Whenever a function is called, it's possible that uninitialized values are passed as parameters, or even pointers to a buffer which is unaddressable. In either case, Memcheck will log this:

$ valgrind --read-var-info=yes --leak-check=full ./unsyscall
==6848== Memcheck, a memory error detector
==6848== Copyright (C) 2002-2015, and GNU GPL'd, by Julian Seward et al.
==6848== Using Valgrind-3.11.0 and LibVEX; rerun with -h for copyright info
==6848== Command: ./unsyscall
==6848== 
==6848== Syscall param write(buf) points to uninitialised byte(s)
==6848==    at 0x4F306E0: __write_nocancel (syscall-template.S:84)
==6848==    by 0x4005EF: main (unsyscall.cpp:7)
==6848==  Address 0x5203040 is 0 bytes inside a block of size 10 alloc'd
==6848==    at 0x4C2DB8F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==6848==    by 0x4005C7: main (unsyscall.cpp:5)
==6848== 
==6848== Syscall param exit_group(status) contains uninitialised byte(s)
==6848==    at 0x4F05B98: _Exit (_exit.c:31)
==6848==    by 0x4E73FAA: __run_exit_handlers (exit.c:97)
==6848==    by 0x4E74044: exit (exit.c:104)
==6848==    by 0x4005FC: main (unsyscall.cpp:8)
==6848== 
==6848== 
==6848== HEAP SUMMARY:
==6848==     in use at exit: 14 bytes in 2 blocks
==6848==   total heap usage: 2 allocs, 0 frees, 14 bytes allocated
==6848== 
==6848== LEAK SUMMARY:
==6848==    definitely lost: 0 bytes in 0 blocks
==6848==    indirectly lost: 0 bytes in 0 blocks
==6848==      possibly lost: 0 bytes in 0 blocks
==6848==    still reachable: 14 bytes in 2 blocks
==6848==         suppressed: 0 bytes in 0 blocks
==6848== Reachable blocks (those to which a pointer was found) are not shown.
==6848== To see them, rerun with: --leak-check=full --show-leak-kinds=all
==6848== 
==6848== For counts of detected and suppressed errors, rerun with: -v
==6848== Use --track-origins=yes to see where uninitialised values come from
==6848== ERROR SUMMARY: 2 errors from 2 contexts (suppressed: 0 from 0)
  

The preceding log was generated by this code:

#include <cstdlib>
 #include <unistd.h> 

 int main() {  
    char* arr  = (char*) malloc(10);  
    int*  arr2 = (int*) malloc(sizeof(int));  
    write(1, arr, 10 ); 
    exit(arr2[0]);
 } 

Much like the general use of uninitialized values as detailed in the previous section, the passing of uninitialized, or otherwise dodgy, parameters is, at the very least, risky, and in the worst case, a source of crashes, data corruption, or worse.