Errors with gcc when compiling C program

Question

I have a custom shell program in which I have included signal.h , unistd.h , and stdio.h . I was originally working on this in RedHat Enterprise (not sure exactly what version, but not too old) and I was able to use gcc on my program and it compiled fine and ran fine. Now I moved it over to Ubuntu and gcc is giving me some errors, the first of which is conflicting types for 'getline()' . Some other errors say incompatible implicit declaration of built-in function strlen . I have overridden the functions in question, why was this working in RedHat but not in Ubuntu? Linux is not my thing so please speak plainly. Let me know if you need more error details.

/* 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;
}

Answer 1

incompatible implicit declaration of built-in function strlen

Include <string.h>

conflicting types for 'getline()

<stdio.h> already contains a declaration of getline, so make sure that nowhere in your code you have redeclared/redefined getline() [with a different prototype].

Answer 2

最简单的解决方案是将您的getline()函数重命名，例如重命名为my_getline()

Answer 3

gcc -Wall typoknig.c
typoknig.c:19: error: conflicting types for ‘getline’
//usr/include/stdio.h:671: note: previous declaration of ‘getline’ was here
typoknig.c:21: error: conflicting types for ‘getline’
//usr/include/stdio.h:671: note: previous declaration of ‘getline’ was here

Two separate declarations of getline which Andy had recommended that you use my_getline() since the former is already part of stdio.h .

typoknig.c: In function 'getline':
typoknig.c:27: warning: implicit declaration of function 'exit'
typoknig.c:27: warning: incompatible implicit declaration of built-in function 'exit'

That can't be good, man exit says right at the top:

#include <stdlib.h>
void exit(int status);

perhaps you need to include stdlib.h ? What does gcc assume is the signature of an undeclared function?

typoknig.c: In function 'handle_SIGINT':
typoknig.c:59: warning: implicit declaration of function 'strlen'
typoknig.c:59: warning: incompatible implicit declaration of built-in function 'strlen'

Ouch, man strlen to the rescue:

#include <string.h>

Fortunately, string.h will help out with the next one and we already nailed exit :

typoknig.c: In function 'main':
typoknig.c:70: warning: implicit declaration of function 'strcpy'
typoknig.c:70: warning: incompatible implicit declaration of built-in function 'strcpy'
typoknig.c:85: warning: incompatible implicit declaration of built-in function 'exit'

Ain't that pre-processor nifty?

typoknig.c:87: warning: implicit declaration of function ‘waitpid’
typoknig.c:64: warning: unused variable ‘ret’

Sayeth man waitpid :

#include <sys/types.h>
#include <sys/wait.h>

Line 64 is left as an exercise for the reader.

Answer 4

Getline() is not part of C standard. It is a GNU extension. In c++ Getline() is standard.

So adding to your code right before your #includes

#define _GNU_SOURCE

should fix the warning. Also see "man getline" on linux.