Write an output file with a user defined name in C - c

Question

I am new to C and am trying to define an output filename before the program runs. I am getting a Bus error
Here is my code:
#include <stdio.h>
int main (int argc, const char * argv[]) {
char fname[128];
printf("Enter the file name\n");
scanf("%123s",fname);
strcat("/Users/user/Desktop/learn/", fname);
strcat(fname, ".txt");
FILE *fp;
fp=fopen(fname,"a");
fprintf(fp, "Testing... OK I think it worked\n");
return 0;
}

Answer 1

You didn't #include <string.h> for strcat.
The first argument to strcat must be a pointer, not a string literal.
strcat itself isn't safe, use strncat instead.
Don't forget to check the result of scanf and fopen.
And close fp when you're done with it.
The signature of main should be int main(int argc, char * argv[]).
The use of scanf is also generally discouraged, use fscanf & sscanf instead.

Answer 2

You are using a string literal as the destination pointer in the first call to strcat. so you are concatonating "/Users/user/Desktop/learn/" with fname and storing the result where ever "/Users/user/Desktop/learn/" was stored, which might not even be writable.

Answer 3

That's not how strcat() works.
I see two approaches:
Use fname correctly, together with the file name inputting:
char fname[128];
strcpy(fname, "/Users/user/Desktop/learn/"); // ok as long as you don't make the 128 too small
char * input = fname + strlen(fname); // now points after the final /
printf("Enter the file name\n");
scanf("%123s", input); // the 123 is probably not correct
strncat(fname, ".txt", sizeof fname);
and use it.
Currently, this approach is still suffering from the fact that input is limited to 123 bytes, which might be too large, so better forget it for now. It is just for getting the idea.
Maybe fgets() might be better:
fgets(input, sizeof(fname)-strlen(fname), stdin);
Use command line parameters, which would be my favourite approach:
// first check if argc is >= 2, i. e. if the caller has supplied an argument
char fname[128];
strcpy(fname, "/Users/user/Desktop/learn/");
strncat(fname, argv[1], sizeof fname);
strncat(fname, ".txt", sizeof fname);

Answer 4

Try this, this is worked for me..
http://cboard.cprogramming.com/c-programming/124576-whats-mean-char*-const*-argv.html
#include <stdio.h>
#include <string.h>
int main (int argc, const char*const* argv[])
{
char fname[128];
char path[] = "/home/abc/test/";
printf("Enter the file name\n");
scanf("%123s",fname);
strcat(fname,".txt");
strcat(path,fname);
FILE *fp;
fp=fopen(path,"a");
fprintf(fp, "Testing... OK I think it worked\n");
fclose(fp);
return 0;
}

Answer 5

Thanks for everyone's comments. This was working code:
#include <stdio.h>
#include <string.h>
int main (int argc, const char*const* argv[])
{
char fname[128];
strcpy(fname, "/Users/user/Desktop/learn/");
char * input = fname + strlen(fname);
printf("Enter the file name\n");
scanf("%s", input);
strncat(fname, ".txt", sizeof fname);
printf("The output pathway and file will be called %s\n", fname);
FILE *fp;
fp=fopen(fname,"a");
fprintf(fp, "Testing... OK I think it worked\n");
fclose(fp);
return 0;
}