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A segment is a contiguous range of virtual addresses. Three important segments are:. This section covers how ordinary programs manage storage for their data, including the famous malloc function and some fancier facilities special to the GNU C Library and GNU Compiler. In GNU C, the size of the automatic storage can be an expression that varies. In other C implementations, it must be a constant. A third important kind of memory allocation, dynamic allocation , is not supported by C variables but is available via GNU C Library functions.
Dynamic memory allocation is a technique in which programs determine as they are running where to store some information. You need dynamic allocation when the amount of memory you need, or how long you continue to need it, depends on factors that are not known before the program runs. For example, you may need a block to store a line read from an input file; since there is no limit to how long a line can be, you must allocate the memory dynamically and make it dynamically larger as you read more of the line.
When you use dynamic allocation, the allocation of a block of memory is an action that the program requests explicitly. You call a function or macro when you want to allocate space, and specify the size with an argument. If you want to free the space, you do so by calling another function or macro. You can do these things whenever you want, as often as you want.
The only way to get dynamically allocated memory is via a system call which is generally via a GNU C Library function call , and the only way to refer to dynamically allocated space is through a pointer. Because it is less convenient, and because the actual process of dynamic allocation requires more computation time, programmers generally use dynamic allocation only when neither static nor automatic allocation will serve.
For example, if you want to allocate dynamically some space to hold a struct foobar , you cannot declare a variable of type struct foobar whose contents are the dynamically allocated space. The malloc implementation in the GNU C Library is derived from ptmalloc pthreads malloc , which in turn is derived from dlmalloc Doug Lea malloc. This malloc may allocate memory in two different ways depending on their size and certain parameters that may be controlled by users.
The most common way is to allocate portions of memory called chunks from a large contiguous area of memory and manage these areas to optimize their use and reduce wastage in the form of unusable chunks.
Traditionally the system heap was set up to be the one large memory area but the GNU C Library malloc implementation maintains multiple such areas to optimize their use in multi-threaded applications. Each such area is internally referred to as an arena. As opposed to other versions, the malloc in the GNU C Library does not round up chunk sizes to powers of two, neither for large nor for small sizes.
Neighboring chunks can be coalesced on a free no matter what their size is. This makes the implementation suitable for all kinds of allocation patterns without generally incurring high memory waste through fragmentation. The presence of multiple arenas allows multiple threads to allocate memory simultaneously in separate arenas, thus improving performance. The other way of memory allocation is for very large blocks, i.
This has the great advantage that these chunks are returned to the system immediately when they are freed. The size threshold for mmap to be used is dynamic and gets adjusted according to allocation patterns of the program.
It is possible to use your own custom malloc instead of the built-in allocator provided by the GNU C Library. See Replacing malloc. The most general dynamic allocation facility is malloc.
It allows you to allocate blocks of memory of any size at any time, make them bigger or smaller at any time, and free the blocks individually at any time or never. To allocate a block of memory, call malloc.
The prototype for this function is in stdlib. This function returns a pointer to a newly allocated block size bytes long, or a null pointer setting errno if the block could not be allocated. The contents of the block are undefined; you must initialize it yourself or use calloc instead; see Allocating Cleared Space.
Normally you would convert the value to a pointer to the kind of object that you want to store in the block. Here we show an example of doing so, and of initializing the space with zeros using the library function memset see Copying Strings and Arrays :.
However, a cast is necessary if the type is needed but not specified by context. Remember that when allocating space for a string, the argument to malloc must be one plus the length of the string. For example:. See Representation of Strings , for more information about this. If no more space is available, malloc returns a null pointer. You should check the value of every call to malloc. It is useful to write a subroutine that calls malloc and reports an error if the value is a null pointer, returning only if the value is nonzero.
This function is conventionally called xmalloc. Here it is:. Here is a real example of using malloc by way of xmalloc. The function savestring will copy a sequence of characters into a newly allocated null-terminated string:.
The block that malloc gives you is guaranteed to be aligned so that it can hold any type of data. On GNU systems, the address is always a multiple of eight on bit systems, and a multiple of 16 on bit systems. Note that the memory located after the end of the block is likely to be in use for something else; perhaps a block already allocated by another call to malloc.
If you attempt to treat the block as longer than you asked for it to be, you are liable to destroy the data that malloc uses to keep track of its blocks, or you may destroy the contents of another block. If you have already allocated a block and discover you want it to be bigger, use realloc see Changing Block Size. When you no longer need a block that you got with malloc , use the function free to make the block available to be allocated again.
Freeing a block alters the contents of the block. Do not expect to find any data such as a pointer to the next block in a chain of blocks in the block after freeing it.
