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Not portably in ISO C. A function pointer lets a C program call a function, but casting the address of arbitrary memory to a function pointer does not turn those bytes into a valid C function. To run generated machine code, a program also needs operating-system permission to execute the memory, instructions for the target processor, and code that follows the target platform’s ABI and calling convention. The details differ between Windows and POSIX systems.
What a function pointer does—and does not do
A function pointer holds a value used to call a function through its declared function type. For example, int (*operation)(int) describes a pointer to a function that accepts an int and returns an int. A normal function can be called through a compatible pointer of that type.
That type describes how a call is made; it does not describe or validate the bytes at some address. Data copied into allocated memory does not become a C function merely because the program casts the address to a function-pointer type. WG14 committee material, including C draft N843 and paper N2222, states that calling through an incompatible function type has undefined behavior. Arbitrary memory bytes have no guarantee of being a function with the signature used for the call.
This is why a generic expression such as ((int (*)(void))p)() is not a portable ISO C recipe for executing arbitrary memory. An implementation may offer extensions or platform conventions that make a particular conversion and call usable, but that behavior is implementation-specific.
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What must be true for generated machine code to run
Several independent conditions have to line up. Satisfying only one—for example, changing a page’s protection to executable—does not make the rest automatic.
- The bytes must be instructions for the target. Machine-code encoding is processor- and architecture-specific. Bytes that are instructions on one target may be invalid or mean something else on another.
- The code must obey the ABI. It must use the expected calling convention, register and stack rules, and any other platform requirements for a function with the signature used by the call.
- The operating system must permit execution. The memory mapping must have suitable execute permission. Process configuration or security policy can reject a requested protection.
- The call must use a compatible function type and representation. C does not guarantee that object pointers and function pointers have interchangeable representations or that an arbitrary address can be converted portably into a callable function pointer.
- Instruction-cache behavior may matter. On Windows, Microsoft puts responsibility on the caller to ensure instruction-cache coherency after generated code is placed in executable memory.
Compiler and architecture features can add further constraints. Clang documents pointer authentication on supported targets: the implementation signs and authenticates function pointers across indirect calls. In such environments, treating a function pointer as an ordinary numeric address or assuming the same representation across systems is unsafe.
How do I execute code from memory in C?
There is no single portable C function for making allocated bytes executable. The broad platform-specific pattern is to obtain memory, put target-specific machine code there, arrange the appropriate memory protection, satisfy any instruction-cache requirements, and call it only through a function-pointer form supported by that implementation and ABI.
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- Choose a specific target. Establish the operating system, processor architecture, compiler, ABI, and function signature. Code bytes and calling conventions cannot safely be guessed from a generic C example.
- Use the operating system’s memory APIs. Windows documents
VirtualAllocandVirtualProtectfor dynamically generated code. POSIXmmapaccepts protection flags for a mapping; GNU systems also documentmprotectfor changing protection. - Write the intended instructions and request executable permission. A protection change only changes access rights. It does not check that the bytes are valid instructions or that they implement the declared function.
- Meet platform execution requirements. On Windows, call
FlushInstructionCacheafter setting code in an executable region, as Microsoft’s API documentation directs. - Use only a supported call mechanism. The conversion from the memory address to a function pointer, and the indirect call through it, depend on the compiler and platform. ISO C alone does not make this a portable operation.
Keep writable and executable permissions separate where the platform supports a write-then-execute transition. This is a security-conscious design recommendation, not a promise that every operating system or process configuration permits the same transition. Avoid using a general-purpose heap allocation as though it were a dedicated code region.
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Microsoft’s documented approach for dynamically generated code is to allocate memory with VirtualAlloc, write the code, then use VirtualProtect to grant execute access. Microsoft states: “To execute dynamically generated code, use VirtualAlloc to allocate memory and the VirtualProtect function to grant PAGE_EXECUTE access.” The caller is also responsible for instruction-cache coherency through an appropriate FlushInstructionCache call; Microsoft warns that otherwise execution may produce unpredictable results.
VirtualProtect changes the protection of every page touched by the supplied address-and-size range. The pages must be committed and belong to the same reserved region. The function returns nonzero on success and zero on failure; GetLastError supplies extended error information.
