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Aliases: psx_register(3), psx_register(3), psx_syscall(3), psx_syscall3(3), psx_syscall6(3)


POSIX 1003.1e capabilities (development)


Development files for libcap


psx_syscall3, psx_syscall6 - POSIX semantics for system calls


#include <sys/psx_syscall.h>

long int psx_syscall3(long int syscall_nr, long int arg1, long int arg2, long int arg3);
long int psx_syscall6(long int syscall_nr, long int arg1, long int arg2, long int arg3, long int arg4, long int arg5, long int arg6);
Link with one of these:
ld ... -lpsx -lpthread --wrap=pthread_create
gcc ... -lpsx -lpthread -Wl,-wrap,pthread_create


The libpsx library attempts to fill a gap left by the pthreads(7) implementation on Linux. To be compliant POSIX threads, via the nptl(7) setxid mechanism glibc maintains consistent UID and GID credentials amongst all of the threads associated with the current process. However, other credential state is not supported by this abstraction. To support these extended kernel managed security attributes, libpsx(3) provides a more generic pair of wrapping system call functions: psx_syscall3(3) and psx_syscall6(3). Like the setxid mechanism, the coordination of thread state is arranged by a realtime signal SIGRTMAX which is usurped for this process.
A linker trick of wrapping the pthread_create() call with a psx thread registration function is used to allow libpsx to keep track of all pthreads. If that trick is not usable by your application, then the much more cumbersome and fragile psx_register(pthread_t thread) function is provided to register threads manually with the library. To successfully use this approach an explanation of how to code for it is provided at the top of the <sys/psx_syscall.h> header file.
An inefficient macrology trick supports the psx_syscall() pseudo function which takes 1 to 7 arguments, depending on the needs of the caller. The macrology pads out the call to actually use psx_syscall3(3) or psx_syscall6(3) with zeros filling the missing arguments. While using this in source code will make it appear clean, the actual code footprint is larger. You are encouraged to use the more explicit psx_syscall3(3) and psx_syscall6(3) functions.


The return value for system call functions is generally the value returned by the kernel, or -1 in the case of an error. In such cases errno(3) is set to the detailed error value. The psx_syscall3 and psx_syscall6 functions attempt a single threaded system call and return immediately in the case of an error. Should this call succeed, then the same system calls are executed from a signal handler on each of the other threads of the process.


The needs of libcap(3) for POSIX semantics of capability manipulation.


Please report bugs via:;list_id=1047723&product=Tools&resolution=---
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