Copy whatever you need out of the block before freeing it! Here is an example of the proper way to free all the blocks in a chain, and the strings that they point to:. Occasionally, free can actually return memory to the operating system and make the process smaller.
Usually, all it can do is allow a later call to malloc to reuse the space. In the meantime, the space remains in your program as part of a free-list used internally by malloc. The free function preserves the value of errno , so that cleanup code need not worry about saving and restoring errno around a call to free. Often you do not know for certain how big a block you will ultimately need at the time you must begin to use the block.
For example, the block might be a buffer that you use to hold a line being read from a file; no matter how long you make the buffer initially, you may encounter a line that is longer. You can make the block longer by calling realloc or reallocarray.
These functions are declared in stdlib. The realloc function changes the size of the block whose address is ptr to be newsize. Since the space after the end of the block may be in use, realloc may find it necessary to copy the block to a new address where more free space is available.
The value of realloc is the new address of the block. If the block needs to be moved, realloc copies the old contents. Otherwise, if newsize is zero realloc frees the block and returns NULL. Otherwise, if realloc cannot reallocate the requested size it returns NULL and sets errno ; the original block is left undisturbed. The reallocarray function changes the size of the block whose address is ptr to be long enough to contain a vector of nmemb elements, each of size size.
Portability Note: This function is not part of any standard. It was first introduced in OpenBSD 5. Like malloc , realloc and reallocarray may return a null pointer if no memory space is available to make the block bigger. When this happens, the original block is untouched; it has not been modified or relocated.
In most cases it makes no difference what happens to the original block when realloc fails, because the application program cannot continue when it is out of memory, and the only thing to do is to give a fatal error message. Often it is convenient to write and use subroutines, conventionally called xrealloc and xreallocarray , that take care of the error message as xmalloc does for malloc :.
You can also use realloc or reallocarray to make a block smaller. The reason you would do this is to avoid tying up a lot of memory space when only a little is needed. In several allocation implementations, making a block smaller sometimes necessitates copying it, so it can fail if no other space is available. The function calloc allocates memory and clears it to zero.
It is declared in stdlib. This function allocates a block long enough to contain a vector of count elements, each of size eltsize. Its contents are cleared to zero before calloc returns. But in general, it is not guaranteed that calloc calls reallocarray and memset internally.
For example, if the calloc implementation knows for other reasons that the new memory block is zero, it need not zero out the block again with memset. Also, if an application provides its own reallocarray outside the C library, calloc might not use that redefinition. The address of a block returned by malloc or realloc in GNU systems is always a multiple of eight or sixteen on bit systems. The alignment must be a power of two and size must be a multiple of alignment. The memalign function allocates a block of size bytes whose address is a multiple of boundary.
The boundary must be a power of two! The function memalign works by allocating a somewhat larger block, and then returning an address within the block that is on the specified boundary.
The memalign function returns a null pointer on error and sets errno to one of the following values:. Otherwise the function returns an error value indicating the problem.
The possible error values returned are:. Using valloc is like using memalign and passing the page size as the value of the first argument.
It is implemented like this:. Query Memory Parameters for more information about the memory subsystem. You can adjust some parameters for dynamic memory allocation with the mallopt function. When calling mallopt , the param argument specifies the parameter to be set, and value the new value to be set. Possible choices for param , as defined in malloc.
The maximum number of chunks to allocate with mmap. Setting this to zero disables all use of mmap. All chunks larger than this value are allocated outside the normal heap, using the mmap system call. This way it is guaranteed that the memory for these chunks can be returned to the system on free. Note that requests smaller than this threshold might still be allocated via mmap. If this parameter is not set, the default value is set as KiB and the threshold is adjusted dynamically to suit the allocation patterns of the program.
If the parameter is set, the dynamic adjustment is disabled and the value is set statically to the input value. If non-zero, memory blocks are filled with values depending on some low order bits of this parameter when they are allocated except when allocated by calloc and freed. This can be used to debug the use of uninitialized or freed heap memory. Note that this option does not guarantee that the freed block will have any specific values. It only guarantees that the content the block had before it was freed will be overwritten.
This parameter determines the amount of extra memory to obtain from the system when an arena needs to be extended. It also specifies the number of bytes to retain when shrinking an arena. This provides the necessary hysteresis in heap size such that excessive amounts of system calls can be avoided. This is the minimum size in bytes of the top-most, releasable chunk that will trigger a system call in order to return memory to the system.
