Updated with output from a Linux VM.

Thank you Laurent_R. I just tried with the -d option and root access is not required. There is space followed by a dot after the -l (letter l as in larry).

mkdir /tmp/phfiles

cd /usr/include
h2ph -d /tmp/phfiles -r -l .

cd /tmp/phfiles
grep sendmsg */*.ph
grep recvmsg */*.ph
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Below, the grep output from a 64-bit Linux OS. Thus, the values are 46 and 47 for sendmsg and recvmsg respectively.

$ cd /tmp/phfiles

$ grep sendmsg */*.ph
asm-generic/unistd.ph:eval 'sub __NR_sendmsg () {211;}' unless defined
+(&__NR_sendmsg);
asm/unistd_64.ph:    eval 'sub __NR_sendmsg () {46;}' unless defined(&
+__NR_sendmsg);
asm/unistd_x32.ph:    eval 'sub __NR_sendmsg () {( &__X32_SYSCALL_BIT 
++ 518);}' unless defined(&__NR_sendmsg);
bits/syscall.ph:    eval 'sub SYS_sendmsg () { &__NR_sendmsg;}' unless
+ defined(&SYS_sendmsg);
bits/syscall.ph:    eval 'sub SYS_sendmsg () { &__NR_sendmsg;}' unless
+ defined(&SYS_sendmsg);

$ grep recvmsg */*.ph
asm-generic/unistd.ph:eval 'sub __NR_recvmsg () {212;}' unless defined
+(&__NR_recvmsg);
asm/unistd_64.ph:    eval 'sub __NR_recvmsg () {47;}' unless defined(&
+__NR_recvmsg);
asm/unistd_x32.ph:    eval 'sub __NR_recvmsg () {( &__X32_SYSCALL_BIT 
++ 519);}' unless defined(&__NR_recvmsg);
bits/syscall.ph:    eval 'sub SYS_recvmsg () { &__NR_recvmsg;}' unless
+ defined(&SYS_recvmsg);
bits/syscall.ph:    eval 'sub SYS_recvmsg () { &__NR_recvmsg;}' unless
+ defined(&SYS_recvmsg);
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