I am trying to check for any unaligned reads in my program. I enable unaligned access processor exception via (using x86_64 on g++ on linux kernel 3.19):
asm volatile("pushf \n"
"pop %%rax \n"
"or $0x40000, %%rax \n"
"push %%rax \n"
"popf \n" ::: "rax");
I do an optional forced unaligned read which triggers the exception so i know its working. After i disable that I get an error in a piece of code which otherwise seems fine :
char fullpath[eMaxPath];
snprintf(fullpath, eMaxPath, "%s/%s", "blah", "blah2");
the stacktrace shows a failure via __memcpy_sse2 which leads me to suspect that the standard library is using sse to fulfill my memcpy but it doesnt realize that i have now made unaligned reads unacceptable.
Is my thinking correct and is there any way around this (ie can i make the standard library use an unaligned safe sprintf/memcpy instead)?
thanks
While I hate to discourage an admirable notion, you're playing with fire, my friend.
It's not merely sse2 access but any unaligned access. Even a simple int fetch.
Here's a test program:
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <malloc.h>
void *intptr;
void
require_aligned(void)
{
asm volatile("pushf \n"
"pop %%rax \n"
"or $0x00040000, %%eax \n"
"push %%rax \n"
"popf \n" ::: "rax");
}
void
relax_aligned(void)
{
asm volatile("pushf \n"
"pop %%rax \n"
"andl $0xFFFBFFFF, %%eax \n"
"push %%rax \n"
"popf \n" ::: "rax");
}
void
msg(const char *str)
{
int len;
len = strlen(str);
write(1,str,len);
}
void
grab(void)
{
volatile int x = *(int *) intptr;
}
int
main(void)
{
setlinebuf(stdout);
// minimum alignment from malloc is [usually] 8
intptr = malloc(256);
printf("intptr=%p\n",intptr);
// normal access to aligned pointer
msg("normal\n");
grab();
// enable alignment check exception
require_aligned();
// access aligned pointer under check [will be okay]
msg("aligned_norm\n");
grab();
// this grab will generate a bus error
intptr += 1;
msg("aligned_except\n");
grab();
return 0;
}
The output of this is:
intptr=0x1996010
normal
aligned_norm
aligned_except
Bus error (core dumped)
The program generated this simply because of an attempted 4 byte int fetch from address 0x1996011 [which is odd and not a multiple of 4].
So, once you turn on the AC [alignment check] flag, even simple things will break.
IMO, if you truly have some things that are not aligned optimally and are trying to find them, using printf, instrumenting your code with debug asserts, or using gdb with some special watch commands or breakpoints with condition statements are a better/safer way to go
UPDATE:
I a using my own custom allocator am preparing my code to run on an architecture that doesnt suport unaligned read/writes so I want to make sure my code will not break on that architecture.
Fair enough.
Side note: My curiousity has gotten the better of me as the only [major] arches I can recall [at the moment] that have this issue are Motorola mc68000 and older IBM mainframes (e.g. IBM System 370).
One practical reason for my curiosity is that for certain arches (e.g. ARM/android, MIPS) there are emulators available. You could rebuild the emulator from source, adding any extra checks, if needed. Otherwise, doing your debugging under the emulator might be an option.
I can trap unaligned read/write using either the asm , or via gdb but both cause SIGBUS which i cant continue from in gdb and im getting too many false positives from std library (in the sense that their implementation would be aligned access only on the target).
I can tell you from experience that trying to resume from a signal handler after this doesn't work too well [if at all]. Using gdb is the best bet if you can eliminate the false positives by having AC off in the standard functions [see below].
Ideally i guess i would like to use something like perf to show me callstacks that have misaligned but so far no dice.
This is possible, but you'd have to verify that perf even reports them. To see, you could try perf against my original test program above. If it works, the "counter" should be zero before and one after.
The cleanest way may be to pepper your code with "assert" macros [that can be compiled in and out with a -DDEBUG switch].
However, since you've gone to the trouble of laying the groundwork, it may be worthwhile to see if the AC method can work.
Since you're trying to debug your memory allocator, you only need AC on in your functions. If one of your functions calls libc, disable AC, call the function, and then reenable AC.
A memory allocator is fairly low level, so it can't rely on too many standard functions. Most standard functions rely on being able to call malloc. So, you might also want to consider a vtable interface to the rest of the [standard] library.
