CERT C: Rule SIG30-C

Call only asynchronous-safe functions within signal handlers

expand all in page

Description

Rule Definition

Call only asynchronous-safe functions within signal handlers.¹

Polyspace Implementation

The rule checker checks for these issues:

Function called from signal handler not asynchronous-safe.
Function called from signal handler not asynchronous-safe (strict).

Examples

expand all

Function called from signal handler not asynchronous-safe

Issue

Function called from signal handler not asynchronous-safe occurs when a signal handler calls a function that is not asynchronous-safe according to the POSIX standard. An asynchronous-safe function can be interrupted at any point in its execution, then called again without causing an inconsistent state. It can also correctly handle global data that might be in an inconsistent state.

If a signal handler calls another function that calls an asynchronous-unsafe function, the defect appears on the function call in the signal handler. The defect traceback shows the full path from the signal handler to the asynchronous-unsafe function.

Risk

When a signal handler is invoked, the execution of the program is interrupted. After the handler is finished, program execution resumes at the point of interruption. If a function is executing at the time of the interruption, calling it from within the signal handler is undefined behavior, unless it is asynchronous-safe.

Fix

The POSIX standard defines these functions as asynchronous-safe. You can call these functions from a signal handler.

`_exit()`	`getpgrp()`	`setsockopt()`
`_Exit()`	`getpid()`	`setuid()`
`abort()`	`getppid()`	`shutdown()`
`accept()`	`getsockname()`	`sigaction()`
`access()`	`getsockopt()`	`sigaddset()`
`aio_error()`	`getuid()`	`sigdelset()`
`aio_return()`	`kill()`	`sigemptyset()`
`aio_suspend()`	`link()`	`sigfillset()`
`alarm()`	`linkat()`	`sigismember()`
`bind()`	`listen()`	`signal()`
`cfgetispeed()`	`lseek()`	`sigpause()`
`cfgetospeed()`	`lstat()`	`sigpending()`
`cfsetispeed()`	`mkdir()`	`sigprocmask()`
`cfsetospeed()`	`mkdirat()`	`sigqueue()`
`chdir()`	`mkfifo()`	`sigset()`
`chmod()`	`mkfifoat()`	`sigsuspend()`
`chown()`	`mknod()`	`sleep()`
`clock_gettime()`	`mknodat()`	`sockatmark()`
`close()`	`open()`	`socket()`
`connect()`	`openat()`	`socketpair()`
`creat()`	`pathconf()`	`stat()`
`dup()`	`pause()`	`symlink()`
`dup2()`	`pipe()`	`symlinkat()`
`execl()`	`poll()`	`sysconf()`
`execle()`	`posix_trace_event()`	`tcdrain()`
`execv()`	`pselect()`	`tcflow()`
`execve()`	`pthread_kill()`	`tcflush()`
`faccessat()`	`pthread_self()`	`tcgetattr()`
`fchdir()`	`pthread_sigmask()`	`tcgetpgrp()`
`fchmod()`	`quick_exit()`	`tcsendbreak()`
`fchmodat()`	`raise()`	`tcsetattr()`
`fchown()`	`read()`	`tcsetpgrp()`
`fchownat()`	`readlink()`	`time()`
`fcntl()`	`readlinkat()`	`timer_getoverrun()`
`fdatasync()`	`recv()`	`timer_gettime()`
`fexecve()`	`recvfrom()`	`timer_settime()`
`fork()`	`recvmsg()`	`times()`
`fpathconf()`	`rename()`	`umask()`
`fstat()`	`renameat()`	`uname()`
`fstatat()`	`rmdir()`	`unlink()`
`fsync()`	`select()`	`unlinkat()`
`ftruncate()`	`sem_post()`	`utime()`
`futimens()`	`send()`	`utimensat()`
`getegid()`	`sendmsg()`	`utimes()`
`geteuid()`	`sendto()`	`wait()`
`getgid()`	`setgid()`	`waitpid()`
`getgroups()`	`setpgid()`	`write()`
`getpeername()`	`setsid()`

Functions not in the previous table are not asynchronous-safe, and should not be called from a signal hander.

