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Tcl_Alloc(3tcl) Tcl Library Procedures Tcl_Alloc(3tcl)


Tcl_Alloc, Tcl_Free, Tcl_Realloc, Tcl_AttemptAlloc, Tcl_AttemptRealloc, Tcl_GetMemoryInfo - allocate or free heap memory


#include <tcl.h>

char *


void *
Tcl_Realloc(ptr, size)

void *

void *
Tcl_AttemptRealloc(ptr, size)



unsigned int size (in)
Size in bytes of the memory block to allocate.
char *ptr (in)
Pointer to memory block to free or realloc.
Tcl_DString *dsPtr (in)
Initialized DString pointer.


These procedures provide a platform and compiler independent interface for memory allocation. Programs that need to transfer ownership of memory blocks between Tcl and other modules should use these routines rather than the native malloc() and free() routines provided by the C run-time library.

Tcl_Alloc returns a pointer to a block of at least size bytes suitably aligned for any use.

Tcl_Free makes the space referred to by ptr available for further allocation.

Tcl_Realloc changes the size of the block pointed to by ptr to size bytes and returns a pointer to the new block. The contents will be unchanged up to the lesser of the new and old sizes. The returned location may be different from ptr. If ptr is NULL, this is equivalent to calling Tcl_Alloc with just the size argument.

Tcl_AttemptAlloc and Tcl_AttemptRealloc are identical in function to Tcl_Alloc and Tcl_Realloc, except that Tcl_AttemptAlloc and Tcl_AttemptRealloc will not cause the Tcl interpreter to panic if the memory allocation fails. If the allocation fails, these functions will return NULL. Note that on some platforms, but not all, attempting to allocate a zero-sized block of memory will also cause these functions to return NULL.

When a module or Tcl itself is compiled with TCL_MEM_DEBUG defined, the procedures Tcl_Alloc, Tcl_Free, Tcl_Realloc, Tcl_AttemptAlloc, and Tcl_AttempRealloc are implemented as macros, redefined to be special debugging versions of these procedures.

Tcl_GetMemoryInfo appends a list-of-lists of memory stats to the provided DString. This function cannot be used in stub-enabled extensions, and it is only available if Tcl is compiled with the threaded memory allocator When used in stub-enabled embedders, the stubs table must be first initialized using one of Tcl_InitSubsystems, Tcl_SetPanicProc, Tcl_FindExecutable or TclZipfs_AppHook.


alloc, allocation, free, malloc, memory, realloc, TCL_MEM_DEBUG

9.0 Tcl