C

Internal and external are relative to a function. Internal to a function includes the arguments and variables defined inside the function. Internal variables are automatic variables, because they are allocated to temporary storage, and get out of space when the function returns.

External to a function refers to the external variables and functions. All functions must be external, because C does not allow definition of functions inside function. By default, external variables and functions have the property that, all references by the same name, even if compiled separately in separate files, will refer to the same thing. This is called external linkage. To change the linkage type, use static to limit an external variable or function inside this file.

The scope of a name is the part of program within which the name can be used. Scope of an automatic variable is the body of the function. The scope of an external variable or function lasts from the point it is declared to the end of the file. If a function or variable is not in scope, it must be declared before usage. This is done by a extern keyword. The declaration only announces the properties of it, while the definition allocate the storage. There must be only one definition of an external variable among all files that make up the program. Array size must be specified in the definition, but optional to an extern declaration. Initialization of an external variable goes only with the definition.

For a non-automatic variable (including static variables and external variables), the initialization is done once only, conceptually before the program starts executing. If no initializer, it is initialized to 0. If has, the initializer must be a constant expression. On the other hand, an automatic variable is initialized every time the function or block is entered, and the initializer can be any expression, this is just a shorthand for assignment statement. If there's no initializer, the default value is undefined. An array can be initialized by assigning a list of initializers enclosed in braces separated by commas, and the length, when omitted, can be computed by the length of the initializers. If there are fewer initializers than the size, the rest will be initialized to 0, even for automatic variables. A string constant can serve as an initializer array of characters.

Extern "C" makes a function-name in C++ have 'C' linkage, i.e. compiler will not mangle the function name. C++ can overload function names, thus internally the compiler cannot just use function name to refer to the function. It add argument/parameter type information to the name used for linkage. However, C does not allow function overloading, thus C compiler expect just the original name, but couldn't find one because the name is modified. Thus, when you state that a function has extern "C" linkage in C++, the C++ compiler does not modify it, thus it is suitable to use in C code.

It can be applied to either an individual declaration or definition, or a whole block of code.

extern "C" void foo(int);
extern "C" {
   void g(char);
   int i;
}

A register declaration advises the compiler that this variable will be heavily used.

The restrict keyword is a declaration of intent given by the programmer to the compiler. It says that for the lifetime of the pointer, only it or a value directly derived from it (such as pointer + 1) will be used to access the object to which it points. This limits the effects of pointer aliasing, aiding optimizations. If this intention is not followed and the object is accessed by an independent pointer, this will result in undefined behavior.

The keyword tells the compiler that the value of the variable may change at any time. It may change unexpectedly, so DO NOT optimize the code when you compiler think it would not change.

It is most commonly to declare a pointer to be volatile, and most likely you do not intent to declare the place it points to be volatile.

volatile int foo;
volatile uint8_t *pReg;
int * volatile p; // very rare
volatile int * volatile p; // very rare

The common use-case is

1. The register's value may change by hardware.
2. global variables modified by interrupt
3. global variables accessed by multiple threads

• types:
  • size_t
  • FILE

Macro: NULL

remoev :: String -> Int
rename :: String -> String -> Int
tmpfile :: () -> FILE
tmpnam :: String -> String
fclose :: FILE -> Int
fflush :: FILE -> Int
fopen :: String -> Mode -> FILE
freopen :: String -> Mode -> FILE -> FILE
-- TODO setvbuf
fprintf :: FILE -> FormatString -> Int
fscanf :: FILE -> FormatString -> Int
printf :: FormatString -> Int
snprintf :: String -> Int -> FormatString -> Int
sprintf ::  String -> FormatString -> Int
sscanf :: String -> FormatString -> Int

character IO fgetc :: FILE -> Int fgets :: String -> Int -> FILE -> String fputc :: Int -> FILE -> Int fputs :: String -> FILE -> Int getc ::

int fgetc(FILE *stream);
char *fgets(char *s, int n, FILE *stream);
int fputc(int c, FILE *stream)
int fputs (char *s, FILE *stream)
int getc(FILE *stream)
int getchar(void)
char *gets(char *s)
putc
puts
ungetc

• File
  • fgetc
  • fgets
  • fputc
  • fputs
• stdin
  • getc
  • getchar
  • gets
  • putc
  • putchar
  • puts
• Put Back
  • ungetc
• Low-level
  • fread
  • fwrite
  • fgetpos
  • fseek
  • fsetpos
  • ftell
  • rewind
  • clearerr
  • feof
  • ferror
  • perror

mapping of characters.

isalpha :: Int -> Bool
isblank :: Int -> Bool
iscntrl :: Int -> Bool
isdigit :: Int -> Bool
isgraph :: Int -> Bool
islower :: Int -> Bool
isprint :: Int -> Bool
ispunct :: Int -> Bool
isspace :: Int -> Bool
isupper :: Int -> Bool
isxdigit :: Int -> Bool
tolower :: Int -> Int
toupper :: Int -> Int

First, trigonometric functions. The f-suffix is float, l-suffix is long double

acos :: Double -> Double
asin :: Double -> Double
atan :: Double -> Double
atan2 :: Double -> Double -> Double
cos :: Double -> Double
sin :: Double -> Double
tan :: Double -> Double

exp :: Double -> Double
log :: Double -> Double
log10 :: Double -> Double
hypot :: Double -> Double -> Double -- sqrt (x^2 + y^2)
pow :: Double -> Double -> Double
sqrt :: Double -> Double
ceil :: Double -> Double
floor :: Double -> Double
round :: Double -> Double
trunc :: Double -> Double

• Conversion
  • atof
  • atoi, atol, atoll
  • strtod, strtof, strtold
  • strtol, strtoll, strtoul, strtoull
• Random
  • rand
  • srand
• Memory
  • calloc
  • free
  • malloc
  • realloc
• other
  • abort
  • exit
  • getenv
  • system
• arithmetic
  • abs
  • div

• Copy
  • memcpy
  • memmove
  • strcpy
  • strncpy
  • strcat
  • strncat
• Compare
  • memcmp
  • strcmp
  • strcoll: seems to be strcmp with locale
  • strncmp
  • strxfrm: also some sort of strcmp
• Search
  • memchr
  • strchr
  • strrchr
  • strstr
  • strtok: tokenize
• Other
  • memset
  • strerror
  • strlen

Types

• clock_t
• time_t
• struct tm

functions

• clock
• difftime
• mktime
• time
• asctime
• ctime
• gmtime
• strftime

type:

• DIR

Function

• closedir
• opendir
• readdir
• seekdir
• telldir

• creat
• fcntl
• open
• openat

• chmod
• fchmod
• fstat
• mkdir
• mkfifo
• mknod
• stat
• umask