C语言实现P命令

在C语言中，可以使用system函数来执行系统命令，其中可以通过传递参数来执行P命令。P命令的作用是打印当前进程的进程号（PID），可以使用如下代码实现：

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int main() {
    char cmd[50];
    sprintf(cmd, "echo $$$$");
    system(cmd);
    return 0;
}

在上面的代码中，使用sprintf函数将字符串"echo $$$$"赋值给字符数组cmd，其中"$$$$"是一个特殊的字符串，在shell中表示当前进程的PID。然后使用system函数执行cmd命令，即可打印当前进程的PID。

相关问题

c语言实现ping命令

实现ping命令需要使用系统底层的网络API，这里给出一个基于Linux系统的C语言实现示例：

#include <unistd.h>  
#include <stdlib.h>  
#include <stdio.h>  
#include <string.h>  
#include <errno.h>  
#include <sys/socket.h>  
#include <sys/types.h>  
#include <netinet/in.h>  
#include <netinet/ip.h>  
#include <netinet/ip_icmp.h>  
#include <netdb.h>  
#include <arpa/inet.h>  
#include <sys/time.h>  

#define PACKET_SIZE 4096  
#define MAX_WAIT_TIME   5  
#define MAX_NO_PACKETS  4  

char sendpacket[PACKET_SIZE];  
char recvpacket[PACKET_SIZE];  
int sockfd, datalen = 56;  
int nsend = 0, nreceived = 0;  
struct sockaddr_in dest_addr;  
struct sockaddr_in from;  
struct timeval tvrecv;  

unsigned short cal_chksum(unsigned short *addr, int len)  
{  
    int sum = 0;  
    unsigned short res = 0;  
    while(len > 1)  
    {  
        sum += *addr++;  
        len -= 2;  
    }  
    if(len == 1)  
    {  
        *((unsigned char *)(&res)) = *((unsigned char *)addr);  
        sum += res;  
    }  
    sum = (sum >> 16) + (sum & 0xffff);  
    sum += (sum >> 16);  
    res = ~sum;  
    return res;  
}  

int pack(int pack_no)  
{  
    int i, packsize;  
    struct icmp *icmp;  
    struct timeval *tval;  
    icmp = (struct icmp*)sendpacket;  
    icmp->icmp_type = ICMP_ECHO;  
    icmp->icmp_code = 0;  
    icmp->icmp_cksum = 0;  
    icmp->icmp_seq = pack_no;  
    icmp->icmp_id = getpid();  
    packsize = 8 + datalen;  
    tval = (struct timeval *)icmp->icmp_data;  
    gettimeofday(tval, NULL);  
    icmp->icmp_cksum = cal_chksum((unsigned short *)icmp, packsize);  
    return packsize;  
}  

int unpack(char *buf, int len)  
{  
    int i, iphdrlen;  
    struct ip *ip;  
    struct icmp *icmp;  
    struct timeval *tvsend;  
    double rtt;  
    ip = (struct ip *)buf;  
    iphdrlen = ip->ip_hl << 2;  
    icmp = (struct icmp *)(buf + iphdrlen);  
    len -= iphdrlen;  
    if(len < 8)  
    {  
        printf("ICMP packets\'s length is less than 8\n");  
        return -1;  
    }  
    if((icmp->icmp_type == ICMP_ECHOREPLY) && (icmp->icmp_id == getpid()))  
    {  
        tvsend = (struct timeval *)icmp->icmp_data;  
        gettimeofday(&tvrecv, NULL);  
        tv_sub(&tvrecv, tvsend);  
        rtt = tvrecv.tv_sec * 1000 + tvrecv.tv_usec / 1000.0;  
        printf("%d byte from %s: icmp_seq=%u ttl=%d rtt=%.3f ms\n",  
            len, inet_ntoa(from.sin_addr), icmp->icmp_seq, ip->ip_ttl, rtt);  
        return 0;  
    }  
    else  
    {  
        return -1;  
    }  
}  

void tv_sub(struct timeval *out, struct timeval *in)  
{  
    if((out->tv_usec -= in->tv_usec) < 0)  
    {  
        --out->tv_sec;  
        out->tv_usec += 1000000;  
    }  
    out->tv_sec -= in->tv_sec;  
}  

