The kernel keeps per-process signal information in /proc/PID/status. The relevant fields are hex bitmasks where each bit represents a signal number:

This program blocks SIGINT, then loops for 5 seconds while you can press Ctrl+C. During that time SIGINT becomes pending. We use sigpending() to check and print the pending set.

/* Example: Block SIGINT, check pending signals with sigpending()
   Compile: gcc -o pending_demo pending_demo.c
   Run:     ./pending_demo  then press Ctrl+C quickly */

#include <stdio.h>
#include <signal.h>
#include <unistd.h>
#include <string.h>

int main(void)
{
    sigset_t block_set, pending_set;

    /* Step 1: block SIGINT */
    sigemptyset(&block_set);
    sigaddset(&block_set, SIGINT);
    sigprocmask(SIG_BLOCK, &block_set, NULL);

    printf("SIGINT is now blocked for 5 seconds.\n");
    printf("Press Ctrl+C — it will become PENDING.\n");

    sleep(5);   /* sleep while SIGINT may arrive */

    /* Step 2: check what is pending */
    sigpending(&pending_set);

    if (sigismember(&pending_set, SIGINT))
        printf("YES — SIGINT is currently PENDING!\n");
    else
        printf("No pending SIGINT detected.\n");

    /* Step 3: unblock — signal is delivered NOW */
    printf("Unblocking SIGINT — it will be delivered now...\n");
    sigprocmask(SIG_UNBLOCK, &block_set, NULL);

    /* If SIGINT was pending, process terminates here */
    printf("This line only prints if no SIGINT was pending.\n");

    return 0;
}

This program blocks SIGINT, then reads its own /proc/self/status to show the SigPnd bitmask before and after pressing Ctrl+C.

/* Example: Read SigPnd from /proc/self/status
   Compile: gcc -o proc_sig proc_sig.c
   Run:     ./proc_sig then press Ctrl+C when prompted */

#include <stdio.h>
#include <signal.h>
#include <unistd.h>
#include <string.h>

/* Helper: print SigPnd line from /proc/self/status */
void print_sig_status(void)
{
    FILE *f = fopen("/proc/self/status", "r");
    if (!f) { perror("fopen"); return; }

    char line[256];
    while (fgets(line, sizeof(line), f)) {
        /* Print only lines starting with Sig */
        if (strncmp(line, "Sig", 3) == 0)
            printf("  %s", line);
    }
    fclose(f);
}

int main(void)
{
    sigset_t block_set;

    /* Block SIGINT */
    sigemptyset(&block_set);
    sigaddset(&block_set, SIGINT);
    sigprocmask(SIG_BLOCK, &block_set, NULL);

    printf("=== Signal status BEFORE pressing Ctrl+C ===\n");
    print_sig_status();

    printf("\nNow press Ctrl+C within 5 seconds...\n");
    sleep(5);

    printf("\n=== Signal status AFTER pressing Ctrl+C ===\n");
    print_sig_status();

    /* Unblock to deliver */
    sigprocmask(SIG_UNBLOCK, &block_set, NULL);

    return 0;
}