Bei Big-Endian (wörtlich „Groß-Ender“)
wird das Byte mit den höchstwertigen Bits (d. h. die signifikantesten Stellen) zuerst gespeichert, das heißt an der kleinsten Speicheradresse. Allgemein bedeutet der Begriff, dass Daten mit dem größtwertigen Element zuerst genannt werden, wie etwa bei der deutschen Schreibweise der Uhrzeit: Stunde:Minute:Sekunde.
Bei Little-Endian (wörtlich „Klein-Ender“)
wird dagegen das Byte mit den niederstwertigen Bits (d. h. die am wenigsten signifikanten Stellen) an der kleinsten Speicheradresse gespeichert beziehungsweise das kleinstwertige Element zuerst genannt, wie bei der herkömmlichen deutschen Datumsschreibweise: Tag.Monat.Jahr.

#include <stdio.h>
#include <stdint.h>

typedef struct {
    union {
        uint16_t    num;
        
        /* little-endian */
        struct {
            uint8_t lo;
            uint8_t hi;
        };
        
        struct {
            uint16_t lsb   : 1;
            uint16_t lsb_1 : 1;
            uint16_t lsb_2 : 1;
            uint16_t lsb_3 : 1;
            uint16_t raw   : 8;
            uint16_t msb_3 : 1;
            uint16_t msb_2 : 1;
            uint16_t msb_1 : 1;
            uint16_t msb   : 1;
        };
    };
} endian_t;

void
print_endian(endian_t endian)
{
    printf("---------------------------\n");
    printf("   16: 0x%016lx 0x%04x\n", (uint64_t) &endian.num, endian.num);
    printf("lo  8: 0x%016lx 0x%02x\n", (uint64_t) &endian.lo, endian.lo);
    printf("hi  8: 0x%016lx 0x%02x\n", (uint64_t) &endian.hi, endian.hi);
    printf("MSB: %u\n", endian.msb);
    printf("LSB: %u\n", endian.lsb);
    printf("Bin: %u%u%u%u %u%u%u%u\n", endian.msb,   endian.msb_1, endian.msb_2, endian.msb_3,
                                       endian.lsb_3, endian.lsb_2, endian.lsb_1, endian.lsb);
}

int main(int argc, char *argv[]) {
    
    endian_t endian;
    
    endian.num = 0x8000;
    print_endian(endian);
    endian.num = 0x4000;
    print_endian(endian);
    endian.num = 0x2000;
    print_endian(endian);
    endian.num = 0x1000;
    print_endian(endian);
    endian.num = 0x0008;
    print_endian(endian);
    endian.num = 0x0004;
    print_endian(endian);
    endian.num = 0x0002;
    print_endian(endian);
    endian.num = 0x0001;
    print_endian(endian);
    
    return 0;
}

$ ./endian
---------------------------
   16: 0x00007fffd2223e80 0x8000
lo  8: 0x00007fffd2223e80 0x00
hi  8: 0x00007fffd2223e81 0x80
MSB: 1
LSB: 0
Bin: 1000 0000
---------------------------
   16: 0x00007fffd2223e80 0x4000
lo  8: 0x00007fffd2223e80 0x00
hi  8: 0x00007fffd2223e81 0x40
MSB: 0
LSB: 0
Bin: 0100 0000

[...]

Byte-Reihenfolge
Bitwertigkeit
UART Communication from Linux Userspace
Byteorder

Mit Bit-Feldern

struct {

#ifdef __ONE_ENDIANESS__
    unsigned int    b1:1;
    unsigned int    b2:8;
    unsigned int    b3:7;
    unsigned int    b4:8;
    unsigned int    b5:7;
    unsigned int    b6:1;
#define _STRUCT_FILLED
#endif /* __ONE_ENDIANESS__ */

#ifdef __OTHER_ENDIANESS__
    unsigned int    b6:1;
    unsigned int    b5:7;
    unsigned int    b4:8;
    unsigned int    b3:7;
    unsigned int    b2:8;
    unsigned int    b1:1;
#define _STRUCT_FILLED
#endif /* __OTHER_ENDIANESS__ */

};

#ifndef _STRUCT_FILLED
#  error Endianess uncertain for struct
#else
#  undef _STRUCT_FILLED

Converting Endianess on a bit field structure
bit ordering and endianess

GCC Attributes

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Specifying Attributes of Variables

Suppressing GCC Warnings

What is the best way to supress “Unused variable x”-warning
UNUSED in gcc
Suppressing GCC Warnings

C Function Declaration to Implementation

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Search:          ;
Replace with:    \{\n    \n\}\n

C Preprocessor

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General

Preprocessor directives

Pound / Hash

#define hash #
#define f(x) x
#define label(a) f(hash)a

How to print a pound / hash via C preprocessor?

