file operations FAQ etc

[Douglas G]

I'm looking for some links or something that discusses opening files,
writing files, and any gotchas associated with it. I'm new to programming
and C so any pointers would be helpful.

I know this is a generic question, but what I've found from googling shows
stuff that I've already read on that leaves me skeptical as they use
functions like scanf that from what I've seen, you're not to use. If
anyone has helpful links, it would be most appreciated.

I'd just like some ideas on checking whether a file already exists and baily
out gracefully if it does exist.

 

[Karthik Kumar]

I'm looking for some links or something that discusses opening files,
writing files, and any gotchas associated with it. I'm new to programming
and C so any pointers would be helpful.

Get a good C book. A good book would describe files.

I know this is a generic question, but what I've found from googling shows
stuff that I've already read on that leaves me skeptical as they use
functions like scanf that from what I've seen, you're not to use. If
anyone has helpful links, it would be most appreciated.

Refer to the C FAQ - http://www.eskimo.com/~scs/C-faq/top.html .

I'd just like some ideas on checking whether a file already exists and baily
out gracefully if it does exist.

FILE * fp;

fp = fopen("myfile.txt", "r");
if (fp) {
fclose(fp); /* Release resources */
} else {
perror("I could not open this file ... ");
}

 

[Malcolm]

I'm looking for some links or something that discusses opening files,
writing files, and any gotchas associated with it. I'm new to programming
and C so any pointers would be helpful.

I know this is a generic question, but what I've found from googling shows
stuff that I've already read on that leaves me skeptical as they use
functions like scanf that from what I've seen, you're not to use. If
anyone has helpful links, it would be most appreciated.

The problem is errors in input. C has no exception handling mechanism, and
file formats can be very compicated. Sometimes it is OK to terminate the
program with an error message on bad input, but often this isn't an option.

Examples of gotchas.

lines too long to fit in the fgets() buffer.
strings of digits too long to be converted to numbers.
premature termination of file.
delimiters (eq quotes) not properly positioned.

I'd just like some ideas on checking whether a file already exists and baily
out gracefully if it does exist.

int fileexists(const char *path)
{
FILE *fp;

fp = fopen(path, "rb");
if(!fp)
return 0;
fclose(fp);
return 1;
}

....
if( fileexists("Myfile.txt") )
{
printf("It already exists folks\n");
exit(EXIT_FAILURE);
}
....

 

[CBFalconer]

int fileexists(const char *path)
{
FILE *fp;

fp = fopen(path, "rb");
if(!fp)
return 0;
fclose(fp);
return 1;
}

The fopen can fail even if the file exists. Better to look for a
better program scheme.

...
if( fileexists("Myfile.txt") )
{
printf("It already exists folks\n");
exit(EXIT_FAILURE);
}
...

and even if filexists works, the answer may be different by the
time the program gets here.

 

[Malcolm]

The fopen can fail even if the file exists. Better to look for a
better program scheme

and even if filexists works, the answer may be different by the
time the program gets here.

This is true. I think the fopen() method is good enough for most purposes,
and there isn't a standard library stat() equivalent.