I've coded some slightly different AC bit set/clear functions. I put them into a .S function to eliminate inline asm hassles.
I've coded up a simple sample usage in three files.
Here are the AC set/clear functions:
// acbit/acops.S -- low level AC [alignment check] operations
#define AC_ON $0x00040000
#define AC_OFF $0xFFFFFFFFFFFBFFFF
.text
// acpush -- turn on AC and return previous mask
.globl acpush
acpush:
// get old mask
pushfq
pop %rax
mov %rax,%rcx // save to temp
or AC_ON,%ecx // turn on AC bit
// set new mask
push %rcx
popfq
ret
// acpop -- restore previous mask
.globl acpop
acpop:
// get current mask
pushfq
pop %rax
and AC_OFF,%rax // clear current AC bit
and AC_ON,%edi // isolate the AC bit in argument
or %edi,%eax // lay it in
// set new mask
push %rax
popfq
ret
// acon -- turn on AC
.globl acon
acon:
jmp acpush
// acoff -- turn off AC
.globl acoff
acoff:
// get current mask
pushfq
pop %rax
and AC_OFF,%rax // clear current AC bit
// set new mask
push %rax
popfq
ret
Here is a header file that has the function prototypes and some "helper" macros:
// acbit/acbit.h -- common control
#ifndef _acbit_acbit_h_
#define _acbit_acbit_h_
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <malloc.h>
typedef unsigned long flags_t;
#define VARIABLE_USED(_sym) \
do { \
if (1) \
break; \
if (!! _sym) \
break; \
} while (0)
#ifdef ACDEBUG
#define ACPUSH \
do { \
flags_t acflags = acpush()
#define ACPOP \
acpop(acflags); \
} while (0)
#define ACEXEC(_expr) \
do { \
acoff(); \
_expr; \
acon(); \
} while (0)
#else
#define ACPUSH /**/
#define ACPOP /**/
#define ACEXEC(_expr) _expr
#endif
void *intptr;
flags_t
acpush(void);
void
acpop(flags_t omsk);
void
acon(void);
void
acoff(void);
#endif
Here is a sample program that uses all of the above:
// acbit/acbit2 -- sample allocator
#include <acbit.h>
// mymalloc1 -- allocation function [raw calls]
void *
mymalloc1(size_t len)
{
flags_t omsk;
void *vp;
// function prolog
// NOTE: do this on all "outer" (i.e. API) functions
omsk = acpush();
// do lots of stuff ...
vp = NULL;
// encapsulate standard library calls like this to prevent false positives:
acoff();
printf("%p\n",vp);
acon();
// function epilog
acpop(omsk);
return vp;
}
// mymalloc2 -- allocation function [using helper macros]
void *
mymalloc2(size_t len)
{
void *vp;
// function prolog
ACPUSH;
// do lots of stuff ...
vp = NULL;
// encapsulate standard library calls like this to prevent false positives:
ACEXEC(printf("%p\n",vp));
// function epilog
ACPOP;
return vp;
}
int
main(void)
{
int x;
setlinebuf(stdout);
// minimum alignment from malloc is [usually] 8
intptr = mymalloc1(256);
intptr = mymalloc2(256);
x = *(int *) intptr;
return x;
}
UPDATE #2:
I like the idea of disabling the check before any library calls.
If the AC H/W works and you wrap the library calls, this should yield no false positives. The only exception would be if the compiler made a call to its internal helper library (e.g. doing 64 bit divide on 32 bit machine, etc.).
Be aware/wary of the ELF loader (e.g. /lib64/ld-linux-x86-64.so.2) doing dynamic symbol resolution on "lazy" bindings of symbols. Shouldn't be a big problem. There are ways to force the relocations to occur at program start, if necessary.
I have given up on perf for this as it seems to show me garbage even for a simple program like the one you wrote.
The perf code in the kernel is complex enough that it may be more trouble than it's worth. It has to communicate with the perf program with a pipe [IIRC]. Also, doing the AC thing is [probably] uncommon enough that the kernel's code path for this isn't well tested.
Im using ocperf with misalign_mem_ref.loads and stores but either way the counters dont correlate at all. If i record and look at the callstacks i get completely unrecognizable callstacks for these counters so i suspect either the counter doesnt work on my hardware/perf or it doesnt actually count what i think it counts
I honestly don't know if perf handles process reschedules to different cores properly [or not]--it should [IMO]. But, using sched_setaffinity to lock your program to a single core might help.
