strToHex ( string to its hex representation as string) I want to convert strings to their hex representations as strings too (like hex dump programs), for example "abz" to "61627A".  

char * strToHex( char * str )
{
    int length = strlen ( str );
    char * newStr = malloc( length  * 2 );
    if ( !newStr ) shutDown ( "can't alloc memory" ) ;

    for ( int x = 0; x &lt; length; x++){
        char y = str[ x ];
        sprintf ( newStr + x * 2, "%02X", y );
    }
    return newStr;
}


ShutDown definition is omitted here, it is a function that calls perror and exit()

I designed strToHex to be used like

char * str = "abcdefghijklmnopqrstuvwxyz";
char * hex = strToHex(str);
printf("%s\n",hex);
//outputs : 6162636465666768696A6B6C6D6E6F707172737475767778797A
Bug

As Carsten points out, you need to allocate \$(\text{length}\cdot 2)+1\$ bytes, rather than \$(\text{length}\cdot2)\$ to account for the null terminator sprintf() adds.

Formatting

Most C formatting guides do not include spaces around the arguments to function calls, nor the expressions within an if-statement. For an example of a C style most C programmers would find acceptable, see OpenBSD's style(9) manual.

I choose to associate * with the variable name, rather than floating between the type and name. This disambiguates the following example:

int *a, b;


Here, a is a pointer to an integer, but b is only an integer. By moving the asterisk next to the name, it makes this clearer.

int length = strlen ( str );
char * newStr = malloc (length * 2 );
if ( !newStr) shutDown ( "can't allocate memory" ) ;


Becomes:

int const len = strlen(str);
char *const new_str = malloc(1 + len * 2);

if (new_str == NULL) {
    shutDown("can't allocate memory");
}


Error checking

Rather than calling shutDown() and exit()ing the program, you should instead return an error value which can be checked by the caller of str_to_hex(). Because you return a pointer, you can return NULL to indicate an error occurred and the caller should check errno.

Likewise, on some systems your program can incorrectly exit when length == 0. If we look at the manual page for malloc(3):


  Return Value
  
  The malloc() and calloc() functions return a pointer to the allocated memory that is suitably aligned for any kind of variable. On error, these functions return NULL. NULL may also be returned by a successful call to malloc() with a size of zero, or by a successful call to calloc() with nmemb or size equal to zero.


So by returning NULL we account for the case where malloc(3) returns NULL on success.

if (new_str == NULL) {
    shutDown("can't alloc memory");
}


Becomes:

if (new_str == NULL) {
    return NULL;
}


If you choose, you can also check if str is NULL before calling strlen(). This is up to you, and it's not uncommon in C to ignore this case and leave it as user error.

Looping

Use the size_t type in your loop rather than int. size_t is guaranteed be wide enough to hold any array index, while int is not.

Using i rather than x is more common for looping variables.

The y variable isn't needed. You can simply use str[i] in its place.

In terms of performance there's likely a faster option than using sprintf(). You should look into strtol(3).

Conclusion

Here is the code I ended up with:

#include &lt;stdio.h&gt;
#include &lt;stdlib.h&gt;
#include &lt;string.h&gt;

char *
str_to_hex(char const *const str)
{
    size_t const len = strlen(str);

    char *const new_str = malloc(1 + len * 2);

    if (new_str == NULL) {
        return NULL;
    }

    for (size_t i = 0; i &lt; len; ++i) {
        sprintf(new_str + i * 2, "%02X", str[i]);
    }

    return new_str;
}

int
main(void)
{
    char *str = "abz";
    char *hex = str_to_hex(str);

    if (hex == NULL &amp;&amp; strlen(str) != 0) {
        /* error ... */
    }

    printf("%s\n",hex);

    free(hex);
}


Hope this helps!