(char*)malloc(sizeof(char)) causing segmentation fault, how? - c

Question

The code works fine on most inputs, but for userID's whih are very long I get a segmentation fault. My question is, how can malloc cause a segmentation fault? simply allocating memory shouldn't cause this. I found the problem area by using printf() statements, it seem the malloc within my read_line() function is where the problem is because the second "read_line" does not print, but the first before the malloc does.
thank you.
- Chris
#include <stdio.h>
#include <ctype.h>
#include <string.h>
#include <stdlib.h>
#define DELIM " " /* the delimiter */
#define MAX_CHANGE (10.0/86400.0) /* 10kg/day */
/* seconds in a day is 24 hours * 60 minutes * 60 seconds */
#define MEM_OUT printf("%s","out of memory");
/* Defines Node structure. */
struct Node{
char *id;
float weight;
int time;
struct Node *next;
} *head, *p, *t, *last;
/* Constructor which returns a pointer to a new node. */
struct Node * new_node(int time, char *id, float w)
{ /*note malloc returns a pointer */
struct Node *node = (struct Node *)malloc(sizeof(struct Node));
node->time = time;
node->id = (char *)malloc( (strlen(id) + 1) * sizeof(char));
strcpy(node->id, id); //duplicate the id, so new node has own copy.
node->weight = w;
node->next = NULL;
return node;
}
/* reads in line of characters until either a EOF or '\n' is encountered
then places a the terminator '\0' at the end */
char * read_line(FILE *stream)
{
printf("read_line");
char * temp = (char*)malloc(sizeof(char));
printf("read_line");
char * line = (char*)malloc(sizeof(char));
char c;
*line = '\0';
int i = 1;
//strchr()
while( (c = getc(stream)) != EOF && c != '\n')
{
//if(c == EOF) return NULL;
//realloc(line,++i);
strcpy(temp,line);
line = malloc(++i * sizeof(char));
strcpy(line,temp);
temp = malloc(i * sizeof(char));
*(line + (i-1)) = '\0';
*(line + (i-2)) = c;
}
free(temp);
if( i == 1) return NULL;
return line;
}
main() {
int lasttime = 0, timestamp, duration, tokens;
char * userID = NULL;
char * lastuserID = NULL;
char * line = NULL;
float weight,lastweight,change,changePerTime;
head = new_node(0,"",0.0);
last = head;
FILE *fp = fopen("C:\\Users\\chris\\Desktop\\School\\York\\cse\\2031 Software Tools\\Labs\\6\\input.txt","r");
while( (line = read_line(fp)) != '\0') {
printf("%s\n",line);
//free(userID);
line = strtok(line, " \n");
if (line == NULL || sscanf(line,"%d",&timestamp) < 1 || timestamp == 0){
printf("%s\n","Invalid time");
continue;
}
line = strtok(NULL, " \n");
if(line == NULL || isdigit(line[0]) || line[0] == '.') {
printf("Illegal userID");
//free(line);
continue;
}
userID = (char * )malloc( (strlen(line)+1) * sizeof(char));
strcpy(userID,line);
strcat(userID," ");
do{
line = strtok(NULL," \n");
if(line != NULL && !isdigit(line[0]) && line[0] != '.'){
strcat(userID,line ); // adds ' ' and '\0'
strcat(userID," ");
}
}while(line != NULL && line[0] != '.' && !isdigit(line[0]) );
userID[strlen(userID)-1] = '\0'; //erases the tailing space.
if(strlen(userID) > 179){
printf("Illegal userID\n");
printf("mid");
continue;
printf("%s\n","after" );
}
if(line != NULL)
tokens = sscanf(line,"%f", &weight);
if(line == NULL || tokens < 1 || weight < 30.0 || weight > 300.0)
{printf("Illegal weight\n"); continue; }
if (lasttime >= timestamp){
printf("Nonmonotonic timestamps\n");
continue;
}
lasttime = timestamp;
// record is valid apst this point.
/* t = last occurence of this userID, p = last userID*/
for(p = head, t = NULL; p != NULL; p = p->next)
{
if(strcmp(userID,p->id) == 0)
t=p;
last = p; // set last to last p.
}
if(t == NULL)
printf("OK newuser\n");
else if(t != NULL)
{
duration = timestamp - t->time;
change = weight - t->weight;
changePerTime = change / duration;
if(changePerTime < -MAX_CHANGE || changePerTime > MAX_CHANGE)
printf("Suspiciously large weight change\n");
else
printf("OK\n");
}
/* add new node to end of list */
last->next = new_node(timestamp,userID,weight);
/* update lastnode */
last = last->next;
free(line);
}
fclose(fp);
/* count sum of id's for last valid user*/
int count=0;
for(p = head->next; p !=NULL; p=p->next)
{
if(strcmp(last->id,p->id) == 0)
count++;
}
//fclose(f); // use if input from file is uncommented
// adds 1 to both demensions to hole axis
int tHeight = 11;
int tWidth = count + 1;
int qHeight = 10;
int qWidth= count;
/* builds the barchart */
char bc[tHeight][tWidth]; // + 1 for y-axis
/* draws axis and orgin */
int a,b;
for(a=0; a<tHeight; a++)
{
for(b=0;b<tWidth; b++)
{
if(a == qHeight && b == 0)
bc[a][b] = '+';
else if(a < tHeight && b == 0)
bc[a][b] = '|';
else if(a == qHeight && b > 0)
bc[a][b] = '-';
}
}
/* prints the bars */
int j=1, i, k, bh;
for(p = head; p != NULL, j < tWidth; p=p->next)
{
if(strcmp(last->id,p->id) == 0)
{
for(i = 9, k=0, bh = (int)(p->weight / 30);i >= 0; i--)
{
if(k < bh)
{
bc[i][j] = '*';
k++; // barheight
}
else
bc[i][j] = ' ';
}
j++;
}
}
/* prints the barchart */
int m, n;
for(m=0; m < tHeight; m++)
{
for(n=0; n < tWidth; n++)
{
printf("%c",bc[m][n]);
}
printf("%c",'\n');
}
}

