Segmentation fault caused by fgets() in C - c

Question

This is a problem I don't understand - I am using fgets() in main and it works. I use it (I think) in exactly the same way in a function and I get an error [Segmentation fault core dumped -- exit code 139).
This code is based on a sample program in book Ivor Horton's "Beginning C" (it's a old tile but I'm just wanting to learn the basics from it).
My program is as follows. I am working on *nix using Geany (basically, compiling with GCC). You can see that fgets works in main (output is the string you enter). But it doesn't work in the function str_in(). It gets as far as the second printf() statement to enter a string, no further. Note that in the book, Horton uses gets(). I am trying to implement a safer string input function here, but no joy.
By the way the program is supposed to sort strings stored in an array of string pointers.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define TRUE 1
#define FALSE 0
#define MAX_NUM_STRINGS 50
int str_in(char **); /*Pointer to a string pointer*/
void str_sort(char *[], int n); /*Array of pointers to strings, number of strings in array*/
void str_out (char *[], int n); /*Array of pointers to strings, number of strings in array*/
int main(){
char *pS[MAX_NUM_STRINGS] = { NULL }; /*Array of pointers to strings stored in str_space*/
int numStrings = 0; /*Count of strings*/
char buffer[BUFSIZ];
printf("Enter a string\n");
fgets(buffer, BUFSIZ, stdin);
printf("%s", buffer);
printf("fgets works here\n\n");
/* get string input from user - a pointer to each string is saved in pS */
while ( str_in(&pS[numStrings]) && numStrings < MAX_NUM_STRINGS)
numStrings++;
if ( numStrings > 0 ){
str_sort(pS, numStrings);
str_out(pS, numStrings);
}
return 0;
}
int str_in(char** pString){
char buffer[BUFSIZ];
char *p;
printf ("Enter string:\n");
fgets(buffer, 60, stdin);
printf("fgets doesn't work here!!\n");
if( buffer != NULL ){
printf("here");
if ((p = strchr(buffer, '\n')) != NULL)
*p = '\0'; /*replace newline with null character*/
else
return FALSE;
if ( strlen(buffer) > 0 ){
strcpy(*pString, buffer);
return TRUE;
}
else
return FALSE; /*blank line - end of input*/
}
else
return FALSE;
}
void str_sort(char* pStrings[], int n){
/*sort strings by manipulating array of string pointers*/
char *temp;
int sorted = FALSE;
int i = 0;
while (!sorted){
sorted = TRUE;
for(i = 0; i < n - 1; i++){
temp = pStrings[i];
if ( strcmp(temp, pStrings[i+1]) > 1 ){
pStrings[i] = pStrings[i+1];
pStrings[i+1] = temp;
sorted = FALSE;
break;
}
}
}
}
void str_out(char* pStrings[], int n){
/*print strings to standard output. Free memory as each string is printed */
int i = 0;
printf("Sorted strings:\n");
for(i = 0; i < n; i++){
printf("%s", pStrings[i]);
free(pStrings[i]);
}
}

Answer 1

The segmentation fault is not caused by fgets(), but by strcpy():
strcpy(*pString, buffer);
You try to write to *pString, but you never allocate memory for it. pS in main() is just an array of null pointers.
Another thing is with the test with if( buffer != NULL ), that would never be true since buffer is an array, not a pointer.

Answer 2

You must check for the return value of fgets to see if you have successfully received something, if not then you should never use your buffer as a string as you are not NUL terminating the buffer.
/* Checking for buffer != NULL is of no use */
/* as buffer will always be not NULL since */
/* since you have allocated it as char buffer[BUFSIZ] */
if (fgets(buffer, BUFSIZ, stdin) == NULL) {
/* buffer may not be a valid string */
}
So what you can do it to initialize the buffer to a NUL string, as soon as you enter the function (after your declarations are done
buffer[0] = 0; /* initialize to NUL string */
now you can use buffer as a string anywhere.
Also note than if BUFSIZ is too big greater than a couple of KB, then your might get seg fault due to stack overflow. If they are too big you could make buffer as "static char" instead of "char".