File line replacements using C

[prague14]

Okay, in Perl, this would be a no-brainer. But, I'm trying to do this basic line change using C (call me crazy). Say, for example, I want to swap a line with the text "-enabled\n" to "-disabled\n"; here's what I have to *find* the line, file opens okay and everything:

Code: Select all

while((fgets(line, 1024, fp) != NULL) && (line[0] != '\0')) {
    if(strncmp("-enabled", line, 8) == 0) {
        printf("Line: %s", line);
    }
}

This loop works just fine. The problem is actually doing the replacement. I'm completely stuck. Based on what I have read, it looks like I need to mark the beginning of the line with ftell() and then replace it with fseek()... or something(please, stop me if I'm headed in the wrong direction). However, I am not finding a very well documented case on how this is done exactly. That's where you bad-ass C guys come in.

Thanks for any help you guys can give me.

[rokstar83]

Honestly your best bet is to read the file in to a temp file, replace the string as its reading/writing then delete the old file and move the new file.

Prolly want something like this (my c is a little rusty but this should be close):

Code: Select all

FILE * infile;
FILE * outfile;
char * tmpFileName;
char line[128];

infile = fopen("somefile.txt","r");
tmpFileName = tmpnam(NULL);
outfile = fopen(tmpFileName,"w");

while(fgets(line, sizeof(line), infile)) {
   if(strcmp(line,"sometext") == 0) 
      strcpy(line, "someOtherText");
   fputs(line,outfile);
}

fclose(infile);
fflush(outfile);
fclose(outfile);
remove("somefile.txt");
rename(tmpFileName,"somefile.txt");

[barophobia]

No easy way around it with out some library I don't know about.

1) read file in memory
2) make modifications
3) write file back

There is one exception to the rule that is if you do not change the number of characters in the line you are replacing. Then you can just mmap or something and change those bytes.

[prague14]

rokstar83: I was looking at that option as well, it certainly seems a bit easier.

baraphobia: If this implies something other than what rokstar83 suggested (using a temp file), how is this done exactly? Do you need to store the file lines in an array and go back to the beginning using rewind()? Can you post some example code if that's the case?

Let me know if you guys have any other thoughts/suggestions. Thanks!

[wellwhoopdedooo]

There is no OS-level support for inserting into or deleting from a file. You'll need to use a temp file.

Keep it on the same filesystem, and you can just move the file over the old one; don't do the unlink or any checking of your ability to do it, that's a race condition. Just do the move and check the return value.

[prague14]

Yeah, the temp file was easy enough to implement, so I did that. If anyone wants to post another approach though, please do so. Thanks to all who replied.