Answer 1

The malloc calls are not causing a segmentation fault. But your use of them later on could be.
Some Items of Note
Your printf("read line") statements will not print out immediately when called because the output is buffered. If you want them to print right away, do printf("read line\n"). You'll then see that both execute and your code that uses the tiny buffer you allocated will cause the crash.
In your while loop, you are doing more malloc calls and assigning the returns to variables, like temp and line, without freeing the prior memory pointers that temp and line hold, thus causing some memory leaks. Your commented out realloc was the better thought process: line = realloc(line, ++i * sizeof(*line));. Similarly for temp.
Memory Allocation Problem
One very problematic area is here:
userID = (char * )malloc( (strlen(line)+1) * sizeof(char));
strcpy(userID,line);
strcat(userID," ");
userID can hold the length of the string in line (strlen(line)) plus one more byte. But that one more byte is needed for the null terminator. Your strcat(userID, " ") will write past the length of the allocated buffer for userID by one byte.

Answer 2

I solved the problem! The issue wasn't the read_line function at all, it was the memory allocation for the userID string. Moving the malloc() for the user to the beginning of the loop fixed the problem.
The amount of memory allocated for the userID portion of the line was based on the the length of the whole line. eg: malloc(strlen(line)+2). However this was done after calling strtok() on the line a few times, which would allocate a memory block shorter than the length of the entire line. This is because strtok() places null terminators '\0' 's at every instance of the specified delimiter in the line, and strlen() only counts the length from the passed character pointer to the first '\0' it encounters.
Anyway, thanks for your help guys!
-Chris
#include <stdio.h>
#include <ctype.h>
#include <string.h>
#include <stdlib.h>
#define DELIM " " /* the delimiter */
#define MAX_CHANGE (10.0/86400.0) /* 10kg/day */
/* seconds in a day is 24 hours * 60 minutes * 60 seconds */
/* Defines Node structure. */
struct Node{
char *id;
float weight;
int time;
struct Node *next;
} *head, *p, *t, *last;
/* Constructor which returns a pointer to a new node. */
struct Node * new_node(int time, char *id, float w)
{ /*note malloc returns a pointer */
struct Node *node = (struct Node *)malloc(sizeof(struct Node));
node->time = time;
node->id = malloc(strlen(id) + 1);
strcpy(node->id, id); //duplicate the id, so new node has own copy.
node->weight = w;
node->next = NULL;
return node;
}
/* reads in line of characters until either a EOF or '\n' is encountered
then places a the terminator '\0' at the end */
char * read_line(FILE *in)
{
int i = 1;
char * s = NULL;
char c;
do{
s = realloc(s,i); //strlen does not work on NULL strings
if(s == NULL || s == "")
{
printf("%s\n","out of memory");
exit(1);
}
*(s + (i-1)) = '\0'; // ensures null terminated
if(i > 1)
*(s + (i-2)) = c;
i++;
}
while( (c = getc(in)) != EOF && c != '\n' );
if (c == '\n')
return s;