C Designated Initializers

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Static Struct Initialization

struct student {
    int roll;
    int class;
    char name[50];
};

student mark = {
    .name = "Mark",
    .class  = 10,
    .roll  = 1038
};

GCC: 6.26 Designated Initializers
GCC 4.4.1: 5.23 Designated Initializers
C: Structure Initialization (Advanced)

Static Array Initialization

#define ADDRESS1 0x01
#define ADDRESS2 0x02
#define ADDRESS3 0x03
#define ADDRESS4 0x04

const static uint32_t address_map[] = {
    [0] = ADDRESS1,
    [1] = ADDRESS2,
    [2] = ADDRESS3,
    [3] = ADDRESS4
};


#define REGISTER_MAP(name, reg) [name] = reg

#define NAME1    0x03
#define NAME2    0x01
#define NAME3    0x02
#define NAME4    0x04

#define ADDRESS1 0x01
#define ADDRESS2 0x02
#define ADDRESS3 0x03
#define ADDRESS4 0x04

const static uint32_t address_map[] = {
    REGISTER_MAP(NAME1, ADDRESS1),
    REGISTER_MAP(NAME2, ADDRESS2),
    REGISTER_MAP(NAME3, ADDRESS3),
    REGISTER_MAP(NAME4, ADDRESS4)
};

How to initialize an array in C
Initialization of arrays
c: initializing arrays

Fortran 95

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GFortran
GFortran Getting Started
GFortran Usage

Fortran: Fortran 95: Unterprogramme
Fortran: Fortran 95: Datentypen
Fortran/Fortran examples (en)

RANDOM_NUMBER — Pseudo-random number
RAND — Real pseudo-random number

How can gfortran tell if I am compiling f90 or f95 code?

FAQ

Question

$ make
[FC] src/main.f90
make: gfortran-4.7: Command not found
Makefile.inc:37: recipe for target 'obj/main.o' failed
make: *** [obj/main.o] Error 127

Answer

$ sudo apt-get install gfortran
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following extra packages will be installed:
  gfortran-5 libgfortran-5-dev
Suggested packages:
  gfortran-multilib gfortran-doc gfortran-5-multilib gfortran-5-doc libgfortran3-dbg
The following NEW packages will be installed:
  gfortran gfortran-5 libgfortran-5-dev
[...]

$ vi Makefile
Change gfortran-4.7 to gfortran

$ make
[FC] src/main.f90
[FC] src/pi.f90
[LD] pi

Question

$ make
[MKDIR] obj/hello/./
[FC] src/hello.f

[...]

Error: Non-numeric character in statement label at (1)
src/hello.f:15:3:

   end 
   1
Error: Unclassifiable statement at (1)
f951: Error: Unexpected end of file in ‘src/hello.f’
Makefile.inc:50: recipe for target 'obj/hello/hello.o' failed
make: *** [obj/hello/hello.o] Error 1

Answer

Getting started with gfortran
Options controlling Fortran dialect

-ffree-form
-ffixed-form
Specify the layout used by the source file. The free form layout was introduced in Fortran 90. Fixed form was traditionally used in older Fortran programs. When neither option is specified, the source form is determined by the file extension.

hello_FFLAGS  = -ffree-form

Berechnung der Zahl Pi nach der Monte-Carlo Methode

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Berechnung der Zahl Pi nach der Monte-Carlo Methode
Berechnung von Pi mit MonteCarlo-Methode
Monte-Carlo-Simulation
Monte-Carlo-Algorithmus

FreeBSD Upgrade Procedures

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FreeBSD Upgrade Procedures
Upgrading from 6.2-RELEASE?
Upgrading from 6.2-RELEASE?
6.1-RELEASE to 8.2-RELEASE?

OpenSSL 1.0.1f on FreeBSD without SCTP

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Question

cc -I.. -I../.. -I../modes -I../asn1 -I../evp -I../../include  -fPIC -DOPENSSL_PIC -DZLIB_SHARED -DZLIB -DOPENSSL_THREADS -pthread -D_REENTRANT -DDSO_DLFCN -DHAVE_DLFCN_H -Wa,--noexecstack -DL_ENDIAN -DTERMIOS -O3 -DMD32_REG_T=int -Wall -O2 -pipe -fPIC -funroll-all-loops -march=opteron -fno-strict-aliasing -DOPENSSL_IA32_SSE2 -DOPENSSL_BN_ASM_MONT -DOPENSSL_BN_ASM_MONT5 -DOPENSSL_BN_ASM_GF2m -DSHA1_ASM -DSHA256_ASM -DSHA512_ASM -DMD5_ASM -DAES_ASM -DVPAES_ASM -DBSAES_ASM -DWHIRLPOOL_ASM -DGHASH_ASM -c bss_dgram.c
bss_dgram.c:74:26: netinet/sctp.h: No such file or directory

[...]

*** Error code 1

Stop in /usr/ports/security/openssl/work/openssl-1.0.1f/crypto/bio.
*** Error code 1

Stop in /usr/ports/security/openssl/work/openssl-1.0.1f/crypto.
*** Error code 1

Stop in /usr/ports/security/openssl/work/openssl-1.0.1f.
*** Error code 1

Stop in /usr/ports/security/openssl.

Answer

# make config
deselect SCTP

# vi /etc/make.conf
WITH_OPENSSL_PORT=yes

Andreas' Blog

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C Struct Assignement

Byte-Reihenfolge: Big- oder Little-Endian

Mit Bit-Feldern

GCC Attributes

Suppressing GCC Warnings

C Function Declaration to Implementation

C Preprocessor

General

Pound / Hash

C Designated Initializers

Static Struct Initialization

Static Array Initialization

Fortran 95

FAQ

Question

Answer

Question

Answer

Berechnung der Zahl Pi nach der Monte-Carlo Methode

FreeBSD Upgrade Procedures

OpenSSL 1.0.1f on FreeBSD without SCTP

Question

Answer