C Reference Manual

This is a quick reference manual — no need to read it cover to cover. Just look things up when you need them.

Keyword Quick Reference

C89 Keywords (32)

Keyword	Description
`auto`	Automatic storage duration (default)
`break`	Exit loop or switch
`case`	switch branch label
`char`	Character type
`const`	Constant qualifier
`continue`	Jump to next loop iteration
`default`	switch default branch
`do`	do-while loop
`double`	Double-precision floating point
`else`	if-else else branch
`enum`	Enumeration type
`extern`	External linkage declaration
`float`	Single-precision floating point
`for`	for loop
`goto`	Unconditional jump
`if`	Conditional statement
`int`	Integer type
`long`	Long integer qualifier
`register`	Register storage hint
`return`	Function return
`short`	Short integer qualifier
`signed`	Signed qualifier
`sizeof`	Get type or variable size
`static`	Static storage duration or internal linkage
`struct`	Structure
`switch`	Multi-branch selection
`typedef`	Type alias
`union`	Union
`unsigned`	Unsigned qualifier
`void`	Empty type
`volatile`	Prevent optimization qualifier
`while`	while loop

C99 New Keywords (5)

Keyword	Description
`_Bool`	Boolean type (usually via `bool` macro)
`_Complex`	Complex number type
`_Imaginary`	Imaginary number type
`inline`	Inline function hint
`restrict`	Pointer aliasing restriction

C11 New Keywords (7)

Keyword	Description
`_Alignas`	Alignment specifier
`_Alignof`	Get alignment requirement
`_Atomic`	Atomic type
`_Generic`	Generic selection
`_Noreturn`	Function does not return
`_Static_assert`	Static assertion
`_Thread_local`	Thread-local storage

Format Specifier Quick Reference

printf Format Specifiers

Specifier	Output Type	Example
`%d` / `%i`	Signed decimal integer	`printf("%d", 42)` → 42
`%u`	Unsigned decimal integer	`printf("%u", 42U)` → 42
`%o`	Unsigned octal	`printf("%o", 8)` → 10
`%x`	Unsigned hexadecimal (lowercase)	`printf("%x", 255)` → ff
`%X`	Unsigned hexadecimal (uppercase)	`printf("%X", 255)` → FF
`%f`	Decimal floating point	`printf("%f", 3.14)` → 3.140000
`%e`	Scientific notation (lowercase e)	`printf("%e", 0.001)` → 1.000000e-03
`%E`	Scientific notation (uppercase E)	`printf("%E", 0.001)` → 1.000000E-03
`%g`	Auto-select %f or %e	`printf("%g", 0.0001)` → 0.0001
`%c`	Character	`printf("%c", 'A')` → A
`%s`	String	`printf("%s", "hi")` → hi
`%p`	Pointer address	`printf("%p", ptr)` → 0x7ffd...
`%ld`	long	`printf("%ld", 100000L)`
`%lld`	long long	`printf("%lld", 1LL<<40)`
`%zu`	size_t	`printf("%zu", sizeof(int))`
`%%`	Percent sign itself	`printf("100%%")` → 100%

printf Modifiers

Modifier	Meaning	Example
`%-`	Left-align	`"%-10s"`
`%0`	Pad with zeros	`"%05d"` → 00042
`%m`	Minimum field width	`"%10d"`
`%.n`	Precision	`"%.2f"` → 3.14

scanf Format Specifiers

Specifier	Read Type
`%d`	int
`%u`	unsigned int
`%ld`	long
`%f`	float
`%lf`	double
`%c`	Character
`%s`	String (stops at whitespace)
`%[...]`	Scanset
`%*d`	Read but discard

Escape Character Table

Escape Sequence	Meaning	ASCII Value
`\a`	Alert (bell)	7
`\b`	Backspace	8
`\f`	Form feed	12
`\n`	Newline	10
`\r`	Carriage return	13
`\t`	Horizontal tab	9
`\v`	Vertical tab	11
`\\`	Backslash	92
`\'`	Single quote	39
`\"`	Double quote	34
`\0`	Null character	0
`\ooo`	Octal value	—
`\xhh`	Hexadecimal value	—

Operator Precedence Table

Listed from highest to lowest. Operators on the same line have equal precedence:

Precedence	Operators	Associativity	Description
1	`()` `[]` `->` `.` `++`(postfix) `--`(postfix)	Left→Right	Postfix
2	`++`(prefix) `--`(prefix) `+` `-` `!` `~` `*`(deref) `&`(address) `sizeof` `(type)`	Right→Left	Unary
3	`*` `/` `%`	Left→Right	Multiplicative
4	`+` `-`	Left→Right	Additive
5	`<<` `>>`	Left→Right	Shift
6	`<` `<=` `>` `>=`	Left→Right	Relational
7	`==` `!=`	Left→Right	Equality
8	`&`	Left→Right	Bitwise AND
9	`^`	Left→Right	Bitwise XOR
10	`\|`	Left→Right	Bitwise OR
11	`&&`	Left→Right	Logical AND
12	`\|\|`	Left→Right	Logical OR
13	`?:`	Right→Left	Conditional
14	`=` `+=` `-=` `*=` `/=` `%=` `&=` `^=` `\|=` `<<=` `>>=`	Right→Left	Assignment
15	`,`	Left→Right	Comma

