free(): invalid next size (normal) while executing free - c

Question

I am learning c programming. Below program is showing me the output, But when it execute free method . It is give me error:- free(): invalid next size (normal) . Please let me know what i am missing.
#include<stdio.h>
#include<stdlib.h>
int main() {
FILE *fp;
char r[1024];
fp = popen("/bin/ls /etc/", "r");
if (fp == NULL) {
perror("Failed to run the command");
exit(-1);
}
int totallengthread = 0, alloc_size = 1024;
char *buffer = (char*) calloc(alloc_size , sizeof(char));
int lenofbuff = 0;
while((lenofbuff=fread(r,sizeof(char),1024,fp))>0){
totallengthread += lenofbuff;
if (totallengthread >= alloc_size) {
alloc_size += 1024;
buffer = realloc(buffer, alloc_size);
}
concat(buffer, r);
}
printf("this is the output =>%s", buffer);
pclose(fp);
free(buffer);
return 0;
}
void concat(char *dest, const char *source) {
char *d = dest;
char *s = source;
while (*d != '\0') {
d++;
}
while (*s != '\0') {
*d++ = *s++;
}
*d = '\0';
}

Answer 1

In modern C, routines must be declared before they are used. Either move the definition of concat before main or insert a declaration of concat before main.
Change int main() to int main(void).
fread does not add a null terminator to the data read. Change char r[1024]; to char r[1025]; and, after fread, insert r[lenofbuff] = '\0'; as the first statement inside the while body.
if (totallengthread >= alloc_size) does not account for the null terminator. Change it to if (totallengthread+1 >= alloc_size)`.
In concat, change char *s = source; to const char *s = source;.
Turn on compiler warnings and pay attention to them. They should have warned you about 1 and 5 above.
After char *buffer = (char*) calloc(alloc_size, sizeof(char));, test buffer == NULL. If it is, print an error and exit. Also, a better form for this statement is char *buffer = calloc(alloc_size, sizeof *buffer);. Casting the result of calloc is not needed in C, and basing the size on the thing being allocated rather than a repetition of the type may be safer if the type is changed in the future.
Change buffer = realloc(buffer, alloc_size); to char *temp = realloc(buffer, alloc_size * sizeof *buffer); if (temp == NULL) { print message and exit } else buffer = temp;.

Answer 2

Attaching the corrected code suggested by Eric Postpischil. It is working fine now.
#include<stdio.h>
#include<stdlib.h>
void concat(char *dest, const char *source);
int main(void) {
FILE *fp;
char r[1024];
fp = popen("/bin/ls /etc/", "r");
if (fp == NULL) {
perror("Failed to run the command");
exit(-1);
}
int totallengthread = 0, alloc_size = 1024;
char *buffer = (char*) calloc(alloc_size, sizeof(char));
if (buffer == NULL) {
perror("Failed allocate memory");
exit(-1);
}
int lenofbuff = 0;
while ((lenofbuff = fread(r, sizeof(char), 1023, fp)) > 0) {
r[lenofbuff] = '\0';
totallengthread += lenofbuff;
if ((totallengthread) >= alloc_size) {
alloc_size += 1024;
buffer = realloc(buffer, alloc_size*sizeof(char));
if (buffer == NULL) {
perror("Failed to extend memory");
exit(-1);
}
}
concat(buffer, r);
}
printf("this is the output =>%s", buffer);
pclose(fp);
free(buffer);
return 0;
}
void concat(char *dest, const char *source) {
char *d = dest;
const char *s = source;
while (*d != '\0') {
d++;
}
while (*s != '\0') {
*d++ = *s++;
}
*d = '\0';
}