Dynamically expanding C code without using realloc

Question

I need to put the names separated by commas from the text into struct that expands dynamically, but I am prohibited from using realloc ().I'm getting a core dumped error in this code. What is the error in this code?

#include <stdio.h> #include <stdlib.h> #include <string.h> struct movie{ double budget; int genre; char* name; double score; int year; }; void recorder_function(struct movie *movies){ FILE*fp; fp=fopen("Movies.txt","r"); struct movie *p; int i,n=0; char line[1000]; char temp_budget[50]; char temp_name[50]; char temp_genre[50]; while (!feof(fp)) { fgets(line,1000,fp); sscanf(line,"%50[^,],%50[^,],%50[^,]",temp_budget,temp_genre,temp_name); //I open the fields in this section movies=(struct movie *)calloc(n+1,sizeof(struct movie)); p=(struct movie *)calloc(n+1,sizeof(struct movie)); p[n].name=(char*)malloc(sizeof(char)*(strlen(temp_name)+1)); movies[n].name=(char*)malloc(sizeof(char)*(strlen(temp_name)+1)); for(i=0;i<n;i++) movies[i]=p[i]; strcpy(movies[n].name,temp_name); free(p); p=movies; n++; } for(i=0;i<n;i++) printf("%s\n",movies[i].name); } int main(){ int choice; struct movie *movies; recorder_function(movies); }

Rousso, Who or what text suggested using while (!feof(fp)) {? — chux
– chux, Commented May 17, 2021 at 16:09
If you're not allowed to use realloc(), you have to read the file twice. First to get the number lines so you can allocate the movies array the proper size. Then the second time to fill it in. — Barmar
– Barmar, Commented May 17, 2021 at 16:13
Why does recorder_function() take movies as a parameter? It immediately overwrites it with calloc(). — Barmar
– Barmar, Commented May 17, 2021 at 16:14
Why does it allocate memory for both movies and p in the loop? — Ian Abbott
– Ian Abbott, Commented May 17, 2021 at 16:28

MikeCAT · Accepted Answer · 2021-05-17 16:32:04Z

It is a bad idea to overwrite the pointer movie by newly allocated clean buffer.

Instead of that, you should

Allocate new buffer only for p.
Put the new element to p[n]
Put existing elements movie[0], ... , movie[n-1] to p[0], ... , p[n-1]
Free the old buffer movie
Assign the new buffer p to movie

Don't forget to initialize movie not to cause troubles at the first freeing.

Also while (!feof(fp)) is wrong and you should check if readings are successful after trying to read and before using what are read.

One more important point is that you should make sure that fopen() succeeded. Passing NULL, which fopen() returns on failure`, to other file manipulation functions may cause troubles.

Another point is that your arrays used for sscanf() outputs should have one more elements for the terminating null-character.

Yet another point is that casting results of malloc() family is considered as a bad practice.

Try this:

void recorder_function(struct movie *movies){ FILE*fp; fp=fopen("Movies.txt","r"); if(fp==NULL){ fputs("file open error\n", stderr); return; } struct movie *p; int i,n=0; char line[1000]; char temp_budget[51]; char temp_name[51]; char temp_genre[51]; movies=NULL; while (fgets(line,1000,fp)) { sscanf(line,"%50[^,],%50[^,],%50[^,]",temp_budget,temp_genre,temp_name); //I open the fields in this section p=calloc(n+1,sizeof(struct movie)); p[n].name=malloc(sizeof(char)*(strlen(temp_name)+1)); strcpy(p[n].name,temp_name); for(i=0;i<n;i++){ p[i]=movies[i]; } free(movies); movies=p; n++; } for(i=0;i<n;i++){ printf("%s\n",movies[i].name); } }

The next stage will be fixing the weird usage of the argument movie. Arguments in C are copies of what are passed, and modifications of arguments in callee functions won't affect what are passed in caller. Your choice is:

Remove the argument movies and convert that to a local variable.
Have recorder_function take a pointer to struct movie* (struct movie**) and have it use the pointer to modify what caller specifies. (You also have to change the statement to call the function to pass a pointer in this case)

@Rousso If you are prohibited from using realloc() for academic purposes then this solution is fine. If it is prohibited for some technical reason, then be aware that this solution is exactly how realloc() works and may therefore be prohibited too.

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