Read and write to binary files in C

Question

Does anyone have an example of code that can write to a binary file  And also code that can read a binary file and output to screen   Looking at examples I can write to a file ok But when I try to read from a file it is not outputting correctly

User · Answer

I really struggled to find a way to read a binary file into a byte array in C   that would output the same hex values I see in a hex editor  After much trial and error  this seems to be the fastest way to do so without extra casts  By default it loads the entire file into memory  but only prints the first 1000 bytes   string Filename    BinaryFile bin   FILE  pFile  pFile   fopen Filename c str     rb    fseek pFile  0L  SEEK END   size t size   ftell pFile   fseek pFile  0L  SEEK SET   uint8 t  ByteArray  ByteArray   new uint8 t size   if  pFile    NULL        int counter   0      do           ByteArray counter    fgetc pFile           counter          while  counter  lt   size       fclose pFile     for  size t i   0  i  lt  800  i          printf   02X    ByteArray i

User · Answer

this questions is linked with the question How to write binary data file on C and plot it using Gnuplot by CAMILO HG  I know that the real problem have two parts  1  Write the binary data file  2  Plot it using Gnuplot   The first part has been very clearly answered here  so I do not have something to add   For the second  the easy way is send the people to the Gnuplot manual  and I sure someone find a good answer  but I do not find it in the web  so I am going to explain one solution  which must be in the real question  but I new in stackoverflow and I can not answer there    After write your binary data file using fwrite    you should create a very simple program in C  a reader  The reader only contains the same structure as the writer  but you use fread   instead fwrite    So it is very ease to generate this program  copy in the reader c file the writing part of your original code and change write for read  and  wb  for  rb    In addition  you could include some checks for the data  for example  if the length of the file is correct  And finally  your program need to print the data in the standard output using a printf     For be clear  your program run like this      reader data dat  X position Y position   it must be a comment for Gnuplot    1 23 2 45  2 54 3 12  5 98 9 52    Okey  with this program  in Gnuplot you only need to pipe the standard output of the reader to the Gnuplot  something like this   plot   lt    reader data dat    This line  run the program reader  and the output is connected with Gnuplot and it plot the data    Because Gnuplot is going to read the output of the program  you must know what can Gnuplot read and plot and what can not

User · Answer

This is an example to read and write binary jjpg or wmv video file      FILE   fout      FILE   fin   Int ch  char  s  fin fopen  D   pic jpg   rb    if fin  NULL          printf   n Unable to open the file              exit 1             fout fopen  D    newpic jpg   wb     ch fgetc fin          while  ch  EOF                                    s  char   ch                    printf   c  s                    ch fgetc  fin                    fputc s fout                    s                             printf  data read and copied            fclose fin           fclose fout

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I m quite happy with my  make a weak pin storage program  solution  Maybe it will help people who need a very simple binary file IO example to follow     ls WeakPin  my pin code pin  weak pin c     WeakPin Pin  45 47 49 32     WeakPin 8 2   Need 4 ints to write a new pin     WeakPin 8 2 99 49 Pin saved      WeakPin Pin  8 2 99 49     cat weak pin c    a program to save and read 4-digit pin codes in binary format   include  lt stdio h gt   include  lt stdlib h gt    define PIN FILE  my pin code pin   typedef struct   unsigned short a  b  c  d    PinCode    int main int argc  const char   argv        if  argc  gt  1      create pin               if  argc    5                        printf  Need 4 ints to write a new pin  n                return -1                    unsigned short  a   atoi argv 1            unsigned short  b   atoi argv 2            unsigned short  c   atoi argv 3            unsigned short  d   atoi argv 4            PinCode pc          pc a    a  pc b    b  pc c    c  pc d    d          FILE  f   fopen PIN FILE   wb        create and or overwrite         if   f                        printf  Error in creating file  Aborting  n                return -2                        write one PinCode object pc to the file  f         fwrite  amp pc  sizeof PinCode   1  f              fclose f           printf  Pin saved  n            return 0                else read existing pin     FILE  f   fopen PIN FILE   rb        if   f                printf  Error in reading file  Abort  n            return -3            PinCode pc      fread  amp pc  sizeof PinCode   1  f       fclose f        printf  Pin          printf   hu    pc a       printf   hu    pc b       printf   hu    pc c       printf   hu n   pc d       return 0

