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).

Just a tip: learn gdb and you won't ever have to ask anyone about segmentation fault errors...you can trace it yourself and see exactly where the problem is... — mmtauqir
– mmtauqir, Commented Dec 12, 2011 at 0:57

AAT · Accepted Answer · 2011-12-12 01:02:21Z

4

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.

answered Dec 12, 2011 at 1:02

AAT

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Comments

Matt Phillips · Accepted Answer · 2011-12-12 01:11:57Z

1

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

answered Dec 12, 2011 at 1:11

Matt Phillips

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