Question

I hava a C function like this ..

func(uint8_t *key,uint8_t keylen) {

FILE *fk;
fk=fopen("akey","wb");
fwrite((char *)key,keylen,1,fk);

puts((char *)key); // for testing

fclose(fk);

sprintf(sstring,"... \"%s\" ... ",(char *)key);
        // ...other irrelevant stuff for here
system(sstring);


}

I want the key provided as input to be used at the
place marked \"%s\" in sprintf command

Input :

  Key=qwerty

Output :

 $cat akey
 qwerty

and output of puts((char *)key) is –

 qwerty�    

 // include newline character as well

Suggest me on this , I tried with this (at their respective position in the code)
but getting segmentation fault

    char *p;
    memcpy(p,(char *)key,keylen);
    puts((char *)p);

Answer 1

You could use the form %.*s to print it out:

printf("%.*s\n", (int)keylen, (char *)key);
...
sprintf(sstring,"... %.*s ... ", ... (int)keylen, (char *)key ...);

Here an extract from the Wikipedia entry on printf:

The width and precision formatting parameters may be omitted, or they
can be a fixed number embedded in the format string, or passed as
another function argument when indicated by an asterisk “*” in the
format string. For example printf(“%*d”, 5, 10) will result in ” 10″
being printed, with a total width of 5 characters, and printf(“%.*s”,
3, “abcdef”) will result in “abc” being printed.

NOTE: Using an 8-bit type (uint8_t) to represent the length limits the length of the string to 255 characters. Better is to use size_t.