte string reentrant to wide character string
`mbsrtowcs_s`	multibyte string reentrant to wide character string safe
`wcsrtombs`	wide character string reentrant to multibyte string
`wcsrtombs_s`	wide character string reentrant to multibyte string safe
`mbrtoc16`	multibyte reentrant to `char16_t`
`c16rtomb`	`char16_t` reentrant to multibyte
`mbrtoc32`	multibyte reentrant to `char32_t`
`c32rtomb`	`char32_t` reentrant to multibyte

Types

Symbol	Meaning
`mbstate_t`	multibyte state type
`char16_t`	character 16-bit type
`char32_t`	character 32-bit type

Null-terminated wide strings

Character classification

Symbol	Meaning
`iswalnum`	is wide alphanumeric
`iswalpha`	is wide alphabetic
`iswlower`	is wide lowercase
`iswupper`	is wide uppercase
`iswdigit`	is wide digit
`iswxdigit`	is wide hexadecimal
`iswcntrl`	is wide control
`iswgraph`	is wide graphic
`iswspace`	is wide space
`iswblank`	is wide blank
`iswprint`	is wide printable
`iswpunct`	is wide punctuation
`iswctype`	is wide character type
`wctype`	wide character type

Character manipulation

Symbol	Meaning
`towlower`	to wide lowercase
`towupper`	to wide uppercase
`towctrans`	to wide character transformation
`wctrans`	wide character transformation

Conversions to numeric formats

Symbol	Meaning
`wcstol`	wide character string to `long`
`wcstoll`	wide character string to `long long`
`wcstoul`	wide character string to `unsigned long`
`wcstoull`	wide character string to `unsigned long long`
`wcstof`	wide character string to `float`
`wcstod`	wide character string to `double`
`wcstold`	wide character string to `long double`
`wcstoimax`	wide character string to `intmax_t`
`wcstoumax`	wide character string to `uintmax_t`

String manipulation

Symbol	Meaning
`wcscpy`	wide character string copy
`wcscpy_s`	wide character string copy safe
`wcsncpy`	wide character string n copy
`wcsncpy_s`	wide character string n copy safe
`wcscat`	wide character string concatenation
`wcscat_s`	wide character string concatenation safe
`wcsncat`	wide character string n concatenation
`wcsncat_s`	wide character string n concatenation safe
`wcsxfrm`	wide character string transformation

String examination

Symbol	Meaning
`wcslen`	wide character string length
`wcslen_s`	wide character string length safe
`wcscmp`	wide character string comparison
`wcsncmp`	wide character string n comparison
`wcscoll`	wide character string collation
`wcschr`	wide character string (find) character
`wcsrchr`	wide character string reverse (find) character
`wcsspn`	wide character string span
`wcscspn`	wide character string complementary span
`wcspbrk`	wide character string pointer break
`wcsstr`	wide character string (find) string
`wcstok`	wide character string tokenization
`wcstok_s`	wide character string tokenization safe

Wide character array manipulation

Symbol	Meaning
`wmemchr`	wide memory (search) character
`wmemcmp`	wide memory comparison
`wmemset`	wide memory set
`wmemcpy`	wide memory copy
`wmemcpy_s`	wide memory copy safe
`wmemmove`	wide memory move
`wmemmove_s`	wide memory move safe

Types

Symbol	Meaning
`wchar_t`	wide character type
`wint_t`	wide integer type
`wctrans_t`	wide character transformation type
`wctype_t`	wide character type type

Algorithms

Symbol	Meaning
`qsort`	quick sort
`qsort_s`	quick sort safe
`bsearch`	binary search
`bsearch_s`	binary search safe

Numerics

Mathematical functions

TODO

Floating-point environment

TODO

Pseudo-random number generation

TODO

Complex number arithmetic

TODO

Type-generic math

TODO

Input/output support

TODO

Localization support

TODO

Atomic operations library

TODO

Thread support library

TODO

POSIX

TODO

Is there documentation that describes what C function names "stand for" (not what they do--rather, what the abreviated name means)?

Standard Library

Type Support

Program support utilities

Program termination

Communicating with the environment

Signals

Signal Types

Non-local jumps

Types

Variadic function

Memory management

Date and time utilities

String Library

Null-terminated byte strings

Null-terminated multibyte strings

Null-terminated wide strings

Algorithms

Numerics

Mathematical functions

Floating-point environment

Pseudo-random number generation

Complex number arithmetic

Type-generic math

Input/output support

Localization support

Atomic operations library

Thread support library

POSIX