how to make this function return a string

Question

I have this code:

static char * display(const u8 array[], int length) {
    int i;
    char *str;
    for (i = 0; i < length; i++) {
        if (i%32 == 0) {
            //printf("\n");
            strcat(str, "\n");
        }
        if (i%8 == 0) {
            //printf(" ");
            strcat(str, " ");
        }
        //printf("%02X", array[i]);
        strcat(str, (char *)array[i]);
    }
    return str;
    /*
    char str[80];
    strcpy (str,"these ");
    strcat (str,"strings ");
    strcat (str,"are ");
    strcat (str,"concatenated.");
    puts (str);
    return 0;
    */
}

Originally this code was printing a string. I dont want it to print a string, I need to make it returning a string. But it gives me this error:

main.c: In function 'display':
main.c:1707:9: warning: passing argument 2 of 'strcat' makes pointer from integer without a cast [ena bled by default]
     strcat(str, array[i]);                                                                      
     ^                                                                                            In file included from main.c:61:0:                                    
/usr/include/string.h:133:14: note: expected 'const char * restrict' but argument is of type 'u8' extern char *strcat (char *__restrict __dest, const char *__restrict __src)
^ sh-4.2# gcc -o main *.c
main.c: In function 'display':
main.c:1707:21: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
strcat(str, (char *)array[i]);

EDIT:

build shared object and load:

VALUE generateIt(char *valor) {
  struct NESSIEstruct w;
  u8 digest[DIGESTBYTES];

  int i;
  for(i=0; valor[i]!='\0'; ++i);
  int sizeo = i;

  NESSIEinit(&w);
  NESSIEadd((u8*)valor, 8*sizeo, &w);
  NESSIEfinalize(&w, digest);
  return displayIt(digest, DIGESTBYTES);
}

on top I do:

#include 'ruby.h'

and I add also this:

void
Init_whirlpool(){
  rb_mWhirlpool = rb_define_module("Whirlpool");
  rb_cClass = rb_define_class_under(rb_mWhirlpool, "Class", rb_cObject);
  rb_define_method(rb_cClass, "generate", generateIt, 1);
}

str is an uninitialized pointer that will have an undefined value. Probably going to crash when you use it. — crashmstr
– crashmstr, Commented Jan 9, 2015 at 19:39
@iharob * u8 -> unsigned integer type, at least 8 bits, equivalent to unsigned char — WeezHard
– WeezHard, Commented Jan 9, 2015 at 19:40
@PSantos What is VALUE is it a generic type used by ruby? something like void *? I am almost sure the problem is you need to use some ruby custom memory allocation funciton that lets the interpreter handle memory allocation. And where did you add the free? — Iharob Al Asimi
– Iharob Al Asimi, Commented Jan 10, 2015 at 0:54
@iharob In Ruby C API, every Ruby type/class is a VALUE C type. In fact VALUE is a just a uintptr_t C type, or void* to simplify things. This is how Ruby’s loose typing is implemented in C. — WeezHard
– WeezHard, Commented Jan 10, 2015 at 1:00

Iharob Al Asimi · Accepted Answer · 2015-01-09 19:57:18Z

2

I believe this is what you need

static char *display(const unsigned char array[], int length)
{
    int  i, k;
    char *str;

    str = malloc(3 * length + 1);
    if (str == NULL)
        return NULL;
    k      = 0;
    str[0] = '\0';
    for (i = 0; i < length; i++) {
        char hex[3];

        if (i % 32 == 0)
            str[k++] = '\n';
        if (i % 8 == 0)
            str[k++] = ' ';
        snprintf(hex, sizeof(hex), "%02X", array[i]);

        str[k++] = hex[0];
        str[k++] = hex[1];
    }
    str[k] = '\0';

    return str;
}

snprintf is POSIX, if it gives a compilation error, change it to _snprintf.

And don't forget to free the returned value in the caller function.

edited Jan 9, 2015 at 19:57

answered Jan 9, 2015 at 19:40

Iharob Al Asimi

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11 Comments

alk Over a year ago

All ints here really should be size_ts.

Iharob Al Asimi Over a year ago

@alk sometimes when I use size_t the OP's ask something like, What is size_t?.

alk Over a year ago

Then tell him/her? However, unsigned would at least be obvious.

Iharob Al Asimi Over a year ago

@alk Oh i've told them, but then they would say something like Oh no, that's too advanced I will just use int.

alk Over a year ago

A signed variable simply makes no sense to count an amount of memory or to measure the length of a "string".

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