How to avoid using printf in a signal handler?

Question

Since printf is not reentrant, it's not supposed to be safe to use it in a signal handler. But I've seen lots of example codes that uses printf this way.

So my question is: when do we need to avoid using printf in a signal handler, and is there a recommended replacement?

Answer 1

Implement your own async-signal-safe snprintf("%d and use write

It is not as bad as I thought, How to convert an int to string in C? has several implementations.

Since there are only two interesting types of data that signal handlers can access:

• sig_atomic_t globals
• int signal argument

this basically covers all interesting use cases.

The fact that strcpy is also signal safe makes things even better.

The POSIX program below prints to stdout the number of times it received SIGINT so far, which you can trigger with Ctrl + C, and the and signal ID.

You can exit the program with Ctrl + \ (SIGQUIT).

main.c:

#define _XOPEN_SOURCE 700
#include <assert.h>
#include <limits.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <string.h>
#include <unistd.h>

/* Calculate the minimal buffer size for a given type.
 *
 * Here we overestimate and reserve 8 chars per byte.
 *
 * With this size we could even print a binary string.
 *
 * - +1 for NULL terminator
 * - +1 for '-' sign
 *
 * A tight limit for base 10 can be found at:
 * https://stackoverflow.com/questions/8257714/how-to-convert-an-int-to-string-in-c/32871108#32871108
 *
 * TODO: get tight limits for all bases, possibly by looking into
 * glibc's atoi: https://stackoverflow.com/questions/190229/where-is-the-itoa-function-in-linux/52127877#52127877
 */
#define ITOA_SAFE_STRLEN(type) sizeof(type) * CHAR_BIT + 2

/* async-signal-safe implementation of integer to string conversion.
 *
 * Null terminates the output string.
 *
 * The input buffer size must be large enough to contain the output,
 * the caller must calculate it properly.
 *
 * @param[out] value  Input integer value to convert.
 * @param[out] result Buffer to output to.
 * @param[in]  base   Base to convert to.
 * @return     Pointer to the end of the written string.
 */
char *itoa_safe(intmax_t value, char *result, int base) {
    intmax_t tmp_value;
    char *ptr, *ptr2, tmp_char;
    if (base < 2 || base > 36) {
        return NULL;
    }

    ptr = result;
    do {
        tmp_value = value;
        value /= base;
        *ptr++ = "ZYXWVUTSRQPONMLKJIHGFEDCBA9876543210123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"[35 + (tmp_value - value * base)];
    } while (value);
    if (tmp_value < 0)
        *ptr++ = '-';
    ptr2 = result;
    result = ptr;
    *ptr-- = '\0';
    while (ptr2 < ptr) {
        tmp_char = *ptr;
        *ptr--= *ptr2;
        *ptr2++ = tmp_char;
    }
    return result;
}

volatile sig_atomic_t global = 0;

void signal_handler(int sig) {
    char key_str[] = "count, sigid: ";
    /* This is exact:
     * - the null after the first int will contain the space
     * - the null after the second int will contain the newline
     */
    char buf[2 * ITOA_SAFE_STRLEN(sig_atomic_t) + sizeof(key_str)];
    enum { base = 10 };
    char *end;
    end = buf;
    strcpy(end, key_str);
    end += sizeof(key_str);
    end = itoa_safe(global, end, base);
    *end++ = ' ';
    end = itoa_safe(sig, end, base);
    *end++ = '\n';
    write(STDOUT_FILENO, buf, end - buf);
    global += 1;
    signal(sig, signal_handler);
}

int main(int argc, char **argv) {
    /* Unit test itoa_safe. */
    {
        typedef struct {
            intmax_t n;
            int base;
            char out[1024];
        } InOut;
        char result[1024];
        size_t i;
        InOut io;
        InOut ios[] = {
            /* Base 10. */
            {0, 10, "0"},
            {1, 10, "1"},
            {9, 10, "9"},
            {10, 10, "10"},
            {100, 10, "100"},
            {-1, 10, "-1"},
            {-9, 10, "-9"},
            {-10, 10, "-10"},
            {-100, 10, "-100"},

            /* Base 2. */
            {0, 2, "0"},
            {1, 2, "1"},
            {10, 2, "1010"},
            {100, 2, "1100100"},
            {-1, 2, "-1"},
            {-100, 2, "-1100100"},

            /* Base 35. */
            {0, 35, "0"},
            {1, 35, "1"},
            {34, 35, "Y"},
            {35, 35, "10"},
            {100, 35, "2U"},
            {-1, 35, "-1"},
            {-34, 35, "-Y"},
            {-35, 35, "-10"},
            {-100, 35, "-2U"},
        };
        for (i = 0; i < sizeof(ios)/sizeof(ios[0]); ++i) {
            io = ios[i];
            itoa_safe(io.n, result, io.base);
            if (strcmp(result, io.out)) {
                printf("%ju %d %s\n", io.n, io.base, io.out);
                assert(0);
            }
        }
    }

    /* Handle the signals. */
    if (argc > 1 && !strcmp(argv[1], "1")) {
        signal(SIGINT, signal_handler);
        while(1);
    }

    return EXIT_SUCCESS;
}

Compile and run:

gcc -std=c99 -Wall -Wextra -o main main.c
./main 1

After pressing Ctrl + C fifteen times, the terminal shows:

^Ccount, sigid: 0 2
^Ccount, sigid: 1 2
^Ccount, sigid: 2 2
^Ccount, sigid: 3 2
^Ccount, sigid: 4 2
^Ccount, sigid: 5 2
^Ccount, sigid: 6 2
^Ccount, sigid: 7 2
^Ccount, sigid: 8 2
^Ccount, sigid: 9 2
^Ccount, sigid: 10 2
^Ccount, sigid: 11 2
^Ccount, sigid: 12 2
^Ccount, sigid: 13 2
^Ccount, sigid: 14 2

where 2 is the signal number for SIGINT.

Tested on Ubuntu 18.04. GitHub upstream.