In C how should I read a text file and print all strings

Question

I have a text file named test txt  I want to write a C program that can read this file and print the content to the console  assume the file contains only ASCII text    I don t know how to get the size of my string variable  Like this   char str 999   FILE   file  file   fopen   test txt     r    if  file        while  fscanf file    s   str   EOF          printf   s  str       fclose file       The size 999 doesn t work because the string returned by fscanf can be larger than that   How can I solve this

User · Answer

There are plenty of good answers here about reading it in chunks, I'm just gonna show you a little trick that reads all the content at once to a buffer and prints it.

I'm not saying it's better. It's not, and as Ricardo sometimes it can be bad, but I find it's a nice solution for the simple cases.

I sprinkled it with comments because there's a lot going on.

#include <stdio.h>
#include <stdlib.h>

char* ReadFile(char *filename)
{
   char *buffer = NULL;
   int string_size, read_size;
   FILE *handler = fopen(filename, "r");

   if (handler)
   {
       // Seek the last byte of the file
       fseek(handler, 0, SEEK_END);
       // Offset from the first to the last byte, or in other words, filesize
       string_size = ftell(handler);
       // go back to the start of the file
       rewind(handler);

       // Allocate a string that can hold it all
       buffer = (char*) malloc(sizeof(char) * (string_size + 1) );

       // Read it all in one operation
       read_size = fread(buffer, sizeof(char), string_size, handler);

       // fread doesn't set it so put a \0 in the last position
       // and buffer is now officially a string
       buffer[string_size] = '\0';

       if (string_size != read_size)
       {
           // Something went wrong, throw away the memory and set
           // the buffer to NULL
           free(buffer);
           buffer = NULL;
       }

       // Always remember to close the file.
       fclose(handler);
    }

    return buffer;
}

int main()
{
    char *string = ReadFile("yourfile.txt");
    if (string)
    {
        puts(string);
        free(string);
    }

    return 0;
}

Let me know if it's useful or you could learn something from it :)

User · Answer

Use  read    instead o fscanf   ssize t read int fildes  void  buf  size t nbyte        DESCRIPTION      The read   function shall attempt to read nbyte bytes from the file associated with the open file descriptor  fildes  into the buffer pointed to by buf     Here is an example   http   cmagical blogspot com 2010 01 c-programming-on-unix-implementing-cat html  Working part from that example   f open argv 1  O RDONLY   while   n read f l 80    gt  0      write 1 l n       An alternate approach is to use getc putc to read write 1 char at a time  A lot less efficient  A good example  http   www eskimo com  scs cclass notes sx13 html

User · Answer

Instead just directly print the characters onto the console because the text file maybe very large and you may require a lot of memory    include  lt stdio h gt   include  lt stdlib h gt   int main          FILE  f      char c      f fopen  test txt   rt         while  c fgetc f    EOF           printf   c  c              fclose f       return 0

User · Answer

You can use fgets and limit the size of the read string   char  fgets char  str  int num  FILE  stream     You can change the while in your code to   while  fgets str  100  file      printf   s   str

User · Answer

The simplest way is to read a character  and print it right after reading   int c  FILE  file  file   fopen  test txt    r    if  file        while   c   getc file      EOF          putchar c       fclose file       c is int above  since EOF is a negative number  and a plain char may be unsigned   If you want to read the file in chunks  but without dynamic memory allocation  you can do    define CHUNK 1024    read 1024 bytes at a time    char buf CHUNK   FILE  file  size t nread   file   fopen  test txt    r    if  file        while   nread   fread buf  1  sizeof buf  file    gt  0          fwrite buf  1  nread  stdout       if  ferror file                deal with error              fclose file       The second method above is essentially how you will read a file with a dynamically allocated array   char  buf   malloc chunk    if  buf    NULL           deal with malloc   failure          otherwise do this   Note  chunk  instead of  sizeof buf     while   nread   fread buf  1  chunk  file    gt  0           as above        Your method of fscanf   with  s as format loses information about whitespace in the file  so it is not exactly copying a file to stdout

User · Answer

Two approaches leap to mind   First  don t use scanf  Use fgets   which takes a parameter to specify the buffer size  and which leaves any newline characters intact  A simple loop over the file that prints the buffer content should naturally copy the file intact   Second  use fread   or the common C idiom with fgetc    These would process the file in fixed-size chunks or a single character at a time   If you must process the file over white-space delimited strings  then use either fgets or fread to read the file  and something like strtok to split the buffer at whitespace  Don t forget to handle the transition from one buffer to the next  since your target strings are likely to span the buffer boundary   If there is an external requirement to use scanf to do the reading  then limit the length of the string it might read with a precision field in the format specifier  In your case with a 999 byte buffer  then say scanf   998s   str   which will write at most 998 characters to the buffer leaving room for the nul terminator  If single strings longer than your buffer are allowed  then you would have to process them in two pieces  If not  you have an opportunity to tell the user about an error politely without creating a buffer overflow security hole   Regardless  always validate the return values and think about how to handle bad  malicious  or just malformed input

User · Answer

You could read the entire file with dynamic memory allocation  but isn t a good idea because if the file is too big  you could have memory problems   So is better read short parts of the file and print it    include  lt stdio h gt   define BLOCK   1000  int main         FILE  f fopen  teste txt   r        int size      char buffer BLOCK                  while  size fread buffer BLOCK sizeof char  f  gt 0              fwrite buffer size sizeof char  stdout       fclose f                  return 0

[c] In C, how should I read a text file and print all strings

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