Example - Call to printf() Inside Signal Handler

#include <signal.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <setjmp.h>
#include <syslog.h>
#include <unistd.h>

#define SIZE20 20

extern volatile sig_atomic_t e_flag;

void display_info(const char *info)
{
    if (info)
    {
        (void)fputs(info, stderr);
    }
}

void sig_handler(int signum)
{
    /* Call function printf() that is not
	asynchronous-safe */
	printf("signal %d received.", signum);  //Noncompliant
    e_flag = 1;
}

int main(void)
{
    e_flag = 0;
    if (signal(SIGINT, sig_handler) == SIG_ERR)
    {
        /* Handle error */
    }
    char *info = (char *)calloc(SIZE20, sizeof(char));
    if (info == NULL)
    {
        /* Handle Error */
    }
    while (!e_flag)
    {
        /* Main loop program code */
        display_info(info);
        /* More program code */
    }
    free(info);
    info = NULL;
    return 0;
}

In this example, sig_handler calls printf() when catching a signal. If the handler catches another signal while printf() is executing, the behavior of the program is undefined.

Correction — Set Flag Only in Signal Handler

Use your signal handler to set only the value of a flag. e_flag is of type volatile sig_atomic_t. sig_handler can safely access it asynchronously.

#include <signal.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <setjmp.h>
#include <syslog.h>
#include <unistd.h>

#define SIZE20 20

extern volatile sig_atomic_t e_flag;

void display_info(const char *info)
{
    if (info)
    {
        (void)fputs(info, stderr);
    }
}

void sig_handler1(int signum)
{
    int s0 = signum;
    e_flag = 1;       
}

int func(void)
{
    e_flag = 0;
    if (signal(SIGINT, sig_handler1) == SIG_ERR)
    {
        /* Handle error */
    }
    char *info = (char *)calloc(SIZE20, 1);
    if (info == NULL)
    {
        /* Handle error */
    }
    while (!e_flag)
    {
        /* Main loop program code */
        display_info(info);
        /* More program code */
    }
    free(info);
    info = NULL;
    return 0;
}

Function called from signal handler not asynchronous-safe (strict)

Issue

Function called from signal handler not asynchronous-safe (strict) occurs when a signal handler calls a function that is not asynchronous-safe according to the C standard. An asynchronous-safe function can be interrupted at any point in its execution, then called again without causing an inconsistent state. It can also correctly handle global data that might be in an inconsistent state.

When you select the checker Function called from signal handler not asynchronous-safe, the checker detects calls to functions that are not asynchronous-safe according to the POSIX standard. Function called from signal handler not asynchronous-safe (strict) does not raise a defect for these cases. Function called from signal handler not asynchronous-safe (strict) raises a defect for functions that are asynchronous-safe according to the POSIX standard but not according to the C standard.

Risk

Fix

The C standard defines the following functions as asynchronous-safe. You can call these functions from a signal handler:

abort()
_Exit()
quick_exit()
signal()

Example - Call to raise() Inside Signal Handler

#include <signal.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <setjmp.h>
#include <syslog.h>
#include <unistd.h>

void SIG_ERR_handler(int signum)
{
    int s0 = signum;
    /* SIGTERM specific handling */
}

void sig_handler(int signum)
{
    int s0 = signum;
	/* Call raise() */ 
    if (raise(SIGTERM) != 0) {  //Noncompliant
        /* Handle error */
    }
}

int finc(void)
{
    if (signal(SIGTERM, SIG_ERR_handler) == SIG_ERR)
    {
        /* Handle error */
    }
    if (signal(SIGINT, sig_handler) == SIG_ERR)
    {
        /* Handle error */
    }
    /* Program code */
    if (raise(SIGINT) != 0)
    {
        /* Handle error */
    }
    /* More code */
    return 0;
}

In this example, sig_handler calls raise() when catching a signal. If the handler catches another signal while raise() is executing, the behavior of the program is undefined.

Correction — Remove Call to raise() in Signal Handler

According to the C standard, the only functions that you can safely call from a signal handler are abort(), _Exit(), quick_exit(), and signal().

#include <signal.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <setjmp.h>
#include <syslog.h>
#include <unistd.h>

void SIG_ERR_handler(int signum)
{
    int s0 = signum;
    /* SIGTERM specific handling */
}
void sig_handler(int signum)
{
    int s0 = signum;
	
  
}

int func(void)
{
    if (signal(SIGTERM, SIG_ERR_handler) == SIG_ERR)
    {
        /* Handle error */
    }
    if (signal(SIGINT, sig_handler) == SIG_ERR)
    {
        /* Handle error */
    }
    /* Program code */
    if (raise(SIGINT) != 0)
    {
        /* Handle error */
    }
    /* More code */
    return 0;
}

Check Information

Group: Rule 11. Signals (SIG)

Version History

Introduced in R2019a