User · Answer

Reading and writing binary files is pretty much the same as any other file  the only difference is how you open it   unsigned char buffer 10   FILE  ptr   ptr   fopen  test bin   rb        r for read  b for binary  fread buffer sizeof buffer  1 ptr      read 10 bytes to our buffer   You said you can read it  but it s not outputting correctly    keep in mind that when you  output  this data  you re not reading ASCII  so it s not like printing a string to the screen   for int i   0  i lt 10  i        printf   u    buffer i       prints a series of bytes   Writing to a file is pretty much the same  with the exception that you re using fwrite   instead of fread     FILE  write ptr   write ptr   fopen  test bin   wb        w for write  b for binary  fwrite buffer sizeof buffer  1 write ptr      write 10 bytes from our buffer     Since we re talking Linux   there s an easy way to do a sanity check  Install hexdump on your system  if it s not already on there  and dump your file   mike mike-VirtualBox   C  hexdump test bin 0000000 457f 464c 0102 0001 0000 0000 0000 0000 0000010 0001 003e 0001 0000 0000 0000 0000 0000       Now compare that to your output   mike mike-VirtualBox   C    a out  127 69 76 70 2 1 1 0 0 0   hmm  maybe change the printf to a  x to make this a little clearer   mike mike-VirtualBox   C    a out  7F 45 4C 46 2 1 1 0 0 0   Hey  look  The data matches up now   Awesome  we must be reading the binary file correctly    Note the bytes are just swapped on the output but that data is correct  you can adjust for this sort of thing

User · Answer

There are a few ways to do it   If I want to read and write binary I usually use open    read    write    close     Which are completely different than doing a byte at a time   You work with integer file descriptors instead of FILE   variables   fileno will get an integer descriptor from a FILE   BTW   You read a buffer full of data  say 32k bytes at once   The buffer is really an array which you can read from really fast because it s in memory   And reading and writing many bytes at once is faster than one at a time   It s called a blockread in Pascal I think  but read   is the C equivalent  I looked but I don t have any examples handy   OK  these aren t ideal because they also are doing stuff with JPEG images   Here s a read  you probably only care about the part from open   to close     fbuf is the array to read into  sb st size is the file size in bytes from a stat   call      fd   open MASKFNAME O RDONLY       if  fd    -1          read fd fbuf sb st size         close fd         splitmask fbuf  uint32 t sb st size      look at lines  etc       have mask   1         Here s a write   here pix is the byte array  jwidth and jheight are the JPEG width and height so for RGB color we write height   width   3 color bytes    It s the   of bytes to write  void simpdump uint8 t  pix  char  nm       makes a raw aka  data file   int sdfd    sdfd   open nm O WRONLY   O CREAT     if  sdfd    -1        printf  quot bad open n quot        exit -1         printf  quot width   i height   i n quot  jwidth jheight       to the console   write sdfd pix  jwidth jheight 3      close sdfd      Look at man 2 open  also read  write  close   Also this old-style jpeg example c  https   github com LuaDist libjpeg blob master example c  I m reading and writing an entire image at once here   But they re binary reads and writes of bytes  just a lot at once   quot But when I try to read from a file it is not outputting correctly  quot   Hmmm   If you read a number 65 that s  decimal  ASCII for an A   Maybe you should look at man ascii too  If you want a 1 that s ASCII 0x31   A char variable is a tiny 8-bit integer really  if you do a printf as a  i you get the ASCII value  if you do a  c you get the character   Do  x for hexadecimal   All from the same number between 0 and 255

[c] Read and write to binary files in C?

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