Can't malloc from within a function

**synthetix** · 11-02-2011

I have the following code that tries to malloc a pointer to a struct outside of the function. Everything works fine inside the function, but once it completes, it seems my memory is lost.

Code:

typedef struct {
	char name[1024];
} FileList;

void read_files(FileList *filelist){
	
	long filecount = 5;
	long i;

	filelist = (FileList*)malloc(sizeof(FileList)*filecount);
	for(i=0;i<filecount;i++){
		sprintf(filelist[i].name,"filename%d",i);
		fprintf(stderr,"Filename %d was %s\n",i,filelist[i].name); //works
	}
}

int main(int argc,char **argv){
	
	FileList *files;
	read_files(files);
	fprintf(stderr,"Filename 0 was %s\n",files[0].name); //doesn't work (returns NULL)

	return 0;
}

What can I do to fix this?

**stahta01** · 11-02-2011

To change a variable in a function and have the change seen outside a function you need to return it; or, pass the pointer to the variable.

This applies to pointers, you need to pass a pointer to the pointer.

Tim S.

**laserlight** · 11-02-2011

The problem is that you are assigning to the pointer from within the function. Like other parameters, changing it in the function does not affect the variable in the caller because pass by value is used. Hence, if you want to do it this way, you should pass a pointer to a pointer.

**synthetix** · 11-02-2011

Thanks. I think I figured it out:

Code:

typedef struct {
    char name[1024];
} FileList;
 
void read_files(FileList **filelist){
     
    long filecount = 5;
    long i;
 
    *filelist = (FileList*)malloc(sizeof(FileList)*filecount);
    for(i=0;i<filecount;i++){
        sprintf((*filelist)[i].name,"filename%d",i);
        fprintf(stderr,"Filename %d was %s\n",i,(*filelist)[i].name); //works
    }
}
 
int main(int argc,char **argv){
     
    FileList *files;
    read_files(&files);
    fprintf(stderr,"Filename 0 was %s\n",files[0].name); //works!
 
    free(files);
    return 0;
}

**laserlight** · 11-02-2011

Remember to free what you malloc.

**synthetix** · 11-02-2011

Originally Posted by laserlight

Remember to free what you malloc.

Good call. I updated the code.

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