Creating a simple shell in C causes linking error - c

Question

I am creating a simple shell in C that can execute commands, do stdout redirects and piping. So far I got only the redirecting part done but for some reason when i try to compile collect2 throws an error.
The util.c file that I am including:
#include<stdio.h>
#include<unistd.h>
#include<stdlib.h>
#include<string.h>
#include<signal.h>
#include<errno.h>
#include"util.h"
void setSigHandler(){
void (*oldHandler)();
oldHandler = signal(SIGINT, SIG_IGN);
signal(SIGTERM, oldHandler);
}
void prompt(){
char* user = getlogin();
printf("[%s]-->$", user);
}
void getNextCommand(){
errno = 0;
cmd = malloc(sizeof *cmd);
if(cmd == NULL){
char* error = strerror(errno);
printf("malloc:%s\n", error);
exit(EXIT_FAILURE);
}
fgets(cmd->payload, sizeof(cmd->payload), stdin);
if(cmd->payload[strlen(cmd->payload) - 1] == '\n'){
cmd->payload[strlen(cmd->payload) - 1] = '\0';
}
}
void parseCommandString(){
cmd->payloadArgc = 0;
char* buffer = strtok(cmd->payload, " ");
while(buffer != NULL){
cmd->payloadArgv[cmd->payloadArgc] = buffer;
if(!strcmp(cmd->payloadArgv[cmd->payloadArgc], ">")){
cmd->payloadArgv[cmd->payloadArgc] = NULL;
fileName = strtok(NULL, " ");
break;
}
if(!strcmp(cmd->payloadArgv[cmd->payloadArgc], "|")){
cmd->payloadArgv[cmd->payloadArgc] = NULL;
pCount++;
break;
}
buffer = strtok(NULL, " ");
cmd->payloadArgc++;
}
}
And my main file :
#include<stdio.h>
#include<stdlib.h>
#include<unistd.h>
#include<string.h>
#include<errno.h>
#include<signal.h>
#include<sys/file.h>
#include<fcntl.h>
#include"util.h"
int execCommand();
int main(){
setSigHandler();
while(1){
prompt();
getNextCommand();
//pdsh exits on "close"
if(!strcmp(cmd->payloadArgv[0], "close")) break;
parseCommandString();
}
return 0;
}
int execCommand()
{
errno = 0;
int newFd;
// Fork process
pid_t pid = fork();
// Error
if (pid == -1) {
char* error = strerror(errno);
printf("fork: %s\n", error);
return -1;
}
// Child process
else if (pid == 0) {
if(fileName != NULL){
newFd = open(fileName, O_WRONLY | O_CREAT | O_TRUNC, 0755);
if(newFd == -1){
char* error = strerror(errno);
printf("open: %s\n", error);
return -2;
}
dup2(newFd, STDOUT_FILENO);
close(newFd);
}
// Execute command
execvp(cmd->payloadArgv[0], cmd->payloadArgv);
// Error occurred
char* error = strerror(errno);
printf("shell: %s: %s\n", cmd->payloadArgv[0], error);
return -2;
}
// Parent process
else {
close(newFd);
fileName = NULL;
// Wait for child process to finish
int childStatus;
waitpid(pid, &childStatus, 0);
return 0;
}
}
And the error from collect2:
/tmp/ccIXS72e.o:(.bss+0x0): multiple definition of `fileName'
/tmp/ccINCcwP.o:(.bss+0x0): first defined here
/tmp/ccIXS72e.o:(.data+0x0): multiple definition of `pCount'
/tmp/ccINCcwP.o:(.data+0x0): first defined here
/tmp/ccIXS72e.o:(.data+0x4): multiple definition of `newDescriptor'
/tmp/ccINCcwP.o:(.data+0x4): first defined here
collect2: error: ld returned 1 exit status
What is causing this?
I know my main.c code is a little ugly sorry for that.
Edit: The util.h as requested:
#ifndef UTIL_H_
#define UTIL_H_
#define TRUE 1
#define FALSE !TRUE
#define STDOUT 0
#define STDIN 1
