Writing my own shell, undeclared identifier "output" - c

Question

Here is my code, for my own shell in C. When compiling I get an error: use of undeclared identifier 'output'. Here are examples of some of the errors when compiling:
error: use of undeclared identifier 'output' char
input[100];output[100];
test3.c:53:15: error: use of undeclared identifier 'output'
strcpy(output,args[i+1]);
^
test3.c:53:15: error: use of undeclared identifier 'output'
test3.c:60:8: warning: implicit declaration of function 'open' is
invalid in C99
[-Wimplicit-function-declaration] j = open(input, O_RDONLY, 0);
^
test3.c:60:20: error: use of undeclared identifier 'O_RDONLY' j =
open(input, O_RDONLY, 0);
^
test3.c:61:29: error: use of undeclared identifier 'O_RDONLY'
if ((j = open(input, O_RDONLY, 0)) < 0) {
test3.c:70:12: warning: implicit declaration of function 'creat' is
invalid in
C99 [-Wimplicit-function-declaration] if ((i= creat(output , 0644)) < 0) {
test3.c:70:18: error: use of undeclared identifier 'output' if ((i=
creat(output , 0644)) < 0) {
Here is my code:
#include "stdio.h"
#include "stdlib.h"
#include "string.h"
#include "signal.h"
#include "unistd.h"
void prompt(char*);
void execute( char* );
char** parse( char* );
int main( int ac, char* av[] )
{
char input[255]; // buffer for supporting command
signal( SIGINT, SIG_IGN ); // ignore ctrl-c
while(1)
{
prompt(input);
execute( input );
}
};
void execute( char* str)
{
int fork_result, status, i = 0,j=0,in=0,out=0;
char input[100];output[100];
char** args = parse( str ); // splits the user command into arguments
fork_result = fork(); // attempt to fork
if ( fork_result == -1 ) // failure
{
perror("Failed to fork\n");
exit(1);
}
else if ( fork_result == 0 ) // I'm the child
{
for(i=0;args[i]!='\0';i++)
{
if(strcmp(args[i],"<")==0)
{
args[i]=NULL;
strcpy(input,args[i+1]);
in=2;
}
if(strcmp(args[i],">")==0)
{
args[i]=NULL;
strcpy(output,args[i+1]);
out=2;
}
}
if (in)
{
j = open(input, O_RDONLY, 0);
if ((j = open(input, O_RDONLY, 0)) < 0)
{
perror("Couldn't open input file");
exit(0);
}
dup2(j, 0);
close(j);
}
if (out)
{
if ((i= creat(output , 0644)) < 0)
{
perror("Couldn't open the output file");
exit(0);
}
dup2(i, STDOUT_FILENO);
close(i);
}
execvp( args[0], args );
perror("failed to exec\n");
exit(2);
}
else // I'm the parent
{
// wait here
wait(&status); // wait for child to finish
free( args ); // free dynamic memory
}
}
char** parse( char* str )
{
char** args = malloc( 256 );
int i = 0;
args[i] = strtok( str, " " );
while( args[i] )
{
i++;
args[i] = strtok( NULL, " " );
}
return args;
}
void prompt(char* input)
{
printf("$ "); // print prompt
fgets( input, 255, stdin );
input[strlen(input)-1] = '\0'; // overwrite \n with \0
if ( strcmp( input, "exit" ) == 0 ) // shell command
exit(0);
}

Answer 1

char input[100];output[100];
You want:
char input[100], output[100];
Also add: #include <fcntl.h>
In general, man open (and other functions you use) is your friend -- it tells you what #includes to add.
There are many more potential bugs and arbitrary limitations in your code. Some examples:
void execute( char* str)
{
char input[100], output[100];
...
if(strcmp(args[i],"<")==0)
{
args[i]=NULL;
strcpy(input,args[i+1]); // possible stack buffer overflow.
if(strcmp(args[i],">")==0)
{
args[i]=NULL;
strcpy(output,args[i+1]); // possible stack buffer overflow
char** parse( char* str )
{
char** args = malloc( 256 ); // limit of 256/sizeof(char*) parameters.
// on a 64-bit system, if more than 32 parameters are supplied ...
args[i] = strtok( NULL, " " ); // ... possible heap buffer overflow.
fgets( input, 255, stdin ); // arbitrary limit of 254 characters on command line.
There is no guarantee that the string ends with \n:
input[strlen(input)-1] = '\0'; // overwrite \n with \0
If I was grading this "shell", I'd give it an "F".

Answer 2

There are multiple errors in your code.
1. On line 27 you need to separate the two variable definitions of input and output with a comma instead of a semicolon char input[100], output[100]; or specify the type of output like char input[100]; char output[100]; You have done this already in the line above.
2. The compiler complains about missing definitions of the function open and the identifier O_RDONLY. This can be fixed by adding #include "fcntl.h" to your includes at the top of the file.
After these changes, the code compiles fine for me (with gcc 5.4.0):
#include "stdio.h"
#include "stdlib.h"
#include "string.h"
#include "signal.h"
#include "unistd.h"
#include "fcntl.h"
void prompt(char*);
void execute( char* );
char** parse( char* );
int main( int ac, char* av[] )
{
char input[255]; // buffer for supporting command
signal( SIGINT, SIG_IGN ); // ignore ctrl-c
while(1)
{
prompt(input);
execute( input );
}
};
void execute( char* str)
{
int fork_result, status, i = 0,j=0,in=0,out=0;
char input[100], output[100];
char** args = parse( str ); // splits the user command into arguments
fork_result = fork(); // attempt to fork
if ( fork_result == -1 ) // failure
{
perror("Failed to fork\n");
exit(1);
}
else if ( fork_result == 0 ) // I'm the child
{
for(i=0;args[i]!='\0';i++)
{
if(strcmp(args[i],"<")==0)
{
args[i]=NULL;
strcpy(input,args[i+1]);
in=2;
}
if(strcmp(args[i],">")==0)
{
args[i]=NULL;
strcpy(output,args[i+1]);
out=2;
}
}
if (in)
{
j = open(input, O_RDONLY, 0);
if ((j = open(input, O_RDONLY, 0)) < 0)
{
perror("Couldn't open input file");
exit(0);
}
dup2(j, 0);
close(j);
}
if (out)
{
if ((i= creat(output , 0644)) < 0)
{
perror("Couldn't open the output file");
exit(0);
}
dup2(i, STDOUT_FILENO);
close(i);
}
execvp( args[0], args );
perror("failed to exec\n");
exit(2);
}
else // I'm the parent
{
// wait here
wait(&status); // wait for child to finish
free( args ); // free dynamic memory
}
}
char** parse( char* str )
{
char** args = malloc( 256 );
int i = 0;
args[i] = strtok( str, " " );
while( args[i] )
{
i++;
args[i] = strtok( NULL, " " );
}
return args;
}
void prompt(char* input)
{
printf("$ "); // print prompt
fgets( input, 255, stdin );
input[strlen(input)-1] = '\0'; // overwrite \n with \0
if ( strcmp( input, "exit" ) == 0 ) // shell command
exit(0);
}