void ping()  
{  
    int i, packsize;  
    struct timeval tv;  
    tv.tv_sec = MAX_WAIT_TIME;  
    tv.tv_usec = 0;  
    for (i = 0; i < MAX_NO_PACKETS; i++)  
    {  
        nsend++;  
        packsize = pack(nsend);  
        if (sendto(sockfd, sendpacket, packsize, 0, (struct sockaddr *)&dest_addr, sizeof(dest_addr)) < 0)  
        {  
            printf("sendto error: %s\n", strerror(errno));  
            continue;  
        }  
        if (recvfrom(sockfd, recvpacket, sizeof(recvpacket), 0, (struct sockaddr *)&from, (socklen_t *)&fromlen) < 0)  
        {  
            printf("recvfrom error: %s\n", strerror(errno));  
            continue;  
        }  
        if (unpack(recvpacket, sizeof(recvpacket)) == -1)  
        {  
            printf("unpack error\n");  
            continue;  
        }  
        nreceived++;  
        usleep(1000000);  
    }  
}  

int main(int argc, char *argv[])  
{  
    struct hostent *host;  
    struct protoent *protocol;  
    char protoname[] = "icmp";  
    int size = 50 * 1024;  
    if (argc < 2)  
    {  
        printf("usage: ping hostname/IP address\n");  
        return 0;  
    }  
    if ((protocol = getprotobyname(protoname)) == NULL)  
    {  
        printf("getprotobyname() error\n");  
        return 0;  
    }  
    if ((sockfd = socket(AF_INET, SOCK_RAW, protocol->p_proto)) < 0)  
    {  
        printf("socket() error\n");  
        return 0;  
    }  
    setuid(getuid());  
    setsockopt(sockfd, SOL_SOCKET, SO_RCVBUF, &size, sizeof(size));  
    bzero(&dest_addr, sizeof(dest_addr));  
    dest_addr.sin_family = AF_INET;  
    if ((dest_addr.sin_addr.s_addr = inet_addr(argv[1])) == INADDR_NONE)  
    {  
        if ((host = gethostbyname(argv[1])) == NULL)  
        {  
            printf("gethostbyname() error\n");  
            return 0;  
        }  
        memcpy((char *)&dest_addr.sin_addr, host->h_addr, host->h_length);  
    }  
    printf("PING %s(%s) %d bytes of data.\n", argv[1], inet_ntoa(dest_addr.sin_addr), datalen);  
    ping();  
    printf("--- %s ping statistics ---\n", argv[1]);  
    printf("%d packets transmitted, %d received, %%%.2f packet loss\n",  
        nsend, nreceived, (nsend - nreceived) * 100.0 / nsend);  
    close(sockfd);  
    return 0;  
}  

这个实现使用了ICMP协议来模拟ping命令。需要注意的是，这个实现中使用了系统底层的网络API，因此需要在Linux系统下编译运行。如果需要在其他系统上运行，请根据具体系统的网络API进行调整。

用c语言实现ping命令

以下是使用C语言实现ping命令的示例代码：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <netdb.h>
#include <netinet/in.h>
#include <netinet/ip_icmp.h>
#include <arpa/inet.h>
#include <sys/socket.h>
#include <sys/time.h>

#define PACKET_SIZE     4096
#define MAX_WAIT_TIME   5
#define MAX_NO_PACKETS  3

char sendpacket[PACKET_SIZE];
char recvpacket[PACKET_SIZE];
int sockfd, datalen = 56;
int nsend = 0, nreceived = 0;
struct sockaddr_in dest_addr;
struct timeval tvrecv;

unsigned short cal_chksum(unsigned short *addr, int len);

void tv_sub(struct timeval *out, struct timeval *in) {
    if ((out->tv_usec -= in->tv_usec) < 0) {
        --out->tv_sec;
        out->tv_usec += 1000000;
    }
    out->tv_sec -= in->tv_sec;
}

unsigned short cal_chksum(unsigned short *addr, int len) {
    int nleft = len;
    int sum = 0;
    unsigned short *w = addr;
    unsigned short answer = 0;

