Also, you might want to try calloc , which zeroes its memory. A TLB exists, it's a hit and the address is translated. But on access you get a trap, as you're trying to write to memory that's read-only. Supplies the number of high order address bits that must be zero in the base address of the section view.
The value of this argument must be less than or equal to the maximum number of zero bits and is only used when memory management determines where to allocate the view i. If ZeroBits is greater than 0 and less than 32, then it is the number of leading zero bits from bit Bits are also required to be zero. This retains compatibility with bit systems.
If ZeroBits is greater than 32, then it is considered as a mask and then number of leading zero are counted out in the mask. This then becomes the zero bits argument. The specified region should be created at the highest virtual address possible based on ZeroBits. Maximum memory which malloc can allocate. Asked 6 Months ago Answers: 5 Viewed 87 times.
I waited for about an hour and finally I had to force shut down my computer. Some questions: Does malloc allocate memory from HD also? What was the reason for above behaviour? The program will normally crash. The zero pointer will be used as a flag when we get to linked lists. The malloc command is used to allocate a block of memory. It is also possible to deallocate a block of memory when it is no longer needed.
When a block is deallocated, it can be reused by a subsequent malloc command, which allows the system to recycle memory. The command used to deallocate memory is called free , and it accepts a pointer as its parameter. The free command does two things:. The free statement simply returns a pointer to its original uninitialized state and makes the block available again on the heap. The following example shows how to use the heap.
It allocates an integer block, fills it, writes it, and disposes of it:. This code is really useful only for demonstrating the process of allocating, deallocating, and using a block in C.
The malloc line allocates a block of memory of the size specified -- in this case, sizeof int bytes 4 bytes. The sizeof command in C returns the size, in bytes, of any type.
The code could just as easily have said malloc 4 , since sizeof int equals 4 bytes on most machines. Using sizeof , however, makes the code much more portable and readable.
The malloc function returns a pointer to the allocated block. This pointer is generic. Using the pointer without typecasting generally produces a type warning from the compiler. The free statement in C returns a block to the heap for reuse. The second example illustrates the same functions as the previous example, but it uses a structure instead of an integer.
In C, the code looks like this:. The answer has to do with the precedence of operators in C. In C, the. The following two statements are exactly equivalent, but the second is easier to type:. Sign up for our Newsletter!
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You should define this function to look like free ; that is, like:. The value of caller is the return address found on the stack when the free function was called. You should define this function to look like memalign ; that is, like:. The value of caller is the return address found on the stack when the memalign function was called. You must make sure that the function you install as a hook for one of these functions does not call that function recursively without restoring the old value of the hook first!
Otherwise, your program will get stuck in an infinite recursion. Before calling the function recursively, one should make sure to restore all the hooks to their previous value. When coming back from the recursive call, all the hooks should be resaved since a hook might modify itself.
This is a weak variable, so it can be overridden in the application with a definition like the following:. An issue to look out for is the time at which the malloc hook functions can be safely installed. It installs a function that prints out information every time malloc or free is called.
We just assume here that realloc and memalign are not used in our program. The mcheck function see section Heap Consistency Checking works by installing such hooks. You can get information about dynamic memory allocation by calling the mallinfo function. Data Type: struct mallinfo This structure type is used to return information about the dynamic memory allocator.
It contains the following members:. Function: struct mallinfo mallinfo void This function returns information about the current dynamic memory usage in a structure of type struct mallinfo. A complicated task when programming with languages which do not use garbage collected dynamic memory allocation is to find memory leaks. Long running programs must assure that dynamically allocated objects are freed at the end of their lifetime.
If this does not happen the system runs out of memory, sooner or later. The malloc implementation in the GNU C library provides some simple means to detect such leaks and obtain some information to find the location.
To do this the application must be started in a special mode which is enabled by an environment variable. There are no speed penalties for the program if the debugging mode is not enabled.
This variable is supposed to contain a valid file name. The user must have write access. If the file already exists it is truncated. If the environment variable is not set or it does not name a valid file which can be opened for writing nothing is done. The behaviour of malloc etc. If the named file is successfully opened, mtrace installs special handlers for the functions malloc , realloc , and free see section Memory Allocation Hooks. From then on, all uses of these functions are traced and protocolled into the file.
There is now of course a speed penalty for all calls to the traced functions so tracing should not be enabled during normal use. This function is a GNU extension and generally not available on other systems. Function: void muntrace void The muntrace function can be called after mtrace was used to enable tracing the malloc calls. If no succesful call of mtrace was made muntrace does nothing.
Otherwise it deinstalls the handlers for malloc , realloc , and free and then closes the protocol file. No calls are protocolled anymore and the program runs again at full speed. Even though the tracing functionality does not influence the runtime behaviour of the program it is not a good idea to call mtrace in all programs. Just imagine that you debug a program using mtrace and all other programs used in the debugging session also trace their malloc calls. The output file would be the same for all programs and thus is unusable.
Therefore one should call mtrace only if compiled for debugging. A program could therefore start like this:. This is all what is needed if you want to trace the calls during the whole runtime of the program. Alternatively you can stop the tracing at any time with a call to muntrace. It is even possible to restart the tracing again with a new call to mtrace. But this can cause unreliable results since there may be calls of the functions which are not called.
Please note that not only the application uses the traced functions, also libraries including the C library itself use these functions. This last point is also why it is no good idea to call muntrace before the program terminated.
The libraries are informed about the termination of the program only after the program returns from main or calls exit and so cannot free the memory they use before this time. So the best thing one can do is to call mtrace as the very first function in the program and never call muntrace.
So the program traces almost all uses of the malloc functions except those calls which are executed by constructors of the program or used libraries. You know the situation. The program is prepared for debugging and in all debugging sessions it runs well.
