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prague14
n00b
Joined: 09 Nov 2007
Posts: 40
Location: USA
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 Posted: Mon Aug 25, 2008 8:29 pm Post subject: File line replacements using C |
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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: |
while((fgets(line, 1024, fp) != NULL) && (line[0] != '\0')) {
if(strncmp("-enabled", line, 8) == 0) {
printf("Line: %s", line);
}
}
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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. 
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"If I live, I will kill you. If I die, you are forgiven."
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rokstar83
Guru
Joined: 09 Apr 2005
Posts: 423
Location: MD
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| Posted: Mon Aug 25, 2008 9:13 pm Post subject: |
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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: |
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");
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barophobia
Apprentice
Joined: 27 Apr 2004
Posts: 229
Location: somewhere
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| Posted: Tue Aug 26, 2008 4:51 am Post subject: |
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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.
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An apple is an apple unless you say it is not an apple! |
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prague14
n00b
Joined: 09 Nov 2007
Posts: 40
Location: USA
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| Posted: Tue Aug 26, 2008 12:10 pm Post subject: |
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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!
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For a wounded man shall say to his assailant,
"If I live, I will kill you. If I die, you are forgiven."
Such is the rule of honor. |
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wellwhoopdedooo
n00b
Joined: 04 Mar 2005
Posts: 69
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| Posted: Tue Aug 26, 2008 12:59 pm Post subject: |
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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. |
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prague14
n00b
Joined: 09 Nov 2007
Posts: 40
Location: USA
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| Posted: Tue Aug 26, 2008 3:31 pm Post subject: |
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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.
_________________
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"If I live, I will kill you. If I die, you are forgiven."
Such is the rule of honor. |
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