umask chmod open

mode_stat.c

#include <sys/types.h>
#include <sys/stat.h>

#include <fcntl.h>
#include <stdio.h>

#define RW_UGO (S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH| S_IWOTH)

int 
main(int argc, char **argv, char **env) {

        const char *path = "/tmp/test";
        const mode_t process_umask = umask(S_IWGRP );
        umask(S_IWGRP);
        const mode_t mode = (S_IRUSR | S_IWUSR);
        
        
//      final permission: 666       -   022             =   644
//                        RW_UGO    ^   process_umask   =   -rw-r--r--
        
        if (open(path, O_RDWR |  O_CREAT | O_EXCL, mode) == -1)
                err(1, "open for '%s' failed", path);
        
        struct stat sb;
        if (stat(path, &sb) != 0)
                err(2, "stat failed");                
        const mode_t file_mode = sb.st_mode & ACCESSPERMS;

        const mode_t umasked_mode = (RW_UGO ^ process_umask) & mode;
        if (umasked_mode != file_mode)
                printf("umasked mode (%o) and file_mode (%o) are different\n", 
                        umasked_mode, file_mode);

        return 0;
}