Consecutive identical "printf" calls produce different output

Question

I call printf function consecutively and first printf writes what it must write, second printf just writes invalid values and I guess some errors.

Type of "LOC" is defined by me and variable "i" doesn't change after callings. I checked double for any discrepancy between printf and values I give it.

i=2;
printf("x = %f,y = %f,z = %f\n",(*LOC)[0].ProjectionPoints[i].X,(*LOC)[0].ProjectionPoints[i].Y,(*LOC)[0].ProjectionPoints[i].Z); /* Prints perfectly */
printf("x = %f,y = %f,z = %f\n",(*LOC)[0].ProjectionPoints[i].X,(*LOC)[0].ProjectionPoints[i].Y,(*LOC)[0].ProjectionPoints[i].Z); /* Shows some errors and values are "0" */

First printf writes

x = -10.000000,y = -8.000000,z = -10.000000

Second printf writes

x = 0.000000,y = 0.000000,z = -1.#QNAN0

Even defining variables as const isn't change anything.

My compiler is: MS Visual C++ 2012

After I read comments;

typedef struct {
P3C_Point *ProjectionPoints;
uint Distance,LayerID,NumberOfPoints,ModelID;} P3C_LayerOnCurtain;
/* and */
P3C_LayerOnCurtain **LOC = P3C_Compile(Stream);
/* when I try printf in P3C_Compile it has no problems */

I made test before I return value, in function produces "LOC" is printf perfectly.

Answer/Solution;

 P3C_LayerOnCurtain *LOC = *P3C_Compile(Stream);

You'll have to show us more code than this. Where is LOC declared,and how are the arrays allocated? I suspect you're reading from uninitialized or unallocated memory. — Lee Daniel Crocker, May 31 '13 at 09:03
Please construct a minimum test case and put the **complete** source in your question. — Bernhard Barker, May 31 '13 at 09:04
The problem is almost certainly as Medinoc hypothesized; you've returned LOC from a function, but the first call to `printf()` reuses the space LOC points at for its own purposes because you returned a local variable from the function where you returned LOC. You need to show us enough of `P3C_Compile()` to show how the return value is allocated. — Jonathan Leffler, May 31 '13 at 09:08

Medinoc · Accepted Answer · 2013-05-31T09:08:22.460

5

The most probable cause would be that LOC or (*LOC[0].ProjectionPoints) are pointers to a local variable of a function that returned them.

If so, they would be overwritten by printf's local variables.

edited May 31 '13 at 09:08

answered May 31 '13 at 09:03

Medinoc

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are all local variables write in same adress? – user1156885 May 31 '13 at 09:12
Local variables are, for all intents and purposes, destroyed when the function returns. Any subsequent function call will overwrite that space with its own local variables. "same address" is a bit of a misnomer, because local variables can have different sizes. – Medinoc May 31 '13 at 09:16
On a classic x86 architecture, local variables are stored on the stack: They are "pushed" when created and "popped" when destroyed. Function calls also use the stack to store the return address, so functions with no local variables and lots of nested calls would overwrite local variables as well. – Medinoc May 31 '13 at 09:19

Consecutive identical "printf" calls produce different output

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