where is the pthread segfault happening? - c

Question

in my program, I provide a directory which contains text files. Each of the text files contain a few hundred lines in the following format
Username,Password,BloodType,Domain,Number
I then create a thread for each file in the directory which will merge-sort(by number) these lines into the array char* text_lines[6000];
I can't figure out why I'm getting a segmentation fault because I'm getting different output on every run.
Heres my code:
#include <string.h>
#include <stdlib.h>
#include <stdio.h>
#include <pthread.h>
#include <sys/types.h>
#include <dirent.h>
#include <string.h>
void store_line(char* line);
void* my_merge_sort(void* file);
char** text_lines;
int main(int argc, char* argv[])
{
if(argc != 2)
{
fprintf(stderr, "usage: ./coolsort <directory>\n");
}
else
{
text_lines = malloc(6000 * sizeof(char*));
DIR* the_directory;
int filecount = 0;
struct dirent* directory_files[50];
if((the_directory = opendir(argv[1])) != NULL)
{
//make a list of the files in the directory
while((directory_files[filecount++] = readdir(the_directory))) ;
filecount--;
//<<<DEBUGGING INFO>
int i;
fprintf(stderr,"there are %i files in %s:\n", filecount, argv[1]);
for(i = 0; i < filecount; i++)
{
fprintf(stderr, "%s\n",directory_files[i]->d_name);
}
char cwd[512];
chdir(argv[1]);
getcwd(cwd, sizeof(cwd));
fprintf(stderr, "the CWD is: %s\n", cwd);
//<DEBUGGING INFO>>>
//lets start some threads
pthread_t threads[filecount-2];
int x = 0;
for(i = 0; i < (filecount); i++ )
{
if (!strcmp (directory_files[i]->d_name, "."))
continue;
if (!strcmp (directory_files[i]->d_name, ".."))
continue;
pthread_create(&threads[x++], NULL, my_merge_sort, (void*)directory_files[i]->d_name);
}
//do stuff here
//
}
else
{
fprintf(stderr, "Failed to open directory: %s\n", argv[1]);
}
}
}
void* my_merge_sort(void* file)
{
fprintf(stderr, "We got into the function!\n");
FILE* fp = fopen(file, "r");
char* buffer;
char* line;
char delim[2] = "\n";
int numbytes;
//minimize I/O's by reading the entire file into memory;
fseek(fp, 0L, SEEK_END);
numbytes = ftell(fp);
fseek(fp, 0L, SEEK_SET);
buffer = (char*)calloc(numbytes, sizeof(char));
fread(buffer, sizeof(char), numbytes, fp);
fclose(fp);
//now read the buffer by '\n' delimiters
line = strtok(buffer, delim);
fprintf(stderr, "Heres the while loop\n");
while(line != NULL)
{
store_line(line);
line = strtok(buffer, NULL);
}
free(buffer);
}
void store_line(char* line)
{
//extract the ID.no, which is the fifth comma-seperated-token.
char delim[] = ",";
char* buff;
int id;
int i;
strtok(line, delim);
for(i = 0; i < 3; i++)
{
strtok(line, NULL);
}
buff = strtok(line, NULL);
id = atoi(buff);
//copy the line to text_lines[id]
memcpy(text_lines[id], line, strlen(line));
}
edit: I checked to make sure that it would fit into the initial array, and found that the highest ID is only 3000;

Answer 1

You use of strtok() is wrong:
line = strtok(buffer, NULL);
should be
line = strtok(NULL, delim);
Another mistakes should be fixed similarly.
The elements of text_lines are uninitialized:
text_lines = malloc(6000 * sizeof(char*));
this allocated 6000 pointers to char, but none of these pointers are initialized.