If the parameter is set, the dynamic adjustment is disabled and the value is set statically to the provided input. This parameter specifies the number of arenas that can be created before the test on the limit to the number of arenas is conducted. The default value of this tunable is 0 , meaning that the limit on the number of arenas is determined by the number of CPU cores online.
For bit systems the limit is twice the number of cores online and on bit systems, it is eight times the number of cores online. This function is a GNU extension, declared in mcheck. Calling mcheck tells malloc to perform occasional consistency checks. These will catch things such as writing past the end of a block that was allocated with malloc. The abortfn argument is the function to call when an inconsistency is found. If you supply a null pointer, then mcheck uses a default function which prints a message and calls abort see Aborting a Program.
The function you supply is called with one argument, which says what sort of inconsistency was detected; its type is described below. It is too late to begin allocation checking once you have allocated anything with malloc. So mcheck does nothing in that case. The function returns -1 if you call it too late, and 0 otherwise when it is successful. Alternatively you might use a debugger to insert a call to mcheck whenever the program is started, for example these gdb commands will automatically call mcheck whenever the program starts:.
This will however only work if no initialization function of any object involved calls any of the malloc functions since mcheck must be called before the first such function. The mprobe function lets you explicitly check for inconsistencies in a particular allocated block. You must have already called mcheck at the beginning of the program, to do its occasional checks; calling mprobe requests an additional consistency check to be done at the time of the call.
The argument pointer must be a pointer returned by malloc or realloc. The values are described below. This enumerated type describes what kind of inconsistency was detected in an allocated block, if any. Here are the possible values:. No consistency checking can be done. The data immediately before the block was modified. This commonly happens when an array index or pointer is decremented too far. The data immediately after the block was modified. This commonly happens when an array index or pointer is incremented too far.
Not all such errors can be protected against, however, and memory leaks can result. You can get information about dynamic memory allocation by calling the mallinfo2 function. This function and its associated data type are declared in malloc. This structure type is used to return information about the dynamic memory allocator. It contains the following members:. This is the total size of memory allocated with sbrk by malloc , in bytes. This is the number of chunks not in use.
This is the size of the top-most releasable chunk that normally borders the end of the heap i. This function returns information about the current dynamic memory usage in a structure of type struct mallinfo2. Allocate a block of size bytes. See Basic Allocation. Free a block previously allocated by malloc. See Freeing after Malloc.
Make a block previously allocated by malloc larger or smaller, possibly by copying it to a new location. See Changing Block Size. See Allocating Cleared Space. Allocate a block of size bytes, starting on a page boundary.
See Aligned Memory Blocks. Allocate a block of size bytes, starting on an address that is a multiple of alignment. Allocate a block of size bytes, starting on an address that is a multiple of boundary. Adjust a tunable parameter. See Malloc Tunable Parameters. Tell malloc to perform occasional consistency checks on dynamically allocated memory, and to call abortfn when an inconsistency is found.
See Heap Consistency Checking. Return information about the current dynamic memory usage. See Statistics of Malloc.
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 ensure 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. The mtrace function provides a way to trace memory allocation events in the program that calls it. 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 behavior of malloc etc. If the named file is successfully opened, mtrace installs special handlers for the functions malloc , realloc , and free. 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. The prototype can be found in mcheck. The muntrace function can be called after mtrace was used to enable tracing the malloc calls.
If no successful 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 behavior 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 that 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 not a good idea to call muntrace before the program terminates. 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. In this case the program tst-mtrace was run and it produced a trace file log. The message printed by mtrace shows there are no problems with the code, all allocated memory was freed afterwards.
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. The GNU C Library supports replacing the built-in malloc implementation with a different allocator with the same interface. For static linking, the malloc replacement library must be linked in before linking against libc. Note: Failure to provide a complete set of replacement functions that is, all the functions used by the application, the GNU C Library, and other linked-in libraries can lead to static linking failures, and, at run time, to heap corruption and application crashes.
Replacement functions should implement the behavior documented for their counterparts in the GNU C Library; for example, the replacement free should also preserve errno. The minimum set of functions which has to be provided by a custom malloc is given in the table below. The malloc implementation in the GNU C Library provides additional functionality not used by the library itself, but which is often used by other system libraries and applications.
A general-purpose replacement malloc implementation should provide definitions of these functions, too. Their names are listed in the following table.
Follow Post Reply. Banfa 9, Expert Mod 8TB. If you call free d when d is not pointed at allocated memory you will get an exception and you program will crash. It is not safe to free d without a check to see if it is actuall allocated so you code should be in the general case Expand Select Wrap Line Numbers.
Post Reply. Pointer being set to null not by me?? How come?? Set a pointer to null when deleting?
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