get C String from argv[]

[justin82]

Hey, I gots a question.

in C im just writing a small little file input test program.

part of the code that is of importance follows

...
...
int inFile, outFile;
char *inFileName = argv[1];
char *outFileName = argv[2];
...
...
inFile = open(inFileName, O_RDONLY);
outFile = open(outFileName, O_WRONLY);
...
...

now my thoughts are: there is something wrong with giving the end user the power to choose name and length of filename. Is this true? and What if any checks can I put in place to keep that from being abused?

Thanks

 

[justin82]

no one?

 

[Earp]

You should always validate your command line arguments right away. There is nothing wrong with letting the user choose filename and length, after all it's just a string. Some filesystems may have limitiations regarding max name length, which you should check for and report to the user as an error.

 

[pedant]

As better practice, shouldn't you be using FILE *infile, *outfile?

Also, I've always used 'r', 'w', or 'a', but each to their own.

 

[HHunt]

As better practice, shouldn't you be using FILE *infile, *outfile?

Also, I've always used 'r', 'w', or 'a', but each to their own.

Depends on what he wants to do. He's using open (correctly), you're (also correctly) describing fopen. Don't confuse them.

 

[Shadow2531]

Here's an example using fopen() and strlen() for checking the length of the arguments. Plus, I threw in a bunch of error messages. You'd set the max value to your desired limit, but you'd keep it <= the max path for the OS.

#include <stdio.h>
#include <string.h>

int main( int argc, char** argv) {
    if ( argc != 3 ) {
        printf( "\nUsage: this infile outfile\n" );
        return 1;
    } 
    const size_t max = 256;
    if ( strlen( argv[1] ) > max ) {
        printf("\ninfile path too long\n");
        return 1;
    } else if ( strlen( argv[2] ) > max ) {
        printf("\noutfile path too long\n");
        return 1;
    }
    FILE* in = fopen( argv[1], "r");
    if ( !in ) {
        printf("\nError reading %s\n", argv[1] );
        return 1;
    }
    // read in from the file and store the data etc.
    fclose( in );
    FILE* out = fopen( argv[2], "w");
    if ( !out ) {
        printf("\nError writing to %s\n", argv[2] );
        return 1;
    }
    // write stuff to the file etc.
    fclose( out );
    return 0;
}

 

[mikeblas]

now my thoughts are: there is something wrong with giving the end user the power to choose name and length of filename. Is this true? and What if any checks can I put in place to keep that from being abused?

If you want the user to specify their own file, then you're fine. I think you're asking for trouble, trying to see if the file name is valid when you could rely on the C runtime (which relies on the OS) to just try and open the file.

You might want to check the opened file to make sure it's really a file and not a device; that may or may not be the right thing to do depending on your application.

In particular, I can't see a reason to cap the input to a length limit, as Shadow2531 is suggesting.