编译C程序时gcc出错

我有一个自定义shell程序,其中包括 signal.h , unistd.h ,和 stdio.h . 我最初是在RedHat Enterprise中进行这项工作的(不确定具体是哪个版本,但不是太旧),我能够在我的程序上使用gcc,它编译得很好,运行得也很好。现在我把它移到了Ubuntu上,gcc给了我一些错误,第一个是 conflicting types for 'getline()' . 还有一些错误说 incompatible implicit declaration of built-in function strlen

/* define a global input buffer */
#include <signal.h>
#include <unistd.h>
#include <stdio.h>

#define MAXARG 512
#define MAXBUF 512
#define BUFFER_SIZE 50
#define MAX_COMMANDS 10
char buffer [BUFFER_SIZE];
static char *prompt = "MYSHELL>";
static char inpbuf[MAXBUF];
static char *arg[MAXARG+1];
static char tokbuf[2*MAXBUF];
static char *tok = tokbuf;
char history[MAX_COMMANDS][MAXBUF];
int cmd_num;

void getline(void);

void getline() {
int length;

length = read(0, inpbuf, MAXBUF);
if (length == 0) {
    printf("\n");
    exit(0);
}
inpbuf[length] = '\0';

}

void processline() {
char *ptr = inpbuf;
int narg;
for (narg=0;;) {    
    arg[narg] = tok;
    for (; *ptr == ' ' || *ptr == '\t'; ptr++)
        ;
    while(*ptr != ' ' && *ptr != '\t' && *ptr != '\n' && 
          *ptr != '\0' && *ptr != ';' && *ptr != '&') 
        *tok++ = *ptr++;
    *tok++ = '\0';
    if (narg < MAXARG)
        narg++;
    if (*ptr == '\n')
        break;
}
// clear the input buffer
for (ptr = inpbuf; *ptr != '\n'; ptr++)
    *ptr = ' ';
if (narg != 0) {
    arg[narg] = NULL;
}
}

void handle_SIGINT()
{
write(STDOUT_FILENO, buffer, strlen(buffer));
}

int main()
{
    int pid, exitstat, ret;
    struct sigaction handler;
    handler.sa_handler = handle_SIGINT;
    handler.sa_flags = 0;
    sigemptyset(&handler.sa_mask);
    sigaction(SIGINT, &handler, NULL);
    strcpy(buffer, "Caught Control C\n");

    while (1) {
        printf("%s ", prompt);
        fflush(stdout);
        getline();
        processline();
        if ((pid = fork()) < 0){
            fprintf(stderr, "myshell: error\n");
            return (-1);
        }

        if (pid == 0) {
            execvp(*arg, arg);
            fprintf(stderr, "%s\n", *arg);
            exit(127);
        }
        waitpid(pid, &exitstat, 0);
    }
    return 0;
}