How can I copy the contents of argv[] into a c style string?

char* filename[64] creates an array of 64 pointers. You intend to create space for a string with 64 characters - this would be char filename[64]. Because you only allocated space for pointers, and never made the pointers point to any memory, you get a seg fault.

Solution: use char filename[64];

This creates a block of 64 bytes for your string; the value filename points to the start of this block and can be used in a copy operation

strcpy(filename, argv[2]);

I would strongly recommend using the "copy no more than n characters" function - this prevents a really long argument from causing buffer overflow. Thus

strncpy(filename, argv[2], 64);

would be safer. Even better

strncpy(filename, argv[2], 63);filename[63] = '\0';

This guarantees that the copied string is null terminated.

You have the same problem with message. I don't think you need the code repeating...

Let me know if you need more info.

UPDATE
Today I learnt about the existence of strlcpy - see this answer. It will take care of including the NUL string terminator even when the original string was longer than the allocated space. See this for a more complete discussion, including the reasons why this function is not available on all compilers (which is of course a major drawback if you are trying to write portable code).

Since you have tagged this as C++ (and no one has yet mentioned it):

argv is already a C-style array, so there is no need to copy it to another (unless you just want to waste space). If you really wanted to copy it into something, a std::string object would be a better approach:

int main(int argc, char* argv[]){    // assuming your conditional checks are already done here ...    std::string filename = argv[1];    std::string message = argv[2];    // do something    return 0;}

Your variables filename and message are char pointer arrays, not C-style strings (which should be null-terminated char arrays). So you need to declare their type as:

char filename[64];char message[256];

and use strcpy as:

strcpy(filename, argv[2]);strcpy(message, argv[3]);

the call to open is similar:

fd = open(filename, O_RDWR | O_CREAT, 00000);