C Programming How to read the whole file contents into a buffer

Question

I want to write the full contents of a file into a buffer  The file actually only contains a string which i need to compare with a string   What would be the most efficient option which is portable even on linux   ENV  Windows

User · Answer

A portable solution could use getc    include  lt stdio h gt   char buffer MAX FILE SIZE   size t i   for  i   0  i  lt  MAX FILE SIZE    i        int c   getc fp        if  c    EOF                buffer i    0x00          break             buffer i    c      If you don t want to have a MAX FILE SIZE macro or if it is a big number  such that buffer would be to big to fit on the stack   use dynamic allocation

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Here is what I would recommend   It should conform to C89  and be completely portable  In particular  it works also on pipes and sockets on POSIXy systems   The idea is that we read the input in large-ish chunks  READALL CHUNK   dynamically reallocating the buffer as we need it  We only use realloc    fread    ferror    and free      include  lt stdlib h gt   include  lt stdio h gt   include  lt errno h gt      Size of each input chunk to be    read and allocate for      ifndef  READALL CHUNK  define  READALL CHUNK  262144  endif   define  READALL OK          0     Success     define  READALL INVALID    -1     Invalid parameters     define  READALL ERROR      -2     Stream error     define  READALL TOOMUCH    -3     Too much input     define  READALL NOMEM      -4     Out of memory        This function returns one of the READALL  constants above     If the return value is zero    READALL OK  then         dataptr  points to a dynamically allocated buffer  with        sizeptr  chars read from the file       The buffer is allocated for one extra char  which is NUL       and automatically appended after the data     Initial values of   dataptr  and   sizeptr  are ignored     int readall FILE  in  char   dataptr  size t  sizeptr        char   data   NULL   temp      size t size   0      size t used   0      size t n          None of the parameters can be NULL         if  in    NULL    dataptr    NULL    sizeptr    NULL          return READALL INVALID          A read error already occurred         if  ferror in           return READALL ERROR       while  1             if  used   READALL CHUNK   1  gt  size                size   used   READALL CHUNK   1                  Overflow check  Some ANSI C compilers                may optimize this away  though                 if  size  lt   used                    free data                   return READALL TOOMUCH                             temp   realloc data  size               if  temp    NULL                    free data                   return READALL NOMEM                            data   temp                         n   fread data   used  1  READALL CHUNK  in           if  n    0              break           used    n             if  ferror in             free data           return READALL ERROR             temp   realloc data  used   1       if  temp    NULL            free data           return READALL NOMEM            data   temp      data used      0         dataptr   data       sizeptr   used       return READALL OK      Above  I ve used a constant chunk size  READALL CHUNK    262144  256 1024   This means that in the worst case  up to 262145 chars are wasted  allocated but not used   but only temporarily  At the end  the function reallocates the buffer to the optimal size  Also  this means that we do four reallocations per megabyte of data read   The 262144-byte default in the code above is a conservative value  it works well for even old minilaptops and Raspberry Pis and most embedded devices with at least a few megabytes of RAM available for the process  Yet  it is not so small that it slows down the operation  due to many read calls  and many buffer reallocations  on most systems   For desktop machines at this time  2017   I recommend a much larger READALL CHUNK  perhaps  define READALL CHUNK 2097152  2 MiB    Because the definition of READALL CHUNK is guarded  i e   it is defined only if it is at that point in the code still undefined   you can override the default value at compile time  by using  in most C compilers  -DREADALL CHUNK 2097152 command-line option -- but do check your compiler options for defining a preprocessor macro using command-line options

User · Answer

Portability between Linux and Windows is a big headache  since Linux is a POSIX-conformant system with - generally - a proper  high quality toolchain for C  whereas Windows doesn t even provide a lot of functions in the C standard library   However  if you want to stick to the standard  you can write something like this    include  lt stdio h gt   include  lt stdlib h gt   FILE  f   fopen  textfile txt    rb    fseek f  0  SEEK END   long fsize   ftell f   fseek f  0  SEEK SET       same as rewind f       char  string   malloc fsize   1   fread string  1  fsize  f   fclose f    string fsize    0    Here string will contain the contents of the text file as a properly 0-terminated C string  This code is just standard C  it s not POSIX-specific  although that it doesn t guarantee it will work compile on Windows

[c] C Programming: How to read the whole file contents into a buffer

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