command line input problem

I am getting a "memory could not be written" when gets for file-out name. When a user only enters file-in name, the program prompts for output file name. When user enters it, it gets the string and puts it in argv[2] and this is where I get the error.



void
main(int argc, char *argv[])
{
char fileNameIn[20];
char fileNameOut[20];

if(argc == 1) // only the program name is input
{
printf("\n Usage: pgm10 input_fileName output_fileName\n");
exit(1);
}

if (argc == 2) // two strings input(pgm10 and input
// fileName); output fileName is missing
{
fflush(stdin);
printf("\n Please enter output file name: ");
gets(argv[3]);
}

printf("file name in = %s\n", argv[2]);
printf("file name out = %s\n", argv[3]);

}

Quote:

---

if (argc == 2)

gets(argv[3]);

---

Not sure if I read your code right, hard to read like it is use code tags.
if argc is 2, then there isnt a argv[3], the highest argv will be argv[1](remember array elements start at 0).
if you try and assign data to argv[3] you are trying to access an area of memory that hasn't been initialised, so you get a memory error.
I think thats your problem or part of it. hope that helps some.
:)

Thanks for your help. If the user only inputs program name and filein name (argc =2), I had to prompt him for fileout name and store it in a different variable, not argv[2]).

This is what I had to do. If you can think of better way, please let me know.

void
main(int argc, char *argv[])
{
char fileNameIn[20];
char fileNameOut[20] = { "\0" };


if(argc == 1) // 0 & 1; only the program name is input
{
printf("\n Usage: pgm10 input_fileName output_fileName\n");
exit(1);
}

if (argc == 2) // two strings input(pgm10 and input
// fileName); output fileName is missing
{
printf("\n Please enter output file name: ");
fflush(stdin);
gets(fileNameOut);
}
strcpy (fileNameIn, argv[1]);

printf(" argc = %d\n", argc);
printf(" Program name = %s\n", argv[0]);
printf(" file name in = %s\n", fileNameIn);

if (argv[2] != NULL)
printf(" file name out = %s\n", argv[2]);
else
printf(" file name out = %s\n", fileNameOut);
}