Realloc using extern - c

Question

Hi im trying to figure out how to use realloc. I have made a small test project that im trying to malloc for the creation of the first element in main, followed by adding a number to that location then reallocating the array to hold one more element.
Main.c
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char *argv[]){
int i =0;
int *fileTest = (int*) malloc(sizeof(char * ));
int amtFiles=0;
for(i=0;i<3;i++){
parseInput(fileTest , amtFiles);
printf("%d", fileTest[1]);
}
}
parse.c
#include <stdlib.h>
#include "stddef.h";
#include <stdlib.h>
extern int parseInput(int *fileTest,int amtFiles){
printf("enter number");
scanf("%d",amtFiles);
fileTest = realloc(fileTest, (amtFiles * sizeof(char *)));
}
makefile
CC = gcc
CC_FLAGS = -g -ansi -pedantic -Wall -w
FILES = main.c parse.c
OUT_EXE = Test2
build: $(FILES)
$(CC) $(CC_FLAGS) -o $(OUT_EXE) $(FILES)
clean:
rm -f *.o core *.exe *~
rebuild: clean build

Answer 1

C is pass by value. You should return the pointer value. Otherwise the original one is unchanged. So from parseInput return the pointer or pass address of the variable and change it accordingly (not shown in example).
While reallocating use another pointer variable other than the original one, in case of failure you don't lose reference to the old memory.
Don't print uninitialized value. (in main()).
Don't cast the return value of malloc.
When printing a pointer do this printf("%p",(void*)fileTest[1]);.
Also you should put the extern declaration in main.c. You have passed incorrect number of arguments to the parseInput function. The code didn' compile.
Then after correcting the input - if you try then you might get segmentation fault or some similar error.
I have tried demonstrating something over here. The naming can be much more better. I have tried to keep the general aspects of what you have tried. Multiple source file with realloc used and tested, working with pointers etc.
main.c
#include <stdio.h>
#include <stdlib.h>
#include "myheader.h"
int main(void)
{
int *fileTest = NULL;
int amtFiles;
int weTried = 2;
while(weTried --> 0){
printf("%s\n", "Enter the array size.\n");
if( scanf("%d",&amtFiles) != 1){
fprintf(stderr, "%s\n","Error in input" );
exit(1);
}
else if( amtFiles <= 0){
fprintf(stderr, "%s\n", "Enter positive integral value");
exit(1);
}
fileTest = parseInput(fileTest, amtFiles);
if( fileTest == NULL){
fprintf(stderr, "%s\n","Realloc failed." );
exit(1);
}
for(int i = 0; i < amtFiles; i++){
printf("[%d]\n",fileTest[i]);
}
}
return 0;
}
parse.c
#include <stdlib.h>
#include <stdio.h>
#include "myheader.h"
int* parseInput(int *fileTest,int amtFiles)
{
if( amtFiles <= 0){
fprintf(stderr, "%s\n", "Error in number of inputs");
exit(1);
}
int *t = realloc(fileTest, (amtFiles * sizeof * t));
if( t == NULL ){
fprintf(stderr, "%s\n", "Execution exception");
return NULL;
}
fileTest = t;
for(int i = 0; i < amtFiles; i++){
fileTest[i] = 2017+i;
}
return fileTest;
}
myheader.h
extern int* parseInput(int *,int );
Compiled and run
gcc -Wall -Wextra -Werror main.c parse.c
./a.out