#define MAX_CMD_LENGHT 500
#define MAX_CMD_ARGS_LENGHT 50
char* fileName = NULL;
int pCount = -1;
int newDescriptor = -1;
Command cmd;
typedef struct commandR* Command;
struct commandR{
char payload[MAX_CMD_LENGHT];
char* payloadArgv[MAX_CMD_ARGS_LENGHT];
int payloadArgc;
};
void setSigHandler();
void prompt();
void getNextCommand();
void parseCommand();
#endif
Edit 2: After a suggestion I have modified my code. Error still goes off.
util.h
#ifndef UTIL_H_
#define UTIL_H_
#define TRUE 1
#define FALSE !TRUE
#define STDOUT 0
#define STDIN 1
#define MAX_CMD_LENGHT 500
#define MAX_CMD_ARGS_LENGHT 50
extern int newDescriptor;
extern char* fileName;
extern int pCount;
typedef struct commandR* Command;
struct commandR{
char payload[MAX_CMD_LENGHT];
char* payloadArgv[MAX_CMD_ARGS_LENGHT];
int payloadArgc;
};
extern Command cmd;
void setSigHandler();
void prompt();
void getNextCommand();
void parseCommand();
#endif
util.c
#include<stdio.h>
#include<unistd.h>
#include<stdlib.h>
#include<string.h>
#include<signal.h>
#include<errno.h>
#include"util.h"
char* fileName = NULL;
int pCount = -1;
int newDescriptor = -1;
Command cmd;
void setSigHandler(){
void (*oldHandler)();
oldHandler = signal(SIGINT, SIG_IGN);
signal(SIGTERM, oldHandler);
}
void prompt(){
char* user = getlogin();
printf("[%s]-->$", user);
}
void getNextCommand(){
errno = 0;
cmd = malloc(sizeof *cmd);
if(cmd == NULL){
char* error = strerror(errno);
printf("malloc:%s\n", error);
exit(EXIT_FAILURE);
}
fgets(cmd->payload, sizeof(cmd->payload), stdin);
if(cmd->payload[strlen(cmd->payload) - 1] == '\n'){
cmd->payload[strlen(cmd->payload) - 1] = '\0';
}
}
void parseCommandString(){
cmd->payloadArgc = 0;
char* buffer = strtok(cmd->payload, " ");
while(buffer != NULL){
cmd->payloadArgv[cmd->payloadArgc] = buffer;
if(!strcmp(cmd->payloadArgv[cmd->payloadArgc], ">")){
cmd->payloadArgv[cmd->payloadArgc] = NULL;
fileName = strtok(NULL, " ");
break;
}
if(!strcmp(cmd->payloadArgv[cmd->payloadArgc], "|")){
cmd->payloadArgv[cmd->payloadArgc] = NULL;
pCount++;
break;
}
buffer = strtok(NULL, " ");
cmd->payloadArgc++;
}
}

Answer 1

The problem is that you have variables defined in the header file. So when you compile each .c file, they both contain a copy of those variables. This causes the linker error of multiple definitions.
What you need to do is declare them in the header file with the extern keyword. That allows the variables to be used without being defined. Then you define them in exactly one .c file.
util.h:
extern char* fileName;
extern int pCount;
extern int newDescriptor;
extern Command cmd;
util.c:
char* fileName = NULL;
int pCount = -1;
int newDescriptor = -1;
Command cmd;
Also note that the variables are initialized where they are defined, not where they are declared.
EDIT:
It looks like the problem is how you're compiling. You need to compile main.c and util.c to object files, then link the resulting object files:
gcc -Wall -Wextra -g -c main.c
gcc -Wall -Wextra -g -c util.c
gcc -Wall -Wextra -g -o main main.o util.o
Also, you need to put the declaration of parseCommandString into util.h, and you need to include sys/types.h and sys/wait.h in main.c for waitpid.