Correct way to read a text file into a buffer in C

Question

I m dealing with small text files that i want to read into a buffer while i process them  so i ve come up with the following code       char source 1000000    FILE  fp   fopen  TheFile txt    r    if fp    NULL        while  symbol   getc fp      EOF                strcat source   amp symbol             fclose fp           Is this the correct way of putting the contents of the file into the buffer or am i abusing strcat     I then iterate through the buffer thus   for int x   0   c   source x        0   x            Process chars

User · Answer

Yes - you would probably be arrested for your terriable abuse of strcat !

Take a look at getline() it reads the data a line at a time but importantly it can limit the number of characters you read, so you don't overflow the buffer.

Strcat is relatively slow because it has to search the entire string for the end on every character insertion. You would normally keep a pointer to the current end of the string storage and pass that to getline as the position to read the next line into.

User · Answer

Have you considered mmap    You can read from the file directly as if it were already in memory   http   beej us guide bgipc output html multipage mmap html

User · Answer

If you re on a linux system  once you have the file descriptor you can get a lot of information about the file using fstat    http   linux die net man 2 stat  so you might have     include   lt unistd h gt   void main         struct stat stat      int fd        get file descriptor     fstat fd   amp stat         the size of the file is now in stat st size     This avoids seeking to the beginning and end of the file

User · Answer

Not tested  but should work   And yes  it could be better implemented with fread  I ll leave that as an exercise to the reader    define DEFAULT SIZE 100  define STEP SIZE 100  char  buffer DEFAULT SIZE   size t buffer sz DEFAULT SIZE  size t i 0  while  feof fp      buffer i  fgetc fp     i      if i gt  buffer sz       buffer sz  STEP SIZE      void  tmp buffer      buffer realloc buffer buffer sz       if buffer  null   free tmp   exit 1      ensure we don t have a memory leak       buffer i  0

User · Answer

Methinks you want fread   http   www cplusplus com reference clibrary cstdio fread

User · Answer

char source 1000000    FILE  fp   fopen  TheFile txt    r    if fp    NULL        while  symbol   getc fp      EOF                strcat source   amp symbol             fclose fp       There are quite a few things wrong with this code    It is very slow  you are extracting the buffer one character at a time   If the filesize is over sizeof source   this is prone to buffer overflows  Really  when you look at it more closely  this code should not work at all  As stated in the man pages            The strcat   function appends a copy of the null-terminated string s2 to the end of the null-terminated string s1  then add a terminating   0        You are appending a character  not a NUL-terminated string   to a string that may or may not be NUL-terminated  The only time I can imagine this working according to the man-page description is if every character in the file is NUL-terminated  in which case this would be rather pointless  So yes  this is most definitely a terrible abuse of strcat     The following are two alternatives to consider using instead   If you know the maximum buffer size ahead of time    include  lt stdio h gt   define MAXBUFLEN 1000000  char source MAXBUFLEN   1   FILE  fp   fopen  foo txt    r    if  fp    NULL        size t newLen   fread source  sizeof char   MAXBUFLEN  fp       if   ferror  fp      0             fputs  Error reading file   stderr         else           source newLen        0      Just to be safe                fclose fp       Or  if you do not    include  lt stdio h gt   include  lt stdlib h gt   char  source   NULL  FILE  fp   fopen  foo txt    r    if  fp    NULL           Go to the end of the file         if  fseek fp  0L  SEEK END     0               Get the size of the file             long bufsize   ftell fp           if  bufsize    -1       Error                  Allocate our buffer to that size             source   malloc sizeof char     bufsize   1                Go back to the start of the file             if  fseek fp  0L  SEEK SET     0       Error                  Read the entire file into memory             size t newLen   fread source  sizeof char   bufsize  fp           if   ferror  fp      0                 fputs  Error reading file   stderr             else               source newLen        0      Just to be safe                         fclose fp      free source      Don t forget to call free   later

User · Answer

See this article from JoelOnSoftware for why you don t want to use strcat   Look at fread for an alternative  Use it with 1 for the size when you re reading bytes or characters

User · Answer

Why don t you just use the array of chars you have  This ought to do it      source i    getc fp       i

[c] Correct way to read a text file into a buffer in C?

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