Insérer une ligne dans un fichier texte

johand@osiris: ~/tmp $ cat data.txt
ligne1
ligne2
ligne3
ligne4
johand@osiris: ~/tmp $ sed -n -i.bak   -e '1 {s/$/\nNouvelle ligne/ ; p };  2,$ p'   data.txt 
johand@osiris: ~/tmp $ cat  data.txt
ligne1
Nouvelle ligne
ligne2
ligne3
ligne4
johand@osiris: ~/tmp $ cat data.txt.bak 
ligne1
ligne2
ligne3
ligne4

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


#define BUFFSIZE 512
#define LINENUMBER 4
void print_usage(char *progname)
{
  fprintf(stderr, "Usage %s new_string [input_file]\n\tIf no input file, stdin is used and output is stdout.\n", progname );
  return;
}
int main(int argc, char*argv[])
{
  FILE *filein, *fileout ;
  char buffer[BUFFSIZE] ;
  char bakname[BUFFSIZE] ;

  unsigned int namelen, cnt = 1;

  char *line_to_insert = "Une nouvelle ligne" ;

  if (argc < 2)
    {
      print_usage(argv[0]) ;
      return(EXIT_FAILURE) ;
    }

  switch (argc)
    {
    case 2:
      filein = stdin ;
      fileout = stdout;
      break;
   
    case 3:
      line_to_insert = argv[1];
      namelen = strlen(argv[2]);

      strncpy(bakname, argv[2], BUFFSIZE - 1);
      strncat(bakname, ".bak", BUFFSIZE - namelen - 1);

      if ( ( -1 ==  rename(argv[2], bakname)))
	{
	  perror(NULL);
	  return(EXIT_FAILURE);
	}
      else
	{
	  //should not give errors at least if permissions have not been modified by som other process
	  filein = fopen(bakname, "r");
	  fileout = fopen(argv[2], "w");
	}
      break;   
    }

  fgets(buffer, BUFFSIZE, filein);
  while( !feof(filein) )
    {
      if( (LINENUMBER)  ==  cnt )
	{
	  fprintf(fileout, "%s\n", line_to_insert);
	}

      fprintf(fileout, "%s", buffer);
      fgets(buffer, BUFFSIZE, filein);
      cnt++;
    }
  
  fclose(filein);
  fclose(fileout);
  return(EXIT_SUCCESS);
}
johand@osiris: ~/tmp $ gcc -Wall  -o inserline inserline.c
johand@osiris: ~/tmp $ cp data.org data.txt; ./inserline BLAH data.txt; cat data.txt
Ligne 1
Ligne 2
Ligne 3
BLAH
Ligne 4
Ligne 5
Ligne 6
Ligne 7

<real	0m7.078s
user	0m0.260s
sys	0m1.292s
johand@osiris: ~/tmp $ time for iter in $(seq 1 1000 ); do cp data.org data.txt; ./inserline BLAH data.txt; done

real	0m6.351s
user	0m0.224s
sys	0m1.288s