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fork_process.c File Reference
#include "postgres.h"
#include <fcntl.h>
#include <signal.h>
#include <time.h>
#include <sys/stat.h>
#include <unistd.h>
#include "libpq/pqsignal.h"
#include "miscadmin.h"
#include "postmaster/fork_process.h"
Include dependency graph for fork_process.c:

Go to the source code of this file.

Functions

pid_t fork_process (void)
 

Function Documentation

◆ fork_process()

pid_t fork_process ( void  )

Definition at line 33 of file fork_process.c.

34{
35 pid_t result;
36 const char *oomfilename;
37 sigset_t save_mask;
38
39#ifdef LINUX_PROFILE
40 struct itimerval prof_itimer;
41#endif
42
43 /*
44 * Flush stdio channels just before fork, to avoid double-output problems.
45 */
46 fflush(NULL);
47
48#ifdef LINUX_PROFILE
49
50 /*
51 * Linux's fork() resets the profiling timer in the child process. If we
52 * want to profile child processes then we need to save and restore the
53 * timer setting. This is a waste of time if not profiling, however, so
54 * only do it if commanded by specific -DLINUX_PROFILE switch.
55 */
56 getitimer(ITIMER_PROF, &prof_itimer);
57#endif
58
59 /*
60 * We start postmaster children with signals blocked. This allows them to
61 * install their own handlers before unblocking, to avoid races where they
62 * might run the postmaster's handler and miss an important control
63 * signal. With more analysis this could potentially be relaxed.
64 */
65 sigprocmask(SIG_SETMASK, &BlockSig, &save_mask);
66 result = fork();
67 if (result == 0)
68 {
69 /* fork succeeded, in child */
70 MyProcPid = getpid();
71#ifdef LINUX_PROFILE
72 setitimer(ITIMER_PROF, &prof_itimer, NULL);
73#endif
74
75 /*
76 * By default, Linux tends to kill the postmaster in out-of-memory
77 * situations, because it blames the postmaster for the sum of child
78 * process sizes *including shared memory*. (This is unbelievably
79 * stupid, but the kernel hackers seem uninterested in improving it.)
80 * Therefore it's often a good idea to protect the postmaster by
81 * setting its OOM score adjustment negative (which has to be done in
82 * a root-owned startup script). Since the adjustment is inherited by
83 * child processes, this would ordinarily mean that all the
84 * postmaster's children are equally protected against OOM kill, which
85 * is not such a good idea. So we provide this code to allow the
86 * children to change their OOM score adjustments again. Both the
87 * file name to write to and the value to write are controlled by
88 * environment variables, which can be set by the same startup script
89 * that did the original adjustment.
90 */
91 oomfilename = getenv("PG_OOM_ADJUST_FILE");
92
93 if (oomfilename != NULL)
94 {
95 /*
96 * Use open() not stdio, to ensure we control the open flags. Some
97 * Linux security environments reject anything but O_WRONLY.
98 */
99 int fd = open(oomfilename, O_WRONLY, 0);
100
101 /* We ignore all errors */
102 if (fd >= 0)
103 {
104 const char *oomvalue = getenv("PG_OOM_ADJUST_VALUE");
105 int rc;
106
107 if (oomvalue == NULL) /* supply a useful default */
108 oomvalue = "0";
109
110 rc = write(fd, oomvalue, strlen(oomvalue));
111 (void) rc;
112 close(fd);
113 }
114 }
115
116 /* do post-fork initialization for random number generation */
118 }
119 else
120 {
121 /* in parent, restore signal mask */
122 sigprocmask(SIG_SETMASK, &save_mask, NULL);
123 }
124
125 return result;
126}
sigset_t BlockSig
Definition: pqsignal.c:23
int MyProcPid
Definition: globals.c:46
#define close(a)
Definition: win32.h:12
#define write(a, b, c)
Definition: win32.h:14
static void const char fflush(stdout)
void pg_strong_random_init(void)
static int fd(const char *x, int i)
Definition: preproc-init.c:105
int setitimer(int which, const struct itimerval *value, struct itimerval *ovalue)
Definition: timer.c:86

References BlockSig, close, fd(), fflush(), MyProcPid, pg_strong_random_init(), setitimer(), and write.

Referenced by postmaster_child_launch().