Passing pointer to struct array between functions

Question

I have a program which needs to load some text word for word into an array, so i have a struct for each text defined in

main.h

typedef struct wordTag{
 char name[MAX_WORD];
 char string[1000][MAX_WORD];
 int words;
}text;

main.c

void main(){
   int fileCount = 0;
   text *checkTexts;
   fileCount = getCheckFiles(checkTexts);

   for(i = 0; i < fileCount; i++){
    printf("Tekst navn: %s\n", checkTexts[i].name);
   }
}

file.c

int getCheckFiles(text *checkTexts){

int fileCount, i;

FILE *file;

createFileList(&file);
fileCount = countFiles(file);

createArray(checkTexts, fileCount, file);

return fileCount;



}

void createArray(text *checkTexts, int fileCount, FILE *file){
 int i, wordCount;
 FILE *textFile;
 text localText;
 char fileName[MAX_WORD + 30];


 checkTexts= (text *)malloc(fileCount * sizeof(text));

 readFileNames(file, checkTexts);

 for(i = 0; i < fileCount; i++){
  localText = checkTexts[i];
  strcpy(fileName, "./testFolder/");
  strcat(fileName, localText.name);
  openFile(&textFile, fileName);

  localText.words = countWords(textFile);

  readFileContent(textFile, localText);
  checkTexts[i] = localText;
 }

  for(i = 0; i < fileCount; i++){
  printf("Tekst navn: %s\n", checkTexts[i].name);

  }

}

Now if I print the name in the createArray function every thing works fine, but if i try and print in my main function I get a segmentation fault (core dumped).

Answer 1

You have not initialised the checkTexts pointer which you are using in main() .

In C (or C++) function pointers are passed by value not by reference (with the exception, in C++, of when a function is declared to take a reference to a type as a parameter). So when you call getCheckFiles(checkTexts) it doesn't matter what getCheckFiles() does with the passed-in parameter -- it doesn't change main() 's checkTexts variable.

The same thing then happens down in your call to createArray() . So although you create the array down in createArray() , the pointer to the buffer you malloc'd never propagates back up the call chain.

Answer 2

The problem is that the malloc call within createArray doesn't associate a block of memory with the address you've supplied ( checktexts ) as you may be tacitly assuming, rather it provides a new pointer to a block of memory it has reserved. The value of checkTexts (the memory address) you've supplied is overwritten within createArray ; checkTexts might as well be a local variable. When createArray returns however, it's still the old , uninitialized address that checkTexts refers to within main , and the memory at that address is (probably) just as it was before, namely either unallocated or reserved for someone else. So, segmentation fault