But, using the AC bit is more direct and definitive, IMO. I think that's the better bet.
I've talked about adding "assert" macros in the code.
I've coded some up below. These are what I'd use. They are independent of the AC code. But, they can also be used in conjunction with the AC bit code in a "belt and suspenders" approach.
These macros have one distinct advantage. When properly [and liberally] inserted, they can check for bad pointer values at the time they're calculated. That is, much closer to the true source of the problem.
With AC, you may calculate a bad value, but AC only kicks in [sometime] later, when the pointer is dereferenced [which may not happen in your API code at all].
I've done a complete memory allocator before [with overrun checks and "guard" pages, etc.]. The macro approach is what I used. And, if I had only one tool for this, it is the one I'd use. So, I recommend it above all else.
But, as I said, it can be used with the AC code as well.
Here's the header file for the macros:
// acbit/acptr.h -- alignment check macros
#ifndef _acbit_acptr_h_
#define _acbit_acptr_h_
#include <stdio.h>
typedef unsigned int u32;
// bit mask for given width
#define ACMSKOFWID(_wid) \
((1u << (_wid)) - 1)
#ifdef ACDEBUG2
#define ACPTR_MSK(_ptr,_msk) \
acptrchk(_ptr,_msk,__FILE__,__LINE__)
#else
#define ACPTR_MSK(_ptr,_msk) /**/
#endif
#define ACPTR_WID(_ptr,_wid) \
ACPTR_MSK(_ptr,(_wid) - 1)
#define ACPTR_TYPE(_ptr,_typ) \
ACPTR_WID(_ptr,sizeof(_typ))
// acptrfault -- pointer alignment fault
void
acptrfault(const void *ptr,const char *file,int lno);
// acptrchk -- check pointer for given alignment
static inline void
acptrchk(const void *ptr,u32 msk,const char *file,int lno)
{
#ifdef ACDEBUG2
#if ACDEBUG2 >= 2
printf("acptrchk: TRACE ptr=%p msk=%8.8X file='%s' lno=%d\n",
ptr,msk,file,lno);
#endif
if (((unsigned long) ptr) & msk)
acptrfault(ptr,file,lno);
#endif
}
#endif
Here's the "fault" handler function:
// acbit/acptr -- alignment check macros
#include <acbit/acptr.h>
#include <acbit/acbit.h>
#include <stdlib.h>
// acptrfault -- pointer alignment fault
void
acptrfault(const void *ptr,const char *file,int lno)
{
// NOTE: it's easy to set a breakpoint on this function
printf("acptrfault: pointer fault -- ptr=%p file='%s' lno=%d\n",
ptr,file,lno);
exit(1);
}
And, here's a sample program that uses them:
// acbit/acbit3 -- sample allocator using check macros
#include <acbit.h>
#include <acptr.h>
static double static_array[20];
// mymalloc3 -- allocation function
void *
mymalloc3(size_t len)
{
void *vp;
// get something valid
vp = static_array;
// do lots of stuff ...
printf("BEF vp=%p\n",vp);
// check pointer
// NOTE: these can be peppered after every [significant] calculation
ACPTR_TYPE(vp,double);
// do something bad ...
vp += 1;
printf("AFT vp=%p\n",vp);
// check again -- this should fault
ACPTR_TYPE(vp,double);
return vp;
}
int
main(void)
{
int x;
setlinebuf(stdout);
// minimum alignment from malloc is [usually] 8
intptr = mymalloc3(256);
x = *(int *) intptr;
return x;
}
Here's the program output:
BEF vp=0x601080
acptrchk: TRACE ptr=0x601080 msk=00000007 file='acbit/acbit3.c' lno=22
AFT vp=0x601081
acptrchk: TRACE ptr=0x601081 msk=00000007 file='acbit/acbit3.c' lno=29
acptrfault: pointer fault -- ptr=0x601081 file='acbit/acbit3.c' lno=29
I left off the AC code in this example. On your real target system, the dereference of intptr in main would/should fault on an alignment, but notice how much later that is in the execution timeline.
Like I commented on the question, that asm isn't safe, because it steps on the red-zone. Instead, use
asm volatile ("add $-128, %rsp\n\t"
"pushf\n\t"
"orl $0x40000, (%rsp)\n\t"
"popf\n\t"
"sub $-128, %rsp\n\t"
);
(-128 fits in a sign-extended 8bit immediate, but 128 doesn't, hence using add $-128 to subtract 128.)
