Can unexecuted code cause a segfault? - c

Question

Yeah I know that sounds crazy but that is the only way I can describe it at this point. I’m writing a program for a class that mimics a terminal, in that it takes commands as inputs and executes them. (I’ll put some code below) As you will see, the program holds a history of commands historyArgs so that the user can execute recent commands.
Command history is listed when the user performs Ctrl-C. Recent commands are accessed with the command 'r' (for most recent) and r 'x' (where x is matches the first letter of a command in recent history). When I started implementing the 'r' command, I started getting this segfault. I then reverted all my changes and added one line at a time. I found that adding even primitive variable declaration causes a segfault (int temp = 10;) But this is where it gets stranger. I believe the line that causes a segfault (int temp = 10;) is never accessed. I put printf statements and flush the output at the beginning of the if block to see if the block has been entered, but they don't execute.
setup was provided for us. It takes the user input and puts it in char *args[] i.e. input = ls -a -C, args = {"ls", "-a", "-C", NULL, ... NULL}. I marked the line in main that somehow leads to a segfault.
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <signal.h>
#include <string.h>
#define BUFFER_SIZE 50
static char buffer[BUFFER_SIZE];
#define MAX_LINE 80 /* 80 chars per line, per command, should be enough. */
char *historyArgs[10][MAX_LINE/2 + 1];
int historyCount = 0;
int indexOfLatestCommand = 0;
/* the signal handler function */
void handle_SIGINT() {
//write(STDOUT_FILENO,buffer,strlen(buffer));
if(historyCount > 0){
printf("\n%i command(s), printing most recent:\n", historyCount);
int i;
for(i = 0; i < historyCount && i < 10; i++){
printf("%i.] %s ", i+1, historyArgs[i][0]);
//print args
int j = 1;
while(historyArgs[i][j] != NULL){
printf("%s ", historyArgs[i][j]);
j++;
}
printf("\n");
}
}
else{
printf("\nNo recent commands.\n");
}
fflush(stdout);
}
/**
* setup() reads in the next command line, separating it into distinct tokens
* using whitespace as delimiters. setup() sets the args parameter as a
* null-terminated string.
*/
void setup(char inputBuffer[], char *args[],int *background)
{
int length, /* # of characters in the command line */
i, /* loop index for accessing inputBuffer array */
start, /* index where beginning of next command parameter is */
ct; /* index of where to place the next parameter into args[] */
ct = 0;
/* read what the user enters on the command line */
length = read(STDIN_FILENO, inputBuffer, MAX_LINE);
start = -1;
if (length == 0)
//exit(0); /* ^d was entered, end of user command stream */
if (length < 0){
perror("error reading the command");
exit(-1); /* terminate with error code of -1 */
}
/* examine every character in the inputBuffer */
for (i = 0; i < length; i++) {
switch (inputBuffer[i]){
case ' ':
case '\t' : /* argument separators */
if(start != -1){
args[ct] = &inputBuffer[start]; /* set up pointer */
ct++;
}
inputBuffer[i] = '\0'; /* add a null char; make a C string */
start = -1;
break;
case '\n': /* should be the final char examined */
if (start != -1){
args[ct] = &inputBuffer[start];
ct++;
}
inputBuffer[i] = '\0';
args[ct] = NULL; /* no more arguments to this command */
break;
case '&':
*background = 1;
inputBuffer[i] = '\0';
break;
default : /* some other character */
if (start == -1)
start = i;
}
}
args[ct] = NULL; /* just in case the input line was > 80 */
}
int main(void)
{
int i;
char inputBuffer[MAX_LINE]; /* buffer to hold the command entered */
int background; /* equals 1 if a command is followed by '&' */
char* args[MAX_LINE/2+1];/* command line (of 80) has max of 40 arguments */
int status;
struct sigaction handler;
handler.sa_handler = handle_SIGINT;
sigaction(SIGINT, &handler, NULL);
strcpy(buffer,"Caught <ctrl><c>\n");
while (1){ /* Program terminates normally inside setup */
background = 0;
printf("COMMAND->");
fflush(0);
setup(inputBuffer, args, &background); /* get next command */
//If command wasn't empty
if(args[0] != NULL){
if(strcmp(args[0], "r") != 0){
//Copy into history if not a recent call
for(i = 0; i < MAX_LINE/2+1 && args[i] != NULL; i++){
historyArgs[historyCount%10][i] = malloc(strlen(args[i]));
strcpy(historyArgs[historyCount%10][i], args[i]);
}
indexOfLatestCommand = historyCount%10;
historyCount++;
}
//Create child process
int pid = fork();
//In child process
if(pid == 0){
if(strcmp(args[0], "r") == 0){
//If only "r" was entered, execute most recent command
if(args[1] == NULL){
printf("Entering recent?\n");
fflush(stdout);
int temp = 10; //SEGFAULTS HERE IF THIS IS INCLUDED
execvp(historyArgs[indexOfLatestCommand][0], &historyArgs[indexOfLatestCommand][0]);
}
else{
//Find in args[1][0] history, run if found
for(i = indexOfLatestCommand; i >= 0; i--){
if(historyArgs[i][0][0] == args[1][0]){
execvp(historyArgs[i][0], &historyArgs[i][0]);
break;
}
}
if(i == -1){
for(i = historyCount > HISTORY_SIZE ? HISTORY_SIZE : historyCount; i > indexOfLatestCommand; i--){
if(historyArgs[i][0][0] == args[1][0])
execvp(historyArgs[i][0], &historyArgs[i][0]);
break;
}
}
}
}
else execvp(args[0], &args[0]);
}
//In parent process
else if (pid > 0){
/*If child isn't a background process,
wait for child to terminate*/
if(background == 0)
while(wait(&status) != pid);
}
}
}
}
Another thing worth mentioning is that declaring a variable in that spot doesn't cause a segfault. Only assigning a value to a new variable does. Reassigning globals in that section also doesn't cause a segfault.
EDIT: What triggers a crash. Commands execute correctly. When you run it, you can type in any command, and that should work. It isn't until I perform Ctrl-C and print out the history that the program segfaults.
Example input:
ls
ls -a
grep
Ctrl-C
HEADS UP: if you decide to run this, know that to end the task, you will probably need to use the kill command because I haven't implement "q" to quit.

Answer 1

Symptoms like you see (unrelated code changes appear to affect the nature of a crash) usually mean that your program caused undefined behaviour earlier. The nature of the behaviour changes because your program is relying on garbage values it has read or written at some stage.
To debug it, try and remove all sources of undefined behaviour in your program. The most obvious one is the content of your void handle_SIGINT() function. The only things you can portably do in a signaler are:
Set a variable of type volatile sig_atomic_t, or other lock-free type, and return
Do other stuff and call _Exit, abort or similar.
Especially, you cannot call any library functions as they may not be re-entrant.
For a full specification see section 7.14.1 of the current C Standard. If you are also following some other standard, e.g. POSIX, it may specify some other things which are permitted in a signal handler.
If you do not intend to exit then you must set a flag , and then test that flag from your main "thread" later to see if a signal arose.