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I am facing a problem in C to edit a specific line of a file. There is a number in the beginning of the file which is followed by a number of lines. It kind of looks like this.

2
Nasif  20  BUET  130
Oishi  24  KMC  131

After each execution, I append one more line in the file. And the first number in the file (which actually indicates the number of lines) increases by 1. This process seems not to be working.

data=fopen("highscore.txt","r");
fscanf(data,"%d",&number_of_scores);
fclose(data);
if(number_of_scores<10){
    data=fopen("highscore.txt","a");
    fprintf(data,"%s  %s  %s  %s\n", user[current_user].name,
            user[current_user].age, user[current_user].college,result);
    number_of_scores++;
    fseek(data,0,0);
    fprintf(data,"%d",number_of_scores);
    fclose(data);
}
else{

}

So, what should be the right approach?

Jonathan Leffler
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Nasif Imtiaz Ohi
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    Why not just open the file once, before the loop, and then write out all of the data in one go? Why do you have to write one line at a time and update the counter manually? – Some programmer dude May 30 '13 at 15:46
  • Why not check for errors? Why not use a debugger to see why it doesn't work after all? –  May 30 '13 at 15:48
  • i will check errors in my final code. It's just a test for what i am trying to do. – Nasif Imtiaz Ohi May 30 '13 at 16:00
  • Note that if you "edit" (in update mode) the first line and change the number of characters in it, you will muck up the next line. – Hot Licks May 30 '13 at 16:07
  • if(number_of_scores<10)- this condition intends to handle that issue ! – Nasif Imtiaz Ohi May 30 '13 at 16:31
  • Look at the code for [Insert into the middle of a file](http://stackoverflow.com/a/10468278/15168). Although it talks about binary files, it will work on Unix for text files too. On Windows, it might need some care -- open the file in binary rather than text mode to avoid issues with CRLF to NL mapping. – Jonathan Leffler May 30 '13 at 17:19

4 Answers4

3

For fopen modes see http://www.cplusplus.com/reference/cstdio/fopen/. I think you need to use the option r+ because you are modifying the file in a random-access way fior both read and write.

"r+" read/update: Open a file for update (both for input and output). The file must exist.

"w+" write/update: Create an empty file and open it for update (both for input and output). If a file with the same name already exists its contents are discarded and the file is treated as a new empty file.

"a+" append/update: Open a file for update (both for input and output) with all output operations writing data at the end of the file. Repositioning operations (fseek, fsetpos, rewind) affects the next input operations, but output operations move the position back to the end of file. The file is created if it does not exist.

I would suggest storing the number of lines in the file as an unsigned integer and not a string. The reason is that as a string 0-9 lines take one byte, but the minute you have 10 lines you need two byes, 100, 3 bytes and so on. In each case when an extra character is required you would have to re-write the entire file. I presume this is why you check that number of scores is less than 10.

A better solution would be to keep the first 4 bytes of the file as an unsigned integer and then start the ascii text after.

int      result;
uint32_t number_of_scores;
size_t   bytesRead;
FILE     *data;

...

/* Open a file for update (both for input and output).
* The file must exist. */
data = fopen("highscore.txt","r+");
if( !data ) 
    exit(SOME_ERROR_CODE);

/* Read a 32-bit unsigned integer from the file. NOTE there is no endianess
 * "protection" here... ignoring this issue for the sake of simplicity and relevance */
bytesRead = fread (&number_of_scores, sizeof(number_of_scores), 1, data);   
if( bytesRead != 1 )
    exit(SOME_ERROR_CODE);

/* Seek to end of file */
result = fseek(data, 0, SEEK_END);    
if( result )
    exit(SOME_ERROR_CODE);

/* Write in the next line */
result = fprintf(data,
                 "%s  %s  %s  %s\n", 
                 user[current_user].name,
                 user[current_user].age, 
                 user[current_user].college,
                 resultVariableRenamedToAvoidNameCollision);

 /* Up the number of scores and write it back to the start of the file */
 number_of_scores++;
 result = fseek(data, 0, SEEK_SET);      
 if( result )
     exit(SOME_ERROR_CODE);

 bytesRead = fwrite (data, sizeof(number_of_scores), 1, data);
 if( bytesRead != 1 )
    exit(SOME_ERROR_CODE);

 fclose(data);

Doh, and I've just realised how late this answer is... never mind :S

Jimbo
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You need to open the file in read-and-write mode ("r+"), not append ("a"). The write position will be at the start, initially, so you might as well update the line count first. Then you can fseek(data, 0, SEEK_END) before appending your new line.

sh1
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Try it replacing "a" with "w+" on the second fopen() call. "a" is intended for appending information at the end of the file. =)

Edit: However I would also pad the written number with some fixed spaces, otherwise, as the code is currently written, when the count exceeds 9 you will probably overwrite the end-of-line marker of the first line.

  • The op's intent _is_ to write the new record at the end of the file, so "a" is correct. – lurker May 30 '13 at 15:51
  • @mbratch, yes you are correct. But the OP also intends to replace the content of the first line. Opening as "w+" would enable both operations, by means of an additional `fseek()` call. As @JoachimPileborg suggested, it would be easier to overwrite the complete file in this case. –  May 30 '13 at 15:56
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  1. No need to open the file multiple times.
  2. Need to seek the file pointer to the accurate point.
  3. If the data from the file has to be read,,the file must be opened in read mode.
akhil
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