Or in this case, there are dedicated instructions for toggling that bit, like there are for the carry and direction flags:
asm("stac"); // Set AC flag
asm("clac"); // Clear AC flag
It's a good idea to have some idea when your code uses unaligned memory. It's not necessarily a good idea to change your code to avoid it in every case. Sometimes better locality from packing data closer together is more valuable.
Given that you shouldn't necessarily aim to eliminate all unaligned accesses anyway, I don't think this is the easiest way to find the ones you do have.
modern x86 hardware has fast hardware support for unaligned loads/stores. When they don't span a cache-line boundary, or lead to store-forwarding stalls, there's literally no penalty.
What you might try is looking at performance counters for some of these events:
misalign_mem_ref.loads [Speculative cache line split load uops dispatched to L1 cache]
misalign_mem_ref.stores [Speculative cache line split STA uops dispatched to L1 cache]
ld_blocks.store_forward [This event counts loads that followed a store to the same address, where the data could not be forwarded inside the pipeline from the store to the load.
The most common reason why store forwarding would be blocked is when a load's address range overlaps with a preceeding smaller uncompleted store.
See the table of not supported store forwards in the Intel? 64 and IA-32 Architectures Optimization Reference Manual.
The penalty for blocked store forwarding is that the load must wait for the store to complete before it can be issued.]
(from ocperf.py list output on my Sandybridge CPU).
There are probably other ways to detect unaligned memory access. Maybe valgrind? I searched on valgrind detect unaligned and found this mailing list discussion from 13 years ago. Probably still not implemented.
The hand-optimized library functions do use unaligned accesses because it's the fastest way for them to get their job done. e.g. copying bytes 6 to 13 of a string to somewhere else can and should be done with just a single 8-byte load/store.
So yes, you would need special slow&safe versions of library functions.
If your code would have to execute extra instructions to avoid using unaligned loads, it's often not worth it. Esp. if the input is usually aligned, having a loop that does the first up-to-alignment-boundary elements before starting the main loop may just slow things down. In the aligned case, everything works optimally, with no overhead of checking alignment. In the unaligned case, things might work a few percent slower, but as long as the unaligned cases are rare, it's not worth avoiding them.
Esp. if it's not SSE code, since non-AVX legacy SSE can only fold loads into memory operands for ALU instructions when alignment is guaranteed.
The benefit of having good-enough hardware support for unaligned memory ops is that software can be faster in the aligned case. It can leave alignment-handling to hardware, instead of running extra instructions to handle pointers that are probably aligned. (Linus Torvalds had some interesting posts about this on the http://realworldtech.com/ forums, but they're not searchable so I can't find it.
You're not going to like it, but there is only one answer: don't link against the standard libraries. By changing that setting you have changed the ABI and the standard library doesn't like it. memcpy and friends are hand-written assembly so it's not a matter of compiler options to convince the compiler to do something else.
I would like to program in threading building blocks with tasks. But how does one do the debugging in practice?
In general the print method is a solid technique for debugging programs.
In my experience with MPI parallelization, the right way to do logging is that each thread print its debugging information in its own file (say "debug_irank" with irank the rank in the MPI_COMM_WORLD) so that the logical errors can be found.
How can something similar be achieved with TBB? It is not clear how to access the thread number in the thread pool as this is obviously something internal to tbb.
Alternatively, one could add an additional index specifying the rank when a task is generated but this makes the code rather complicated since the whole program has to take care of that.
First, get the program working with 1 thread. To do this, construct a task_scheduler_init as the first thing in main, like this:
#include "tbb/tbb.h"
int main() {
tbb::task_scheduler_init init(1);
...
}
Be sure to compile with the macro TBB_USE_DEBUG set to 1 so that TBB's checking will be enabled.
If the single-threaded version works, but the multi-threaded version does not, consider using Intel Inspector to spot race conditions. Be sure to compile with TBB_USE_THREADING_TOOLS so that Inspector gets enough information.
Otherwise, I usually first start by adding assertions, because the machine can check assertions much faster than I can read log messages. If I am really puzzled about why an assertion is failing, I use printfs and task ids (not thread ids). Easiest way to create a task id is to allocate one by post-incrementing a tbb::atomic<size_t> and storing the result in the task.
