mjhjmtu

Why is sigset_t on 64-bit Linux 128 bytes large in glibc and musl?

#include <signal.h>
#include <stdio.h>
int main()

 printf("%zun", sizeof(sigset_t)); //prints 128 with both glibc and musl

Shouldn't 64 / 8 = 8 (number_of_signals / CHAR_BIT) be enough?

I don’t know the original reason; back in 1996, the Linux-specific header was added with the following definition:

/* A `sigset_t' has a bit for each signal. Having 32 * 4 * 8 bits gives 
 us up to 1024 signals. */ 
#define _SIGSET_NWORDS 32 
typedef struct 
 
 unsigned int __val[_SIGSET_NWORDS]; 
 __sigset_t;

and this “1024 signal” limit has been preserved in the current definition:

/* A `sigset_t' has a bit for each signal. */ 

#define _SIGSET_NWORDS (1024 / (8 * sizeof (unsigned long int))) 
typedef struct 
 
 unsigned long int __val[_SIGSET_NWORDS]; 
 __sigset_t;

which makes the 1024-based calculation clearer (and results in 16 unsigned longs on 64-bit x86, i.e. 128 bytes).

Presumably the glibc maintainers wanted to leave room for growth...

musl aims for ABI compatibility with glibc for sigaction, so it uses the same 1024-bit (128-byte) size:

TYPEDEF struct __sigset_t unsigned long __bits[128/sizeof(long)]; sigset_t;