Premise

Each thread in a process has its own stack, which is a memory area that stores local variables, function parameters, and control data. The stack is characterized by its LIFO (last-in, first-out) access pattern and typically has a limited size. When a thread ends, its stack is reclaimed. The data segment is a memory area used to store global variables and static variables that are allocated at compile time and exist for the lifetime of the process. It is shared across threads. The heap is a dynamically allocated memory that is managed at runtime through functions like malloc in C. Unlike the stack, the heap's lifetime is controlled manually by the programmer via allocation and deallocation. When a pointer to a memory area in the stack is passed to a thread, the risk of losing the reference arises because the stack space is local to the calling thread. If the calling thread terminates or changes the stack content before the newly created thread accesses the memory location, the reference becomes invalid or points to garbage data. The data segment, however, remains consistent throughout the life of the process. Pointers to global or static variables (residing in the data segment) retain their validity across threads since this memory is not reclaimed until the process terminates. For memory allocated on the heap, the duration of the memory's validity is controlled explicitly by the programmer through allocation and deallocation. Since heap memory is not automatically reclaimed upon thread exit (like stack memory), a pointer to a heap memory area remains valid across threads until it is explicitly freed.

From a philosophical standpoint, reflecting on the Marxist dialectic, the distinctions between stack, data segment, and heap memory can be seen as a metaphor for different societal structures and their permanence or transience. The stack, with its temporary and volatile nature, mirrors the ephemeral conditions of individual labor and existence within a capitalist framework, where workers' stability and security are often precarious. The data segment, by contrast, represents the enduring structures of society —laws, institutions, and norms— that persist beyond the lifespans of individual actors, much like global and static variables in a program. The heap represents a domain of managed resources, akin to the means of production that must be consciously allocated and freed. Just as the heap allows for dynamic allocation and deallocation of memory, Marxist theory emphasizes the importance of conscious control and distribution of resources within society. This dynamic allocation can be seen as a representation of the potential for change and redistribution in society, where resources (or memory) can be managed to meet the needs of processes (or people) dynamically.

Explanation

When using pthread_create to create a new thread in a C program, the function requires a pointer to the routine that the new thread will execute and an optional argument that can be passed to this routine. An unpredictable behavior, where references to memory areas on the stack become invalid, versus the stability of references to memory allocated on the heap or in the data segment, can be explained by understanding the memory model of a C program and the nature of threads. Each thread in a C program has its own stack, which is a region of memory that stores local variables, function parameters, and return addresses. The stack is limited in size and is intended for temporary storage that is only valid within the scope of a function call. When a function exits, its stack frame (the section of the stack containing its variables) is considered no longer valid, and the memory can be reused by subsequent function calls. When you pass a pointer to a stack-allocated variable to a thread, you're passing a reference to a memory area that is temporary and scoped to a particular function's execution. If the function that allocated the variable on the stack returns before the newly created thread has a chance to use the pointer, the thread may attempt to access memory that is no longer valid, leading to undefined behavior such as segmentation faults. On the other hand, variables allocated on the heap (using malloc, calloc, or similar functions) or global/static variables stored in the data segment of a program have a lifetime that extends beyond the scope of individual function calls. Heap memory remains valid until it is explicitly freed, and global/static variables remain valid for the duration of the program. This means that pointers to these types of memory are safe to pass to threads, as the memory they reference will not be reclaimed or reused by the system until the program explicitly does so or terminates. In the context of a typical process, memory is divided into three primary sections: the text (code) segment, the initialized data segment, and the uninitialized data (stack) segment. The text segment contains the executable code of the program, while the initialized data segment stores global and static variables with predefined values. Conversely, the stack segment serves as a temporary storage area for local variables, function parameters, and return addresses during program execution. When a function is called, a new frame is pushed onto the stack, allocating space for local variables, and preserving the state of the calling function. This frame remains active until the function returns, at which point it is removed from the stack, and the previously stored state is restored. If a pointer to a variable residing in the stack is passed as a parameter to another function, the value of the pointer itself is copied, whereas the actual memory location it refers to remains unaffected. However, since the stack is a volatile region of memory, once the function returns, the allocated space is deallocated, rendering the pointer invalid. On the other hand, memory allocated on the heap via dynamic memory allocation functions such as malloc or calloc persists throughout the lifetime of the process unless explicitly freed. Since heap memory is managed independently of the call stack, passing a pointer to a heap-allocated memory area between functions or threads maintains the validity of the pointer, allowing access to the same memory location irrespective of the invoking context. Similarly, the data segment, containing initialized global and static variables, retains its contents across function calls and thread creation due to its persistent nature.