If I'm having a really bad day and the printfs are changing program behavior so that the error does not show up, I use "delayed printfs". Stuff the printf arguments in a circular buffer, and run printf on the records later after the failure is detected. Typically for the buffer, I use an array of structs containing the format string and a few word-size values, and make the array size a power of two. Then an atomic increment and mask suffices to allocate slots. E.g., something like this:
const size_t bufSize = 1024;
struct record {
const char* format;
void *arg0, *arg1;
};
tbb::atomic<size_t> head;
record buf[bufSize];
void recf(const char* fmt, void* a, void* b) {
record* r = &buf[head++ & bufSize-1];
r->format = fmt;
r->arg0 = a;
r->arg1 = b;
}
void recf(const char* fmt, int a, int b) {
record* r = &buf[head++ & bufSize-1];
r->format = fmt;
r->arg0 = (void*)a;
r->arg1 = (void*)b;
}
The two recf routines record the format and the values. The casting is somewhat abusive, but on most architectures you can print the record correctly in practice with printf(r->format, r->arg0, r->arg1) even if the the 2nd overload of recf created the record.
~
~
I try to write a very simple os to better understand the basic principles. And I need to implement user-space malloc. So at first I want to implement and test it on my linux-machine.
At first I have implemented the sbrk() function by the following way
void* sbrk( int increment ) {
return ( void* )syscall(__NR_brk, increment );
}
But this code does not work. Instead, when I use sbrk given by os, this works fine.
I have tryed to use another implementation of the sbrk()
static void *sbrk(signed increment)
{
size_t newbrk;
static size_t oldbrk = 0;
static size_t curbrk = 0;
if (oldbrk == 0)
curbrk = oldbrk = brk(0);
if (increment == 0)
return (void *) curbrk;
newbrk = curbrk + increment;
if (brk(newbrk) == curbrk)
return (void *) -1;
oldbrk = curbrk;
curbrk = newbrk;
return (void *) oldbrk;
}
sbrk invoked from this function
static Header *morecore(unsigned nu)
{
char *cp;
Header *up;
if (nu < NALLOC)
nu = NALLOC;
cp = sbrk(nu * sizeof(Header));
if (cp == (char *) -1)
return NULL;
up = (Header *) cp;
up->s.size = nu; // ***Segmentation fault
free((void *)(up + 1));
return freep;
}
This code also does not work, on the line (***) I get segmentation fault.
Where is a problem ?
Thanks All. I have solved my problem using new implementation of the sbrk. The given code works fine.
void* __sbrk__(intptr_t increment)
{
void *new, *old = (void *)syscall(__NR_brk, 0);
new = (void *)syscall(__NR_brk, ((uintptr_t)old) + increment);
return (((uintptr_t)new) == (((uintptr_t)old) + increment)) ? old :
(void *)-1;
}
The first sbrk should probably have a long increment. And you forgot to handle errors (and set errno)
The second sbrk function does not change the address space (as sbrk does). You could use mmap to change it (but using mmap instead of sbrk won't update the kernel's view of data segment end as sbrk does). You could use cat /proc/1234/maps to query the address space of process of pid 1234). or even read (e.g. with fopen&fgets) the /proc/self/maps from inside your program.
BTW, sbrk is obsolete (most malloc implementations use mmap), and by definition every system call (listed in syscalls(2)) is executed by the kernel (for sbrk the kernel maintains the "data segment" limit!). So you cannot avoid the kernel, and I don't even understand why you want to emulate any system call. Almost by definition, you cannot emulate syscalls since they are the only way to interact with the kernel from a user application. From the user application, every syscall is an atomic elementary operation (done by a single SYSENTER machine instruction with appropriate contents in machine registers).
You could use strace(1) to understand the actual syscalls done by your running program.
BTW, the GNU libc is a free software. You could look into its source code. musl-libc is a simpler libc and its code is more readable.
At last compile with gcc -Wall -Wextra -g and use the gdb debugger (you can even query the registers, if you wanted to). Perhaps read the x86/64-ABI specification and the Linux Assembly HowTo.