💡 Tip: Mnemonic: Unary → Multiplicative → Additive → Shift → Relational → Equality → Bitwise AND → XOR → Bitwise OR → Logical AND → Logical OR → Conditional → Assignment → Comma

Common Header File Function Reference

stdio.h

Function	Purpose
`printf`	Formatted output to stdout
`fprintf`	Formatted output to file
`sprintf`	Formatted output to string
`snprintf`	Formatted output to string (safe)
`scanf`	Formatted read from stdin
`fscanf`	Formatted read from file
`sscanf`	Formatted read from string
`fopen`	Open file
`fclose`	Close file
`fgetc`	Read one character
`fputc`	Write one character
`fgets`	Read one line
`fputs`	Write string
`fread`	Binary read
`fwrite`	Binary write
`fseek`	Set file position
`ftell`	Get file position
`rewind`	Reset file position to beginning
`feof`	Check end of file
`ferror`	Check file error
`fflush`	Flush buffer
`remove`	Delete file
`rename`	Rename file
`tmpfile`	Create temporary file
`perror`	Print error message

stdlib.h

Function	Purpose
`malloc`	Dynamic memory allocation
`calloc`	Allocate and zero memory
`realloc`	Reallocate memory
`free`	Release memory
`atoi`	String to int
`atol`	String to long
`atof`	String to double
`strtol`	String to long (safe)
`strtod`	String to double (safe)
`rand`	Generate random number
`srand`	Set random seed
`qsort`	Quick sort
`bsearch`	Binary search
`abs`	Integer absolute value
`labs`	long absolute value
`exit`	Terminate program
`atexit`	Register exit function
`system`	Execute system command
`getenv`	Get environment variable

string.h

Function	Purpose
`strcpy`	String copy
`strncpy`	String copy (safe)
`strcat`	String concatenation
`strncat`	String concatenation (safe)
`strcmp`	String comparison
`strncmp`	String comparison (with length)
`strlen`	String length
`strchr`	Find character (from left)
`strrchr`	Find character (from right)
`strstr`	Find substring
`strtok`	String tokenization
`memcpy`	Memory copy
`memmove`	Memory move (allows overlap)
`memset`	Memory fill
`memcmp`	Memory comparison

math.h

Function	Purpose
`fabs`	Floating-point absolute value
`sqrt`	Square root
`pow`	Power
`ceil`	Round up
`floor`	Round down
`round`	Round to nearest
`fmod`	Floating-point remainder
`log`	Natural logarithm
`log10`	Common logarithm
`sin`	Sine
`cos`	Cosine
`tan`	Tangent
`asin`	Arc sine
`acos`	Arc cosine
`atan`	Arc tangent
`atan2`	Arc tangent (two arguments)

ctype.h

Function	Purpose
`isalpha`	Is letter
`isdigit`	Is digit
`isalnum`	Is letter or digit
`isupper`	Is uppercase
`islower`	Is lowercase
`isspace`	Is whitespace
`ispunct`	Is punctuation
`isprint`	Is printable
`toupper`	Convert to uppercase
`tolower`	Convert to lowercase

time.h

Function/Macro	Purpose
`time`	Get current time
`clock`	Get CPU clock ticks
`localtime`	Convert to local time
`gmtime`	Convert to UTC time
`strftime`	Format time
`difftime`	Calculate time difference
`mktime`	Convert tm to time_t
`CLOCKS_PER_SEC`	Clock ticks per second

assert.h

Function/Macro	Purpose
`assert(expr)`	Assert; terminate on failure
`NDEBUG`	Disable assert when defined

errno.h

Symbol	Purpose
`errno`	Global error code variable
`EDOM`	Domain error
`ERANGE`	Range error

Data Type Range Quick Reference

Type	Size (bytes)	Minimum	Maximum
`char`	1	-128	127 or 0/255
`signed char`	1	-128	127
`unsigned char`	1	0	255
`short`	2	-32768	32767
`unsigned short`	2	0	65535
`int`	4	-2147483648	2147483647
`unsigned int`	4	0	4294967295
`long`	4/8	Platform-dependent	Platform-dependent
`unsigned long`	4/8	0	Platform-dependent
`long long`	8	-9223372036854775808	9223372036854775807
`float`	4	~1.2e-38	~3.4e+38
`double`	8	~2.2e-308	~1.8e+308

C Reference Manual

Keyword Quick Reference

C89 Keywords (32)

C99 New Keywords (5)

C11 New Keywords (7)

Format Specifier Quick Reference

printf Format Specifiers

printf Modifiers

scanf Format Specifiers

Escape Character Table

Operator Precedence Table

Common Header File Function Reference

stdio.h

stdlib.h

string.h

math.h

ctype.h

time.h

assert.h

errno.h

Data Type Range Quick Reference

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