...
The sig_atomic_t type is the integer type of an object that can be accessed as an atomic entity even in the presence of asynchronous interrupts. The type of sig_atomic_t is implementation-defined, though it provides some guarantees. Integer values ranging from SIG_ATOMIC_MIN through SIG_ATOMIC_MAX may be safely stored to a variable of the type. In addition, when sig_atomic_t is a signed integer type, SIG_ATOMIC_MIN must be no greater than -127 and SIG_ATOMIC_MAX no less than 127. Otherwise, SIG_ATOMIC_MIN must be 0 and SIG_ATOMIC_MAX must be no less than 255. The macros SIG_ATOMIC_MIN and SIG_ATOMIC_MAX are defined in the header <stdint.h>.
...
ISO/IEC 9899:2011 Section 6.7.3, "Type qualifiers," and Section section 7.14, "Signal handling <signal.h>"
...