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Why am I getting a seg fault?

Posted on 2015-02-04
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Last Modified: 2015-05-20
Here's the code

int main(int argc, char* argv[1])
{
char charOld = argv[4][0];
char charNew = argv[5][0];
}
~

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I know I'm not seeing something simple. What am I over looking in terms of the seg fault??
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Question by:cgunix
7 Comments
 
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Accepted Solution

by:
Mark Bullock earned 250 total points
ID: 40590398
argv is an array of char*, which point to strings.
argv[0] contains a pointer to the first string, the name or pathname of the program.
argv[0][1] contains the second character of that string.
So swap the indexes in your code, e.g. argv[0][4]
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Expert Comment

by:käµfm³d 👽
ID: 40590400
Why aren't you checking the value of argc? How would you know how many arguments were passed to your program?
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Expert Comment

by:Mark Bullock
ID: 40590412
argc is always >= 1 the first argument is the name or pathname of the program.
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LVL 84

Assisted Solution

by:ozo
ozo earned 250 total points
ID: 40590436
If you don't pass at least 5 command line arguments to  your program, argv[5] will be undefined.

If the program name is not at least 5 characters long, argv[0][5] will be undefined.
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Expert Comment

by:sarabande
ID: 40591617
I think Mark is right and you mixed up the array indices. the first index counts the pointers to char, the second index counts the characters in the argument string.

If you don't pass at least 5 command line arguments to  your program, argv[5] will be undefined.
it is not only undefined but argv[5] is an invalid pointer value - probably 0 - what causes the segmentation fault. when accessing the argv array elements you always should follow the advice kaufmed has given and check for the argc argument before accessing argv elements:

if (argc > 1)
{
       // now it is safe to use the argv[1] argument
       int len = strlen(argv[1];
       ....
}
if (argc > 2)
{
       // now it is safe to use the argv[2] argument
       std::cout << argv[2] << std::endl;
       ...
}
....

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or

if (argc <= 3)
{
       std::cout << "program requires at least 3 arguments" << std::endl;
       return -1;
}
// here it is safe to access argv[1], argv[2], or argv[3]

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Sara
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