else if(c == EOF)
return NULL;
}
main() {
int lasttime = 0, timestamp, duration, tokens;
char * userID = NULL;
char * lastuserID = NULL;
char * line = NULL;
float weight,lastweight,change,changePerTime;
head = new_node(0,"",0.0);
last = head;
FILE *fp = fopen("C:\\Users\\chris\\Desktop\\School\\York\\cse\\2031 Software Tools\\Labs\\6\\tests\\04.in","r");
while((line = read_line(fp)) != NULL) {
userID = malloc(strlen(line)+2); // max userID length is line length**
line = strtok(line, " \n");
if (line == NULL || sscanf(line,"%d",&timestamp) < 1 || timestamp == 0){
printf("%s\n","Invalid time");
continue;
}
line = strtok(NULL, " \n");
if(line == NULL || isdigit(line[0]) || line[0] == '.') {
printf("%s\n","Illegal userID");
//free(line);
continue;
}
strcpy(userID,line);
strcat(userID," ");
do{
line = strtok(NULL," \n");
if(line != NULL && !isdigit(line[0]) && line[0] != '.'){
strcat(userID,line ); // adds ' ' and '\0'
strcat(userID," ");
}
}while(line != NULL && line[0] != '.' && !isdigit(line[0]) );
userID[strlen(userID)-1] = '\0'; //erases the tailing space.
if(strlen(userID) > 179){
printf("Illegal userID\n");
free(userID);
free(line);
continue;
}
if(line != NULL)
tokens = sscanf(line,"%f", &weight);
if(line == NULL || tokens < 1 || weight < 30.0 || weight > 300.0)
{printf("Illegal weight\n"); continue; }
if (lasttime >= timestamp){
printf("Nonmonotonic timestamps\n");
continue;
}
lasttime = timestamp;
// record is valid apst this point.
/* t = last occurence of this userID, p = last userID*/
for(p = head, t = NULL; p != NULL; p = p->next)
{
if(strcmp(userID,p->id) == 0)
t=p;
last = p; // set last to last p.
}
if(t == NULL)
printf("OK newuser\n");
else if(t != NULL)
{
duration = timestamp - t->time;
change = weight - t->weight;
changePerTime = change / duration;
if(changePerTime < -MAX_CHANGE || changePerTime > MAX_CHANGE)
printf("Suspiciously large weight change\n");
else
printf("OK\n");
}
/* add new node to end of list */
last->next = new_node(timestamp,userID,weight);
/* update lastnode */
last = last->next;
free(line);
} // end of input loop
fclose(fp);
/* count sum of id's for last valid user*/
int count=0;
for(p = head->next; p !=NULL; p=p->next)
{
if(strcmp(last->id,p->id) == 0)
count++;
}
//fclose(f); // use if input from file is uncommented
// adds 1 to both demensions to hole axis
int tHeight = 11;
int tWidth = count + 1;
int qHeight = 10;
int qWidth= count;
/* builds the barchart */
char bc[tHeight][tWidth]; // + 1 for y-axis
/* draws axis and orgin */
int a,b;
for(a=0; a<tHeight; a++)
{
for(b=0;b<tWidth; b++)
{
if(a == qHeight && b == 0)
bc[a][b] = '+';
else if(a < tHeight && b == 0)
bc[a][b] = '|';
else if(a == qHeight && b > 0)
bc[a][b] = '-';
}
}
/* prints the bars */
int j=1, i, k, bh;
for(p = head; p != NULL, j < tWidth; p=p->next)
{
if(strcmp(last->id,p->id) == 0)
{
for(i = 9, k=0, bh = (int)(p->weight / 30);i >= 0; i--)
{
if(k < bh)
{
bc[i][j] = '*';
k++; // barheight
}
else
bc[i][j] = ' ';
}
j++;
}
}
/* prints the barchart */
int m, n;
for(m=0; m < tHeight; m++)
{
for(n=0; n < tWidth; n++)
{
printf("%c",bc[m][n]);
}
printf("%c",'\n');
}
}