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Do not casually apply this approach to memory returned by HeapAlloc, GlobalAlloc, or LocalAlloc. Multiple heap objects can share a page, while the heap manager assumes at least read/write access. Changing protection at page granularity can therefore affect more than the particular object a program meant to change.
POSIX and GNU systems: mmap, mprotect, and page boundaries
POSIX mmap creates a mapping in the process address space, with its prot flags specifying whether read, write, or execute access is allowed. GNU libc documents PROT_EXEC and changing protections with mprotect.
For GNU mprotect, the starting address must be aligned to a page boundary; the specified length is rounded up to page units. GNU libc cautions that, for portable use, mprotect should be limited to regions obtained with mmap or mmap64, even though it can work on many other process-memory regions on some systems.
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A requested protection can fail because of system security policy. GNU libc gives write-and-execute permission as an example of a mapping the policy might disallow, and describes the error this way: “The system security policy does not allow a mapping with the specified flags.” Exact behavior depends on the operating system and process environment; a successful mapping request should not be assumed in advance.
How the platform approaches compare
| Issue | Windows | POSIX / GNU systems |
|---|---|---|
| Documented memory approach | VirtualAlloc to allocate; VirtualProtect to grant execute access (Microsoft Learn Windows API documentation). |
mmap protection flags; GNU libc documents mprotect for changing protection (POSIX.1-2024 and GNU C Library 2.39 manual). |
| Page and region constraints | VirtualProtect affects all pages touched; those pages must be committed and within the same reserved region (Microsoft Learn). |
GNU mprotect requires a page-aligned start address; its length is rounded up to page units. |
| Write-and-execute policy | Protection is set through Windows memory APIs; support for a particular transition depends on the requested protection and environment. | GNU libc documents that policy can reject requested flags; write-plus-execute is one example that may be disallowed. |
| Instruction-cache synchronization | Microsoft assigns the caller responsibility for an appropriate FlushInstructionCache call after code is set in an executable region. |
Not stated in the cited POSIX mmap or GNU libc protection material; requirements depend on the target architecture and implementation. |
| Function-pointer conversion and representation | Not established as portable ISO C by the Windows memory APIs; compiler and ABI behavior governs the call. | Not established as portable ISO C by mmap or mprotect; compiler and ABI behavior governs the call. |
These APIs address memory allocation and protection, not the validity of the generated instructions or the portability of the function-pointer conversion. They are platform facilities, not interchangeable ISO C solutions.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Can I cast memory to a function pointer?
A cast can express what a particular implementation is being asked to do, but it does not guarantee that the result is valid or callable under ISO C. The WG14 materials distinguish function-pointer conversions from object-pointer conversions and identify calls through incompatible function types as undefined behavior. Whether a conversion from an address supplied by an operating-system API can be called safely is a matter for that compiler, platform, and ABI.
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There may also be implementation-specific pointer representation details. For example, Clang’s pointer-authentication documentation describes targets where function pointers carry authentication information used during indirect calls. Code that assumes every function pointer is simply an unadorned address can fail on such a target.
Why does calling memory as a function segfault?
A segmentation fault is one possible symptom, not a diagnosis. The call can fail for different reasons, including:
- The page is not executable, or the requested protection was refused.
- The bytes are not valid instructions for the current architecture, or execution enters them at the wrong offset.
- The instructions do not follow the ABI or calling convention expected by the call site.
- The function-pointer type does not match the actual callable interface.
- The platform requires instruction-cache synchronization that was not performed.
- The implementation uses pointer-representation or authentication rules that the program bypasses.
Changing memory protection can address only an access-permission problem. It cannot repair invalid machine code, an incompatible call, or an ABI mismatch.
When this technique is—and is not—a good fit
Executing generated instructions is a low-level, platform-specific technique. It may be appropriate when a program has a concrete need for runtime code generation and can control its target architecture, compiler behavior, memory policy, and ABI requirements. It is a poor fit for portable application code that merely wants to call a function: ordinary compiled functions and compatible function pointers are the portable C mechanism for that job.
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The relevant primary references are Microsoft Learn’s VirtualAlloc and VirtualProtect documentation, the POSIX.1-2024 mmap specification, the GNU C Library 2.39 manual’s memory protection documentation, WG14 committee material (including draft N843 and paper N2222), and Clang’s pointer-authentication documentation. Their guarantees are scoped to the respective language, operating system, library, or compiler; none makes arbitrary memory bytes portable C functions.
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