File write vs append mode|Append without in append mode.

File Append mode

Write mode either overwrites the existing file or creates a new one if the file is non-existent. Append mode opens the file and sets the file pointer/cursor to the end of the file so that any write operation may start from the very end of that file. It cannot overwite an existing file.

fopen() Syntax

  #include <stdio.h>

  FILE *fopen(const char *path, const char *mode);
  FILE *fdopen(int fd, const char *mode);
  FILE *freopen(const char *path, const char *mode, FILE *stream);

  FILE *fp = fopen(<path>, "a");
  FILE *fp = fopen(<path>, "a+");

fopen() DESCRIPTION

  The fopen() function opens the file whose name is the string pointed
  to by path and associates a stream with it.

  The argument mode points to a string beginning with one of the
  following sequences (possibly followed by additional characters, as
  described below):

   "r"      Open text file for reading.  The stream is positioned at the
  beginning of the file.

   "r+"     Open for reading and writing.  The stream is positioned at the
  beginning of the file.

   "w"      Truncate file to zero length or create text file for writing.
  The stream is positioned at the beginning of the file.

   "w+"     Open for reading and writing.  The file is created if it does
  not exist, otherwise it is truncated.  The stream is
  positioned at the beginning of the file.

   "a"      Open for appending (writing at end of file).  The file is
  created if it does not exist.  The stream is positioned at the
  end of the file.

   "a+"     Open for reading and appending (writing at end of file).  The
  file is created if it does not exist.  The initial file
  position for reading is at the beginning of the file, but
  output is always appended to the end of the file.

  The mode string can also include the letter 'b' either as a last
  character or as a character between the characters in any of the two-
  character strings described above.  This is strictly for
  compatibility with C89 and has no effect; the 'b' is ignored on all
  POSIX conforming systems, including Linux.  (Other systems may treat
  text files and binary files differently, and adding the 'b' may be a
  good idea if you do I/O to a binary file and expect that your program
  may be ported to non-UNIX environments.)

  Opening a file in append mode (a as the first character of mode)
  causes all subsequent write operations to this stream to occur at
  end-of-file, as if preceded the call:

  fseek(stream, 0, SEEK_END);

Append mode with fread and fwrite

We can mimic the operation of append mode in a C program using the following steps:

  1. open the file in read mode
  2. read the size attribute
  3. copy the entire file into buffer
  4. Open file in write mode
  5. Write existing buffer
  6. append new buffer
  7. close the file

Append mode example

Append mode is used to open a file and jump to the last location for further writing or appending new contents.

Logging modules are the best examples. Syslog, windows event logger are live examples where files are opened in append mode. A simple application like Putty, application servers or GUI based applications can log critical data and errors in log file in the background.

Now lets understand this small logger application. This takes a line from user and appends the timestamp and the text in the log file.

#include <stdio.h>
#include <string.h>
#include <time.h>

int main (int argc, char *argv[])
{
  FILE *fp;
  char line[1024];
  time_t rawtime;
  struct tm * timeinfo;

  do {
  
    printf("Enter text to add in log file : ");
    gets(line);
    
    if(strcmp("quit", line) == 0)
      break;
      
    fp = fopen("/var/log/logger.log", "a+");
    if(fp == NULL)
    {
      printf("Error opening log file \n");
      return (-1);
    }
    time ( &rawtime );
    timeinfo = localtime ( &rawtime );
    fprintf(fp, "%s%s\n", asctime (timeinfo), line);
    fclose(fp);
    
  } while (1);
  return 0;
}

Fopen append output

$ gcc logging.c -o logging
$ sudo ./logging 
Enter text to add in log file : This is a sample text in log 1
Enter text to add in log file : This is a sample text in log 2
Enter text to add in log file : quit

$ sudo cat /var/log/logger.log 
Sat May  9 15:17:06 2020
This is a sample text in log 1
Sat May  9 15:17:18 2020
This is a sample text in log 2
$

File append using fdopen

#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <time.h>
int main (int argc, char *argv[])
{
  FILE *fp;
  int fd;
  char buffer[1025] = { 0 };
  time_t rawtime;
  struct tm * timeinfo;
  size_t len;

  fd = open("/var/log/logger.log", O_RDWR | O_CREAT);
  if(fd < 0)
  {
      printf("open : error log file \n");
      return (-1);
  }
  printf("log file : \n");
  while((len = read(fd, buffer, sizeof(buffer)-1)) > 0)
  {
    puts(buffer);
    memset(buffer, 0, sizeof(buffer));
  }

  fp = fdopen(fd, "a+");
  if(fp == NULL)
  {
      printf("fdopen : error log file \n");
      return (-1);
  }
  printf("Enter text to append to : ");
  gets(buffer);
  if(strcmp("quit", buffer) == 0)
      goto quit;
  time ( &rawtime );
  timeinfo = localtime ( &rawtime );
  fprintf(fp, "%s%s\n", asctime (timeinfo), buffer);

quit:
  fclose(fp);
  close(fd);
  return 0;
}



Fdopen append output

$ gcc fdopen.c -o fdopen
$ sudo ./fdopen
log file : 
Enter text to append to : this text to append 1
$ sudo ./fdopen
log file : 
Sat Oct 10 18:32:07 2020
this text to append 1

Enter text to append to : this text to append 2
$ sudo ./fdopen
log file : 
Sat Oct 10 18:32:07 2020
this text to append 1
Sat Oct 10 18:32:23 2020
this text to append 2


Enter text to append to : quit
$

File append using freopen

#include <stdio.h>
#include <string.h>
#include <time.h>
int main (int argc, char *argv[])
{
  FILE *fp, *fpr;
  char buffer[1025] = { 0 };
  time_t rawtime;
  struct tm * timeinfo;
  size_t len;
  printf("log file : \n");
  fp = fopen("/var/log/logger.log", "rb");
  if(fp == NULL)
  {
      printf("fopen: error fopen log file \n");
      return (-1);
  }
  while((len = fread(buffer, 1, sizeof(buffer)-1, fp)) > 0)
  {
    fwrite(buffer, len, 1, stdout);
    memset(buffer, 0, sizeof(buffer));
  }
  fpr = freopen("/var/log/logger.log", "a+", fp);
  if(fpr == NULL)
  {
      printf("freopen : error log file \n");
      return (-1);
  }
  printf("Enter text to append to : ");
  gets(buffer);
  if(strcmp("quit", buffer) == 0)
      goto quit;
  time ( &rawtime );
  timeinfo = localtime ( &rawtime );
  fprintf(fpr, "%s%s\n", asctime (timeinfo), buffer);

quit:
  fclose(fp);
  fclose(fpr);
  return 0;
}

Freopen append output

$ gcc freopen.c -o freopen
$ sudo ./freopen
log file : 
Sat Oct 10 18:32:07 2020
this text to append 1
Sat Oct 10 18:32:23 2020
this text to append 2
Sat Oct 10 18:33:12 2020


Enter text to append to : this text to appen 3
$ sudo ./freopen
log file : 
Sat Oct 10 18:32:07 2020
this text to append 1
Sat Oct 10 18:32:23 2020
this text to append 2
Sat Oct 10 18:33:12 2020

Sat Oct 10 18:37:01 2020
this text to appen 3


Enter text to append to : quit
$

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