How to pass an integer input into a program in c?-CodePudding

I am wanting to write a program in c that takes in an integer, an input file and an output file when compiled. The execution code should be as follows:

./example N input_file output_file

I'm trying to work out how to configure this in c. So far I have managed to successfully take in the input_file and output_file and code is as follows:

int main(int argc, char *argv[]){
FILE *M, *fin, *fout;
int *N;

// check if we have 3 arguments - remember that the first one is the file name
if(argc!=4){
    printf("ERROR: not enough input parameters\n");
    printf("USE: %s input output\n",argv[0]);
    exit(1);
}

N = argv[1];
printf("%f \n",DECIMATION_FACTOR);
// open input files as binary read-only
fin=fopen(argv[2],"rb");
if(fin == NULL) {
    printf("ERROR: %s does not exist\n",argv[1]);
    exit(1);
}

// open output files as binary - overwrite the file if it alredy exists
fout=fopen(argv[3],"w b");
if(fout == NULL) {
    printf("ERROR: %s cannot be created\n",argv[2]);
    exit(1);
}

}

How can I input an integer where N is?

CodePudding user response：

All arguments are C strings (zero terminated pointers to char). You need to convert the string to integer. The standard library offers many facilities to do so, which differ based on the type of control that you want to have on strings which should be integers and are in fact something else. One option is:

char *endptr;
long int val = strtol(argv[1], &endptr, 0);
if (*endptr != 0) {
    // Failed to convert the whole string (at least something was left behind)
}
else {
    // Everything was converted: now you can trust the input, or you need to 
    // check errno for ERANGE. In this case set errno=0 before calling strtol.
}

CodePudding user response：

The lines

int *N;
[...]
N = argv[1];

will not work, because argv[1] is a pointer to a string, not an integer or a pointer to an integer.

In order to convert a string to an integer, you can use the function strtol, for example like this:

int N;
char *p;

N = strtol( argv[1], &p, 10 );

if ( p == argv[1] )
{
    fprintf( stderr, "error converting value!\n" );
    exit( EXIT_FAILURE );
}

printf( "The number was successfully converted to %d.\n", N );

Note that this program will not check whether the input is in the representable range of an int. It will simply return LONG_MAX (e.g. 2,147,483,647 or 9,223,372,036,854,775,807, depending on your platform) and truncate it to an int. However, as described in the documentation of the function strtol (link see above), it is possible to check for such a range error, by checking the value of errno.