Using malloc in function? - c

Question

I am new to malloc. So I wanted to test this small code.
I'm getting the "segmentation fault error".
**EDIT: The problem was from not allocation char* name (Thanks to #Zack)
You can see what caused the problem:
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
typedef struct info { char *name; } fileinfo;
void mal (fileinfo **ptr)
{
if ((*ptr = (fileinfo*) malloc(3*sizeof(fileinfo))) == NULL) {
fprintf(stderr,"Memory allocating error #1\n");
exit(1);
}
//that what was causing the error - START
for (j = 0; j < 3; j += 1)
{
(*ptr)[j].name = (char*) malloc(10*sizeof(char*));
}
//that what was causing the error - END
strcpy(ptr[0]->name,"1. A\n"); //line 13
strcpy(ptr[1]->name,"2. B\n");
strcpy(ptr[2]->name,"3. C\n");
}
int main (int argc, char *argv[])
{
fileinfo *files;
int i;
mal(&files); //line 22
for (i = 0; i < 3; i += 1)
{
printf("name=%s\n",files[i].name);
}
free(files);
return 0;
}
Using valgrind:
==5751== Use of uninitialised value of size 8
==5751== at 0x4006EB: mal (mal1.c:13)
==5751== by 0x400747: main (mal1.c:22)
==5751==
==5751== Invalid write of size 4
==5751== at 0x4006EB: mal (mal1.c:13)
==5751== by 0x400747: main (mal1.c:22)
==5751== Address 0x0 is not stack'd, malloc'd or (recently) free'd
What am I doing wrong??!
Thanks in advance.

Answer 1

Your problem is that you need to allocate ptr[i]->name before copying to it. A pointer by itself points to some random area of memory. To use it, you must get good memory that is safe and that is what malloc and calloc, and (new in C++) all do. The example you gave is still a mess, and my correction isn't that great, but you get the idea that every pointer in C and C++ must be allocated from somewhere. And, on the flipside, once you are done with the pointer, you have to free them.
void mal (fileinfo **ptr, int count)
{
if ((*ptr = (fileinfo*) malloc(count*sizeof(fileinfo))) == NULL) {
fprintf(stderr,"Memory allocating error #1\n");
exit(1);
}
for (int i = 0; i < 3; i++) {
char buff[2];
buff[0] = 'A' + i;
buff[1] = 0;
ptr[i]->name = malloc( 10 );
strcpy(ptr[0]->name,"1. %s\n", buff);
}
}

Answer 2

Try with
void mal (fileinfo **ptr) {
if ((*ptr = malloc(3*sizeof(fileinfo))) == NULL) {
perror("Memory allocating error #1\n");
exit(EXIT_FAILURE);
}
if (!(ptr[0]->name = strdup("1. A\n")))
{ perror("string 1"); exit(EXIT_FAILURE);};
if (!(ptr[1]->name = strdup("2. B\n")))
{ perror("string 2"); exit(EXIT_FAILURE);};
if (!(ptr[2]->name = strdup("3. C\n")))
{ perror("string 3"); exit(EXIT_FAILURE);} ;
}
BTW, it would be better to make it a function returning a pointer, like
fileinfo* make() {
fileinfo* ptr = malloc(3*sizeof(fileinfo)));
if (!ptr) { perror("make fileinfo"); exit(EXIT_FAILURE); };
if (!(ptr[0]->name = strdup("1. A\n")))
{ perror("string 1"); exit(EXIT_FAILURE);};
if (!(ptr[1]->name = strdup("2. B\n")))
{ perror("string 2"); exit(EXIT_FAILURE);};
if (!(ptr[2]->name = strdup("3. C\n")))
{ perror("string 3"); exit(EXIT_FAILURE);} ;
return ptr;
}
Then, you need a destructor function like
void destroy(fileinfo*ptr) {
free (ptr[0]->name);
free (ptr[1]->name);
free (ptr[2]->name);
free (ptr);
}
to be called instead of your free at the end of your main
Notice that your code is very ad hoc: the number (i.e. 3) of names is completely built-in. This is wrong design. At the very least, that constant should have been named:
#define NB_FILES 3
Actually, a much better approach would be to declare your structure with a flexible array member:
typedef struct fileinfo_st {
unsigned nbfiles;
char* filenames[];
} Fileinfo;
See this for more. You could even have a variadic function to make such structures.
FIleinfo *make(int, ...);
Which you would call like
Fileinfo* ptr = make (3, "abc","def","ghijklmnop");
or you could decide that that last variadic argument is NULL
BTW, read about undefined behavior. You might be unlucky to have your program not crashing but still be very wrong. And compile with gcc -Wall -g and learn more how to use the gdb debugger and valgrind