But once it is started without debugging the error shows up. A typical example is a memory leak that becomes visible only when we turn off the debugging. If you foresee such situations you can still win. Simply use something equivalent to the following little program:. The output will of course not show the allocations which happened before the first signal but if there is a memory leak this will show up nevertheless. What this all means is not really important since the trace file is not meant to be read by a human.
Therefore no attention is given to readability. Instead there is a program which comes with the GNU C library which interprets the traces and outputs a summary in an user-friendly way.
The program is called mtrace it is in fact a Perl script and it takes one or two arguments. In any case the name of the file with the trace output must be specified.
If an optional argument precedes the name of the trace file this must be the name of the program which generated the trace. The message printed by mtrace shows there are no problems with the code, all allocated memory was freed afterwards.
If we call mtrace on the example trace given above we would get a different outout:. We have called mtrace with only one argument and so the script has no chance to find out what is meant with the addresses given in the trace.
We can do better:. Suddenly the output makes much more sense and the user can see immediately where the function calls causing the trouble can be found. Interpreting this output is not complicated.
There are at most two different situations being detected. First, free was called for pointers which were never returned by one of the allocation functions. This is usually a very bad problem and what this looks like is shown in the first three lines of the output.
Situations like this are quite rare and if they appear they show up very drastically: the program normally crashes. The other situation which is much harder to detect are memory leaks.
Whether this is a real problem remains to be investigated. An obstack is a pool of memory containing a stack of objects. You can create any number of separate obstacks, and then allocate objects in specified obstacks. Within each obstack, the last object allocated must always be the first one freed, but distinct obstacks are independent of each other. Aside from this one constraint of order of freeing, obstacks are totally general: an obstack can contain any number of objects of any size.
They are implemented with macros, so allocation is usually very fast as long as the objects are usually small. And the only space overhead per object is the padding needed to start each object on a suitable boundary. Data Type: struct obstack An obstack is represented by a data structure of type struct obstack. This structure has a small fixed size; it records the status of the obstack and how to find the space in which objects are allocated.
It does not contain any of the objects themselves. You should not try to access the contents of the structure directly; use only the functions described in this chapter. You can declare variables of type struct obstack and use them as obstacks, or you can allocate obstacks dynamically like any other kind of object.
Dynamic allocation of obstacks allows your program to have a variable number of different stacks. You can even allocate an obstack structure in another obstack, but this is rarely useful.
All the functions that work with obstacks require you to specify which obstack to use. In the following, we often say "an obstack" when strictly speaking the object at hand is such a pointer. The objects in the obstack are packed into large blocks called chunks. The struct obstack structure points to a chain of the chunks currently in use.
The obstack library obtains a new chunk whenever you allocate an object that won't fit in the previous chunk. Since the obstack library manages chunks automatically, you don't need to pay much attention to them, but you do need to supply a function which the obstack library should use to get a chunk. Usually you supply a function which uses malloc directly or indirectly. You must also supply a function to free a chunk. These matters are described in the following section.
These macros should appear before any use of obstacks in the source file. Usually these are defined to use malloc via the intermediary xmalloc see section Unconstrained Allocation. This is done with the following pair of macro definitions:. Though the memory you get using obstacks really comes from malloc , using obstacks is faster because malloc is called less often, for larger blocks of memory. See section Obstack Chunks , for full details. Here are two examples of how to allocate the space for an obstack and initialize it.
First, an obstack that is a static variable:. The default action is to print a message and abort. You should supply a function that either calls exit see section Program Termination or longjmp see section Non-Local Exits and doesn't return.
Here obstack-ptr specifies which obstack to allocate the block in; it is the address of the struct obstack object which represents the obstack. Each obstack function or macro requires you to specify an obstack-ptr as the first argument. This extra byte is not counted in the argument size. Here is an example of its use:.
Contrast this with the previous example of savestring using malloc see section Basic Memory Allocation. Since the obstack is a stack of objects, freeing one object automatically frees all other objects allocated more recently in the same obstack. Otherwise, object must be the address of an object allocated in the obstack. Then object is freed, along with everything allocated in obstack since object.
Note that if object is a null pointer, the result is an uninitialized obstack. Recall that the objects in an obstack are grouped into chunks. When all the objects in a chunk become free, the obstack library automatically frees the chunk see section Preparing for Using Obstacks. Then other obstacks, or non-obstack allocation, can reuse the space of the chunk.
The interfaces for using obstacks may be defined either as functions or as macros, depending on the compiler. If you are using an old-fashioned non-ISO C compiler, all the obstack "functions" are actually defined only as macros. You can call these macros like functions, but you cannot use them in any other way for example, you cannot take their address. Calling the macros requires a special precaution: namely, the first operand the obstack pointer may not contain any side effects, because it may be computed more than once.
For example, if you write this:. In ISO C, each function has both a macro definition and a function definition. The function definition is used if you take the address of the function without calling it. An ordinary call uses the macro definition by default, but you can request the function definition instead by writing the function name in parentheses, as shown here:. This is the same situation that exists in ISO C for the standard library functions. See section Macro Definitions of Functions.
Warning: When you do use the macros, you must observe the precaution of avoiding side effects in the first operand, even in ISO C. If you use the GNU C compiler, this precaution is not necessary, because various language extensions in GNU C permit defining the macros so as to compute each argument only once.
Because memory in obstack chunks is used sequentially, it is possible to build up an object step by step, adding one or more bytes at a time to the end of the object. With this technique, you do not need to know how much data you will put in the object until you come to the end of it. We call this the technique of growing objects. The special functions for adding data to the growing object are described in this section.
You don't need to do anything special when you start to grow an object. Using one of the functions to add data to the object automatically starts it.
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