Overwriting lines in file in C
I'm doing a project on filesystems on a university operating systems course, my C program should simulate a simple filesystem in a human-readable file, so the file should be based on lines, a line will be a "sector". I've learned, that lines must be of the same length to be overwritten, so I'll pad them with ascii zeroes till the end of the line and leave a certain amount of lines of ascii zeroes that can be filled later.
Now I'm making a test program to see if it works like I want it to, but it doesnt. The critical part of my code:
file = fopen("irasproba_tesztfajl.txt", "r+"); //it is previously loaded with 10 copies of the line I'll print later in reverse order
/* this finds the 3rd line */
int count = 0; //how much have we gone yet?
char c;
while(count != 2) {
if((c = fgetc(fil开发者_JAVA技巧e)) == '\n') count++;
}
fflush(file);
fprintf(file, "- . , M N B V C X Y Í Ű Á É L K J H G F D S A Ú Ő P O I U Z T R E W Q Ó Ü Ö 9 8 7 6 5 4 3 2 1 0\n");
fflush(file);
fclose(file);
Now it does nothing, the file stays the same. What could be the problem?
Thank you.
From here,
When a file is opened with a "+" option, you may both read and write on it. However, you may not perform an output operation immediately after an input operation; you must perform an intervening "rewind" or "fseek". Similarly, you may not perform an input operation immediately after an output operation; you must perform an intervening "rewind" or "fseek".
So you've achieved that with fflush
, but in order to write to the desired location you need to fseek
back. This is how I implemented it - could be better I guess:
/* this finds the 3rd line */
int count = 0; //how much have we gone yet?
char c;
int position_in_file;
while(count != 2) {
if((c = fgetc(file)) == '\n') count++;
}
// Store the position
position_in_file = ftell(file);
// Reposition it
fseek(file,position_in_file,SEEK_SET); // Or fseek(file,ftell(file),SEEK_SET);
fprintf(file, "- . , M N B V C X Y Í Ű Á É L K J H G F D S A Ú Ő P O I U Z T R E W Q Ó Ü Ö 9 8 7 6 5 4 3 2 1 0\n");
fclose(file);
Also, as has been commented, you should check if your file has been opened successfully, i.e. before reading/writing to file
, check:
file = fopen("irasproba_tesztfajl.txt", "r+");
if(file == NULL)
{
printf("Unable to open file!");
exit(1);
}
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