技術標籤：# C++

shell

$ cat /proc/sys/kernel/pid_max    #檢視系統支援的最大執行緒數
131072

c程式碼

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

#define likely(x)  __builtin_expect((x), 1)
#define unlikely(x)  __builtin_expect((x), 0)
static inline int read_single_number_file(const char *filename,
 
 unsigned long long *result);
static inline int read_file(const char *filename, char *buffer, size_t size);
static inline unsigned long long str2ull(const char *s);

pid_t pid_max = 32768;
pid_t get_system_pid_max(void) {
    static char read = 0;
    if(read) return pid_max;
    read = 1;

    char filename[
 
FILENAME_MAX + 1];
    snprintf(filename, FILENAME_MAX, "/proc/sys/kernel/pid_max");

    unsigned long long max = 0;
    if(read_single_number_file(filename, &max) != 0) {
        printf("Cannot open file '%s'. Assuming system supports %d pids.", filename, pid_max);
        return
 
 pid_max;
    }

    if(!max) {
        printf("Cannot parse file '%s'. Assuming system supports %d pids.", filename, pid_max);
        return pid_max;
    }

    pid_max = (pid_t) max;
    return pid_max;
}

int main() {
    get_system_pid_max();

    printf("pid_max = %d", pid_max);

    return 0;
}


static inline int read_single_number_file(const char *filename, unsigned long long *result) {
    char buffer[30 + 1];

    int ret = read_file(filename, buffer, 30);
    if(unlikely(ret)) {
        *result = 0;
        return ret;
    }

    buffer[30] = '\0';
    *result = str2ull(buffer);
    return 0;
}

static inline int read_file(const char *filename, char *buffer, size_t size) {
    if(unlikely(!size)) return 3;

    int fd = open(filename, O_RDONLY, 0666);
    if(unlikely(fd == -1)) {
        buffer[0] = '\0';
        return 1;
    }

    ssize_t r = read(fd, buffer, size);
    if(unlikely(r == -1)) {
        buffer[0] = '\0';
        close(fd);
        return 2;
    }
    buffer[r] = '\0';

    close(fd);
    return 0;
}
static inline unsigned long long str2ull(const char *s) {
    unsigned long long n = 0;
    char c;
    for(c = *s; c >= '0' && c <= '9' ; c = *(++s)) {
        n *= 10;
        n += c - '0';
    }
    return n;
}