getting free(): invalid size when executing code - c

Question

compiling with gcc hw2.c -o x -lpthread
free(): invalid size
free(): invalid size
free(): invalid size
nano infile.txt
I'm assuming it has something to do with the file pointer maybe? the semaphores have been commented out to fix this issue first. All answers point to the pointer but switching things up isn't much help. I've tried compiling differently that doesn't help either.
#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
#include <semaphore.h>
#include <fcntl.h>
#include <sys/stat.h>
//sem_t X;
void process(){
//sem_open("X", O_CREAT,0777,0);
int ret;
int N = 1;
pid_t pid;
FILE* infile = fopen ("infile.txt", "r");
for(int i = 0; i< 50; i++){
fscanf (infile, "%d", &N);
fclose (infile);
printf("N: %d Process ID: %d",N,pid);
infile = fopen("infile.txt", "w");
N++;
//sem_post(&X);
fprintf(infile,"%d",N);
fflush(infile);
fclose(infile);
}
printf("\n");
int c;
//sem_getvalue(&X,&c);
printf(" \n \n \n%d",c);
}
int main(){
int pid, pid1, pid2;
pid = fork();
if(pid == 0){
//child1, Last
printf("Starting Process C: ");
process();
}
else{
pid1 = fork();
if(pid1 == 0){
//child2, Middle
printf("Starting Process B: ");
process();
}
else{
pid2 = fork();
if(pid2 == 0){
//child 3, First
printf("Starting Process A: ");
process();
}
else{
}
}
}
//sem_close(&X);
//sem_unlink(&X);
}

Answer 1

The most obvious problem is in your loop. I've removed everything but the fopen and fclose calls:
FILE* infile = fopen ("infile.txt", "r");
for(int i = 0; i< 50; i++){
fclose (infile);
infile = fopen("infile.txt", "w");
fclose(infile);
}
As you probably now see, when i is 1, you try to fclose(infile) - but that isn't open, hence the errors you get.
You need to move the first fopen into the loop - and check that opening the file and reading from it succeeds too:
#include <fcntl.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/wait.h>
void process() {
int N = 1;
for(int i = 0; i < 50; i++) {
FILE* infile = fopen("infile.txt", "r");
if(infile) {
bool readok = fscanf(infile, "%d", &N) == 1;
fclose(infile);
if(readok) { /* only do this if a value was read from the file ok */
printf("N: %d Process ID: %d", N, getpid());
infile = fopen("infile.txt", "w");
if(infile) {
N++;
fprintf(infile, "%d", N);
fflush(infile);
fclose(infile);
}
}
}
}
printf("\n");
exit(0); /* terminate this sub process */
}
int main() {
const size_t kPids = 3;
pid_t pids[kPids]; /* simplify keeping a number of background processes */
for(size_t i = 0; i < kPids; ++i) {
pids[i] = fork();
if(pids[i] == 0) {
printf("Starting Process %c:\n", (char)('A' + i));
process();
}
}
/* wait for children to finish */
pid_t pid;
int wstatus;
while((pid = wait(&wstatus)) != -1) {
printf("pid %d is done with status %d\n", pid, wstatus);
}
}

Answer 2

Your loop is problematic
FILE* infile = fopen ("infile.txt", "r"); //1
for(int i = 0; i< 50; i++){
fscanf (infile, "%d", &N);
fclose (infile); //2
printf("N: %d Process ID: %d",N,pid);
infile = fopen("infile.txt", "w"); //3
N++;
//sem_post(&X);
fprintf(infile,"%d",N);
fflush(infile);
fclose(infile); //4
}
You open the file at 1, then enter the loop, close it at 2, re-open it at 3 and re-close it at 4. In the next iteration, when you attempt to close at 2, you would hit into a double-free as it's already been closed at 4.