Recently when I look into how the thread-local storage is implemented in glibc, I found the following code, which implements the API pthread_key_create()
int
__pthread_key_create (key, destr)
pthread_key_t *key;
void (*destr) (void *);
{
/* Find a slot in __pthread_kyes which is unused. */
for (size_t cnt = 0; cnt < PTHREAD_KEYS_MAX; ++cnt)
{
uintptr_t seq = __pthread_keys[cnt].seq;
if (KEY_UNUSED (seq) && KEY_USABLE (seq)
/* We found an unused slot. Try to allocate it. */
&& ! atomic_compare_and_exchange_bool_acq (&__pthread_keys[cnt].seq,
seq + 1, seq))
{
/* Remember the destructor. */
__pthread_keys[cnt].destr = destr;
/* Return the key to the caller. */
*key = cnt;
/* The call succeeded. */
return 0;
}
}
return EAGAIN;
}
__pthread_keys is a global array accessed by all threads. I don't understand why the read of its member seq is not synchronized as in the following:
uintptr_t seq = __pthread_keys[cnt].seq;
although it is syncrhonized when modified later.
FYI, __pthread_keys is an array of type struct pthread_key_struct, which is defined as follows:
/* Thread-local data handling. */
struct pthread_key_struct
{
/* Sequence numbers. Even numbers indicated vacant entries. Note
that zero is even. We use uintptr_t to not require padding on
32- and 64-bit machines. On 64-bit machines it helps to avoid
wrapping, too. */
uintptr_t seq;
/* Destructor for the data. */
void (*destr) (void *);
};
Thanks in advance.
In this case, the loop can avoid an expensive lock acquisition. The atomic compare and swap operation done later (atomic_compare_and_exchange_bool_acq) will make sure only one thread can successfully increment the sequence value and return the key to the caller. Other threads reading the same value in the first step will keep looping since the CAS can only succeed for a single thread.
This works because the sequence value alternates between even (empty) and odd (occupied). Incrementing the value to odd prevents other threads from acquiring the slot.
Just reading the value is fewer cycles than the CAS instruction typically, so it makes sense to peek at the value, before doing the CAS.
There are many wait-free and lock-free algorithms that take advantage of the CAS instruction to achieve low-overhead synchronization.
typedef struct Radios_Frequencia {
char tipo_radio[3];
int qt_radio;
int frequencia;
}Radiof;
typedef struct Radio_Cidade {
char nome_cidade[30];
char nome_radio[30];
char dono_radio[3];
int numero_horas;
int audiencia;
Radiof *fre;
}R_cidade;
void Cadastrar_Radio(R_cidade**q){
printf("%d\n",i);
q[0]=(R_cidade*)malloc(sizeof(R_cidade));
printf("informa a frequencia da radio\n");
scanf("%d",&q[0]->fre->frequencia); //problem here
printf("%d\n",q[0]->fre->frequencia); // problem here
}
i want to know why this function void Cadastrar_Radio(R_cidade**q) does not print the data
You allocated storage for your primary structure but not the secondary one. Change
q[0]=(R_cidade*)malloc(sizeof(R_cidade));
to:
q[0]=(R_cidade*)malloc(sizeof(R_cidade));
q[0]->fre = malloc(sizeof(Radiof));
which will allocate both. Without that, there's a very good chance that fre will point off into never-never land (as in "you can never never tell what's going to happen since it's undefined behaviour).
You've allocated some storage, but you've not properly initialized any of it.
You won't get anything reliable to print until you put reliable values into the structures.
Additionally, as PaxDiablo also pointed out, you've allocated the space for the R_cidade structure, but not for the Radiof component of it. You're using scanf() to read a value into space that has not been allocated; that is not reliable - undefined behaviour at best, but most usually core dump time.
Note that although the two types are linked, the C compiler most certainly doesn't do any allocation of Radiof simply because R_cidade mentions it. It can't tell whether the pointer in R_cidade is meant to be to a single structure or the start of an array of structures, for example, so it cannot tell how much space to allocate. Besides, you might not want to initialize that structure every time - you might be happy to have left pointing nowhere (a null pointer) except in some special circumstances known only to you.
You should also verify that the memory allocation succeeded, or use a memory allocator that guarantees never to return a null or invalid pointer. Classically, that might be a cover function for the standard malloc() function:
#undef NDEBUG
#include <assert.h>
void *emalloc(size_t nbytes)
{
void *space = malloc(nbytes);
assert(space != 0);
return(space);
}
That's crude but effective. I use non-crashing error reporting routines of my own devising in place of the assert:
#include "stderr.h"
void *emalloc(size_t nbytes)
{
void *space = malloc(nbytes);
if (space == 0)
err_error("Out of memory\n");
return space;
}