    while (nleft > 1) {
        sum += *w++;
        nleft -= 2;
    }

    if (nleft == 1) {
        *(unsigned char *)(&answer) = *(unsigned char *)w;
        sum += answer;
    }

    sum = (sum >> 16) + (sum & 0xffff);
    sum += (sum >> 16);
    answer = ~sum;
    return answer;
}

void send_packet() {
    int packetsize;
    struct icmp *icmp;
    icmp = (struct icmp*) sendpacket;
    icmp->icmp_type = ICMP_ECHO;
    icmp->icmp_code = 0;
    icmp->icmp_cksum = 0;
    icmp->icmp_seq = nsend++;
    icmp->icmp_id = getpid();
    packetsize = 8 + datalen;
    gettimeofday((struct timeval*) icmp->icmp_data, NULL);
    icmp->icmp_cksum = cal_chksum((unsigned short *) icmp, packetsize);
    sendto(sockfd, sendpacket, packetsize, 0, (struct sockaddr*) &dest_addr, sizeof(dest_addr));
}

void recv_packet() {
    int n, fromlen;
    extern int errno;
    fromlen = sizeof(struct sockaddr_in);

    while (1) {
        fd_set rfds;
        FD_ZERO(&rfds);
        FD_SET(sockfd, &rfds);
        struct timeval timeout;
        timeout.tv_sec = MAX_WAIT_TIME;
        timeout.tv_usec = 0;
        int retval = select(sockfd + 1, &rfds, NULL, NULL, &timeout);

        if (retval == -1) {
            printf("select error\n");
            break;
        } else if (retval == 0) {
            printf("request timeout.\n");
            break;
        } else {
            n = recvfrom(sockfd, recvpacket, sizeof(recvpacket), 0, (struct sockaddr*) &dest_addr, &fromlen);

            if (n < 0) {
                if (errno == EINTR)
                    continue;

                printf("recvfrom error\n");
                break;
            }

            gettimeofday(&tvrecv, NULL);
            struct ip *ip;
            struct icmp *icmp;
            ip = (struct ip*) recvpacket;
            icmp = (struct icmp*) (recvpacket + (ip->ip_hl << 2));
            int icmplen = n - (ip->ip_hl << 2);

            if (icmp->icmp_type == ICMP_ECHOREPLY) {
                if (icmp->icmp_id != getpid())
                    continue;

                if (icmplen < 8)
                    continue;

                if (icmp->icmp_seq != nreceived)
                    continue;

                nreceived++;
                printf("%d bytes from %s: icmp_seq=%u ttl=%d time=%ld ms\n", icmplen, inet_ntoa(dest_addr.sin_addr), icmp->icmp_seq, ip->ip_ttl, (tvrecv.tv_sec - ((struct timeval*) icmp->icmp_data)->tv_sec) * 1000 + (tvrecv.tv_usec - ((struct timeval*) icmp->icmp_data)->tv_usec) / 1000);
            } else {
                continue;
            }
        }
    }
}

void ping() {
    struct hostent *host;
    struct protoent *protocol;

    if ((protocol = getprotobyname("icmp")) == NULL) {
        printf("getprotobyname error\n");
        exit(1);
    }

    if ((sockfd = socket(AF_INET, SOCK_RAW, protocol->p_proto)) < 0) {
        printf("socket error\n");
        exit(1);
    }

    if ((host = gethostbyname("localhost")) == NULL) {
        printf("gethostbyname error\n");
        exit(1);
    }

    memset(&dest_addr, 0, sizeof(dest_addr));
    dest_addr.sin_family = AF_INET;
    dest_addr.sin_addr = *((struct in_addr*) host->h_addr);

    printf("PING %s (%s): %d data bytes\n", host->h_name, inet_ntoa(dest_addr.sin_addr), datalen);
    send_packet();
    recv_packet();
    close(sockfd);
}

int main(int argc, char **argv) {
    ping();
    return 0;
}

注意：该示例代码只实现了基本的ping功能，没有实现ping命令的全部参数和功能。如果需要实现更完整的ping命令，请参考相关文档或代码库。