SSIZEMAX

SSIZEMAX is commonly used to refer to the maximum value of the signed size type ssize_t in C and POSIX environments. ssize_t is the signed counterpart to size_t and is used by several system calls and library functions to report either a size or an error (typically returning -1 on error). SSIZEMAX thus represents the largest non-error value that such functions can return.

Definition and standards

The macro is defined in standard headers on many Unix-like systems, typically limits.h (and is sometimes exposed via unistd.h). Its value depends on the platform’s signed size type ssize_t, which in turn depends on the architecture and data model. On most 64-bit systems using LP64 (where ssize_t is 64 bits), SSIZEMAX is usually 9223372036854775807 (2^63−1). On many 32-bit systems using ILP32, it is commonly 2147483647 (2^31−1). In many implementations, SSIZEMAX is defined as LONG_MAX.

Relation to other macros

SSIZEMAX is distinct from SIZE_MAX, which is the maximum value of size_t (an unsigned type). Some systems also provide SSIZE_MIN, which is the minimum value representable by ssize_t, commonly computed as -SSIZEMAX−1 on two’s complement architectures.

Usage

SSIZEMAX provides a portable upper bound for operations that involve ssize_t values, helping to prevent overflow when computing sizes or performing arithmetic on counts returned by functions like read, write, or similar interfaces that may return a size or -1 on error.

Notes on naming

The standard macro name is SSIZE_MAX. Some codebases or non-POSIX environments may refer informally to “SSIZEMAX” or use project-specific aliases, but such forms are not guaranteed to be portable. When writing portable code, use SSIZE_MAX and include the appropriate headers (limits.h).