Returning from Signal Handlers

Question

Am I not leaving my signal handler function in the correct way? It does not seem to return to the program normally. Instead it goes into the loop and where it should wait for user input, it skips and reads the length of the "user input" to -1 and errors out. (Will make more sense in code.)

void handle_SIGINT() {

    int k = recent;
    int count = 0;
    int stop;

    if (stringSize >= 10) {
        stop = 10;
    }
    else {
        stop = p;
    }

    printf("\nCommand History:\n");

    for (count = 0; count < stop; count++) {

        if (k < 0) {
            k += 10;
        }
        printf("%s", string[abs(k)]);
        k -= 1;

    }

}



void setup(char inputBuffer[], char *args[],int *background)
{
    //char inputBuffer[MAX_LINE];
    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[] */
    int add = 1;
    ct = 0;

    /* read what the user enters on the command line */
    length = read(STDIN_FILENO, inputBuffer, MAX_LINE);  


        printf("%i",length);
    start = -1;
    if (length == 0)
        exit(0);            /* ^d was entered, end of user command stream */
    if (length < 0){
        perror("error reading the commanddddddddd");
        exit(-1);           /* terminate with error code of -1 */
    }
}



int main(void)
{
    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 */
    FILE *inFile = fopen("pateljay.history", "r");



    if (inFile != NULL) {
        int count = 0;
        char line[MAX_LINE];
        while (fgets(line,sizeof line, inFile) != NULL) {
            string[count] = strdup(line);
            //string[count][strlen(line)] = '\n';
            //string[count][strlen(line) + 1] = '\0';
            printf("%s", string[count]);
            count++;
            stringSize++;
        }


            p = count % 10;
            recent = abs(p - 1);

    }   

    fclose(inFile); 

    /* set up the signal handler */
    struct sigaction handler;
    handler.sa_handler = handle_SIGINT;
    sigaction(SIGINT, &handler, NULL);

    while (1) {/* Program terminates normally inside setup */


        background = 0;
        printf("COMMAND->");

        fflush(0);

        setup(inputBuffer, args, &background);/* get next command */
    }
}

So when ctrl+c is entered it should catch the signal and handle it. Once it returns back to main, it goes into setup and completely skips the read function and makes length equal to -1. This in turn errors out the program. I think the code inside handle_SIGINT is irrelevant as it is right now. Does anyone know any reason why it would be skipping the read function in setup?

Answer 1

read is blocking, waiting for input. SIGINT arrives. The kernel calls your signal handler. When your signal handler returns, the kernel makes read return -1 and set errno to EINTR. You need to check for this case and handle it by calling read again:

   do {
        length = read(STDIN_FILENO, inputBuffer, MAX_LINE);
   } while (length == -1 && errno == EINTR);

Answer 2

The signal handler is supposed to take an int argument:

void handle_sigint(int signum) {}