C - Multithreading count frequency of letters causes memory error

Question

I'm trying to use C multithreading to find out the frequency of each alphabet letter in a text file. Assignment is to: 1) write a function that read every single sentence in a text, ended by '.' 2) write a function that load a sentence in a bidimensional array 3) write a function that generates a pthread for every letter for every sentence (pthread function add 1 to a counter for that letter). EDIT: I figured out with Valgrind that the problem is in sentence function, by I dont understand why.

Here's the code:

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <pthread.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/types.h>

char alphabet[26] = "abcdefghijklmnopqrstuvwxyz";
int count[26];

char* sentence(char * s){
    char* p;
    char* q;
    char* arr;
    int i;
    p = s;
    q = malloc(100);
    arr = q;
    for (i=0; *p != '.'; i++){ 
        *q = *p;
        q++;
        p++; 
    }
    *q = '\0';
    return arr;
}

char** load_sentence(char* p, char** q, int i){
    q[i] = malloc(strlen(p)+1);
    strcpy(q[i], p);
    return q;
}

void* count_letter(void * s){
    char* p = (char*) s;
    int i;
    for (i=0; i<26; i++){
        if (*p == alphabet[i]){
            count[i]++;
        }
    }
}

void frequency(char* str){
    char* s = str;
    int i, j, l;
    l = strlen(str);
    pthread_t tid[l];
    for (i=0; i<l; i++){
        pthread_create(&tid[i], NULL, count_letter, (void*) s);
        s++;
    }
    for (j=0; j<l; j++){
        pthread_join(tid[j], NULL);
    }
}


int main(int argc, char* argv[]){

    int fd;
    char buff[100];
    fd = open(argv[1], O_RDONLY);
    char ** text = malloc(10*sizeof(char*));
    read(fd, buff, sizeof(buff));
    char* start = buff;
    int i = 0; //number of phrases!
    char* p = NULL;

    while (*(p = sentence(start)) != '\0'){
        text = load_sentence(p, text, i);
        start += strlen(p)+1;
        i++;
   }

   int j, k;

   for (k=0; k<i; k++){
        frequency(text[k]);
   }

   for (j=0; j<26; j++){
        printf("%c : %d times\n", alphabet[j], count[j]);
   }
}

It looks like that with cases like this: hope it's a good reading. bye. The output is correct:

a : 2 times
b : 1 times
c : 0 times
d : 2 times
e : 3 times
f : 0 times
g : 3 times
h : 1 times
i : 2 times
j : 0 times
k : 0 times
l : 0 times
m : 0 times
n : 1 times
o : 3 times 
p : 1 times
q : 0 times
r : 1 times
s : 1 times
t : 1 times
u : 0 times
v : 0 times
w : 0 times
x : 0 times
y : 1 times
z : 0 times

With others, a "memory error", that begins with free() : invalid next size (normal). The error has many lines of memory map and ends with abortion.

I'm quite new to C, sorry for my inexperience.

Is it necessary to introduce a mutex in this case?

Answer 1

Erika,

Since I don't really know your assignment please see this as just another way out of a 1000 to count characters. I have not checked it for bugs, rewrite to your needs. Anyhow this is how I would have solved it. If memory is sparse I would read character by character from the file until ".". Anyhow hope it helps you and you get great grades :-)...

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <pthread.h>
#include <stdatomic.h>

#define MAX_THREADS 100
atomic_int threadCount;
#define NCHAR 26
char alphabet[NCHAR] = "abcdefghijklmnopqrstuvwxyz";
atomic_int count[NCHAR];


void* count_letter(void * s){
    threadCount++;
    char* p = (char*) s;
        for (int i=0; i<NCHAR; i++)
            if (*p == alphabet[i])
                count[i]++;
    threadCount--;
    return NULL;
}

int main(int argc, char* argv[]){

    //Init variables
    FILE *file;
    char *myText;
    unsigned long fileLen;
    int deadLockGuard=0;
    threadCount=0;

    //Open the file
    file = fopen(argv[1], "rb");
    if (!file) {
        fprintf(stderr, "Unable to open file %s", argv[1]);
        return EXIT_FAILURE;
    }
    fseek(file, 0, SEEK_END);
    fileLen=ftell(file);
    rewind(file);

    //reserve memory and read the file
    myText=(char *)malloc(fileLen+1);
    if (!myText) {
        fprintf(stderr, "Memory error!");
        fclose(file);
        return EXIT_FAILURE;
    }
    fread(myText, fileLen, 1, file);
    fclose(file);

    //Get each sentence ending with a . and then for each character look at the count for each character in it's own thread.
    char *subString = strtok(myText, "."); //This is your sentence/load_sentence method
    while (subString != NULL) {
        for (int v = 0;v<strlen(subString);v++) { //This is your frequency method
        deadLockGuard=0;
        while (threadCount >= MAX_THREADS) {
            usleep(100); //Sleep 0.1ms
            if(deadLockGuard++ == 10000) {
                printf("Dead-lock guard1 triggered.. Call Bill Gates for help!"); //No free threads after a second.. Either the computer is DEAD SLOW or we got some creepy crawler in da house.
                return EXIT_FAILURE;
            }
        }

        pthread_t tid; //Yes you can overwrite it.. I use a counter to join the workers.
        pthread_create(&tid, NULL, count_letter, (void*) subString+v);
    }
        subString = strtok(NULL, ".");
    }
    deadLockGuard=0;
    //pthread_join all the still woring threads
    while (threadCount) {
        usleep(1000); //sleep a milli
        if(deadLockGuard++ == 2*1000) {
            printf("Dead-lock guard2 triggered.. Call Bill Gates for help!"); //Threads are running after 2 seconds.. Exit!!
            return EXIT_FAILURE;
        }
    }
    //Garbage collect and print the results.
    free(myText);
    for (int j=0; j<NCHAR; j++)
        printf("%c : %d times\n", alphabet[j], count[j]);
    return EXIT_SUCCESS;
}

Answer 2

Your previous version with mutex had undefined behaviour because you initialized mutex multiple times, according to reference:

Attempting to initialize an already initialized mutex results in undefined behavior.

You are accesing count concurrently, so you have to use mutex to make thread-safe code. You called pthread_mutex_init in count_letter it is incorrect, this function is the body of your thread (multiple initialization of mutex without destroying it leads to UB), you should call pthread_mutex_init only once, for instance as first line in main function:

int main() {
 pthread_mutex_init(&mtx,NULL);

before return add

 pthread_mutex_destroy(&mtx);

Critical section in your count_letter function is line

count[i]++;

you should modify it as follows

pthread_mutex_lock(&mtx);
count[i]++;
pthread_mutex_unlock(&mtx);

---

Now, return to sentence implementation, you need to check if *p doesn't point to null terminator before comparing with .:

for (i=0; *p && *p != '.'; i++){ 
          ^^ added

without testing it, \0 != . returns true and your loop continues ...