Yes. For a nonzero allocation request, malloc can return NULL when it cannot provide the requested space. Check the returned pointer before using it. POSIX also requires errno to be set on failure; ISO C does not, so portable C code should rely on the pointer rather than errno.
What does a NULL return mean?
For a nonzero request, a null pointer means the allocation failed: the program must not use the result as allocated storage. POSIX states, “If the space cannot be allocated, a null pointer shall be returned.” See the POSIX Programmer’s Manual for malloc.
Failure is not limited to a machine having no free physical RAM. On Linux, ENOMEM can also reflect process address-space or data limits, such as RLIMIT_AS or RLIMIT_DATA, or the system mapping-count limit, /proc/sys/vm/max_map_count. The causes and diagnostics depend on the operating system and its allocation policy.
Should you check whether malloc returned NULL?
Yes. Save the return value, check it, and only then access the allocated memory. The GNU C Library’s allocation examples likewise check for a null pointer and take an explicit failure path.
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#include <stdint.h>
#include <stdlib.h>
int *items = malloc(count * sizeof *items);
if (items == NULL) {
/* Report failure, unwind, reduce work, or exit as appropriate. */
return ERROR_CODE;
}This example assumes the requested byte count is valid. In production code, check that count * sizeof *items cannot overflow before calling malloc; an overflow could make the requested allocation smaller than intended. What to do after failure depends on the program: it might report an error, release resources already acquired, try a smaller operation, or terminate. There is no single recovery action that is safe for every program.
Does malloc set errno?
It depends on the contract your program targets. POSIX requires allocation functions to set errno on failure, and its malloc documentation names ENOMEM when insufficient storage is available. ISO C does not require errno to be set, so portable C should test the returned pointer first and must not use errno as its failure test. The Linux malloc(3) manual explains this distinction.
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On a POSIX target, after a failed nonzero request, errno can help diagnose the failure. Do not assume that the same diagnostic behavior is guaranteed by every C implementation.
Does malloc(0) return NULL?
It may, but zero-size allocation has separate rules and should not be treated as an ordinary allocation failure. POSIX permits malloc(0) to return either a null pointer or a pointer to a zero-size allocation; the latter must not be used to access an object. Accordingly, malloc(0) == NULL does not prove that memory was exhausted. See the POSIX malloc specification and cppreference’s C malloc summary.
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If zero elements are a normal case, handle that case deliberately—for example, skip the allocation for an empty collection or normalize the requested size according to your program’s design. Do not depend on a particular implementation’s zero-size return value.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Can malloc succeed and memory still become unavailable later?
On Linux, yes. The Linux man-pages documentation says, “By default, Linux follows an optimistic memory allocation strategy.” A non-NULL result therefore does not guarantee that physical memory will be available when the program later touches the allocated region. Under memory pressure, the kernel’s out-of-memory (OOM) killer may terminate one or more processes. This is Linux system behavior layered on top of the C allocation API; it does not change the API’s null return for an allocation failure reported by malloc. The details are in the Linux malloc(3) manual.
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How the rules differ by platform
| Context | Meaning of NULL for a nonzero request | Is errno required on failure? | Important qualification |
|---|---|---|---|
| ISO C | The requested allocation was not provided; the return value is null. | No. | Zero-size allocation behavior is implementation-defined. See cppreference’s C malloc summary. |
| POSIX | Allocation failure returns null. | Yes; ENOMEM is specified for insufficient storage. |
Zero-size requests have separate permitted outcomes. See the POSIX specification. |
| Linux | The API’s null-on-failure behavior still applies. | The Linux manual documents errno on error. |
With Linux’s default optimistic allocation strategy, non-NULL does not guarantee memory will be available when touched later. See the Linux manual. |
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