Yes. For a request larger than zero, malloc can return a null pointer when it cannot allocate the requested space. Always check the returned pointer before using it. POSIX also requires errno to be set on failure; portable ISO C does not. Linux adds an important caveat: a successful allocation does not guarantee the memory will remain available when your program later accesses it.
What does a NULL return mean?
For a nonzero request, a null return means the allocator could not provide the requested space. POSIX states, “If the space cannot be allocated, a null pointer shall be returned.” The POSIX Programmer’s Manual for malloc specifies that failure returns a null pointer and sets errno; ENOMEM indicates insufficient storage.
The return value is the reliable test in C: if it is null, do not dereference it. A null result is not necessarily proof that the machine has run out of physical RAM. On Linux, documented causes also include process address-space or data limits and the system mapping-count limit.
Should you check whether malloc returned NULL?
Yes. Save the result, test it, and only use the pointer after the check succeeds. The GNU C Library’s allocation guidance likewise demonstrates checking for a null pointer and taking an explicit failure path.
#1 Best Overall
#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 that the size calculation is valid. Check separately that count * sizeof *items cannot overflow; otherwise the program may request less memory than intended. What to do after failure depends on what the program can safely undo. It might report an error, release resources already acquired, reduce the work, or terminate rather than continue in an invalid state.
Does malloc set errno?
It depends on the contract your program targets. POSIX requires errno to be set when allocation fails, with ENOMEM identifying insufficient storage. ISO C does not provide a portable guarantee that errno is set by malloc. As the Linux malloc(3) manual explains, POSIX requires this behavior while the C standard does not.
Check the pointer first. If you are writing for POSIX and need a diagnostic, you can inspect errno after a failed nonzero request. For code intended to be portable across C implementations, do not use errno as the failure test.
Does malloc(0) return NULL?
It may, but zero-size allocation is a special case and should not be confused with failure for an ordinary request. POSIX permits malloc(0) to return either a null pointer or a pointer to a zero-size allocation. A non-null result must not be used to access an object. The POSIX specification also allows an implementation-defined errno value in the null-return case.
Do these 3 things before closing this tab:
1Clear out junk files and repair common Windows errors2Fix the driver behind crashes, sound loss and screen glitches3Repair Windows errors before they cause bigger problemsIf an empty collection needs no storage, handle that case explicitly—for example, skip the allocation or use a separate representation for the empty collection—instead of relying on how a particular implementation handles malloc(0).
Why can Linux allocation succeed and still lead to a memory problem?
Linux documents that, by default, it follows an optimistic memory-allocation strategy. A non-null return therefore does not guarantee that memory will be available when the program later touches it. If the system subsequently runs out of memory, the OOM killer may terminate one or more processes. This is Linux system behavior layered on top of the allocation API; it does not change the null-return rule for an allocation failure.
Linux also lists limits that can cause ENOMEM even when “the computer ran out of RAM” is not an adequate explanation. These include reaching RLIMIT_AS or RLIMIT_DATA, or exceeding /proc/sys/vm/max_map_count. The details are described in the Linux malloc(3) manual.
Quick Recap
Best Value
How the rules differ by environment
| Environment | Meaning of NULL for a nonzero request | Is errno required? | Key qualification |
|---|---|---|---|
| ISO C portability | The requested allocation was not provided. | No portable guarantee. | malloc(0) has separate implementation-defined behavior, as summarized by cppreference. |
| POSIX | Allocation failure returns null. | Yes; ENOMEM indicates insufficient storage. |
Zero-size requests have separate rules in the POSIX specification. |
| Linux, default allocation policy | The API still returns null when the allocator reports failure. | The Linux manual documents errno on error. |
A non-null result does not guarantee future availability when memory is accessed; the OOM killer may act, as described in the Linux manual. |
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
