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POFunction pointer location not getting passed
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copied!<p>I've got some C code I'm targeting for an AVR. The code is being compiled with avr-gcc, basically the gnu compiler with the right backend.</p> <p>What I'm trying to do is create a callback mechanism in one of my event/interrupt driven libraries, but I seem to be having some trouble keeping the value of the function pointer.</p> <p>To start, I have a static library. It has a header file (<code>twi_master_driver.h</code>) that looks like this:</p> <pre><code>#ifndef TWI_MASTER_DRIVER_H_ #define TWI_MASTER_DRIVER_H_ #define TWI_INPUT_QUEUE_SIZE 256 // define callback function pointer signature typedef void (*twi_slave_callback_t)(uint8_t*, uint16_t); typedef struct { uint8_t buffer[TWI_INPUT_QUEUE_SIZE]; volatile uint16_t length; // currently used bytes in the buffer twi_slave_callback_t slave_callback; } twi_global_slave_t; typedef struct { uint8_t slave_address; volatile twi_global_slave_t slave; } twi_global_t; void twi_init(uint8_t slave_address, twi_global_t *twi, twi_slave_callback_t slave_callback); #endif </code></pre> <p>Now the C file (<code>twi_driver.c</code>):</p> <pre><code>#include <stdint.h> #include "twi_master_driver.h" void twi_init(uint8_t slave_address, twi_global_t *twi, twi_slave_callback_t slave_callback) { twi->slave.length = 0; twi->slave.slave_callback = slave_callback; twi->slave_address = slave_address; // temporary workaround <- why does this work?? twi->slave.slave_callback = twi->slave.slave_callback; } void twi_slave_interrupt_handler(twi_global_t *twi) { (twi->slave.slave_callback)(twi->slave.buffer, twi->slave.length); // some other stuff (nothing touches twi->slave.slave_callback) } </code></pre> <p>Then I build those two files into a static library (.a) and construct my main program (<code>main.c</code>) #include #include #include #include #include "twi_master_driver.h"</p> <pre><code>// ...define microcontroller safe way for mystdout ... twi_global_t bus_a; ISR(TWIC_TWIS_vect, ISR_NOBLOCK) { twi_slave_interrupt_handler(&bus_a); } void my_callback(uint8_t *buf, uint16_t len) { uint8_t i; fprintf(&mystdout, "C: "); for(i = 0; i < length; i++) { fprintf(&mystdout, "%d,", buf[i]); } fprintf(&mystdout, "\n"); } int main(int argc, char **argv) { twi_init(2, &bus_a, &my_callback); // ...PMIC setup... // enable interrupts. sei(); // (code that causes interrupt to fire) // spin while the rest of the application runs... while(1){ _delay_ms(1000); } return 0; } </code></pre> <p>I carefully trigger the events that cause the interrupt to fire and call the appropriate handler. Using some fprintfs I'm able to tell that the location assigned to <code>twi->slave.slave_callback</code> in the <code>twi_init</code> function is different than the one in the <code>twi_slave_interrupt_handler</code> function.</p> <p>Though the numbers are meaningless, in <code>twi_init</code> the value is 0x13b, and in <code>twi_slave_interrupt_handler</code> when printed the value is 0x100.</p> <p>By adding the commented workaround line in <code>twi_driver.c</code>:</p> <pre><code>twi->slave.slave_callback = twi->slave.slave_callback; </code></pre> <p>The problem goes away, but this is clearly a magic and undesirable solution. What am I doing wrong?</p> <p>As far as I can tell, I've marked appropriate variables <code>volatile</code>, and I've tried marking other portions volatile and removing the volatile markings. I came up with the workaround when I noticed removing <code>fprintf</code> statements after the assignment in <code>twi_init</code> caused the value to be read differently later on.</p> <p>The problem seems to be with how I'm passing around the function pointer -- and notably the portion of the program that is accessing the value of the pointer (the function itself?) is technically in a different thread.</p> <p>Any ideas?</p> <p>Edits:</p> <ul> <li><p>resolved typos in code.</p></li> <li><p>links to actual files: <a href="http://straymark.com/code/" rel="nofollow noreferrer">http://straymark.com/code/</a> [test.c|twi_driver.c|twi_driver.h]</p></li> <li><p>fwiw: compiler options: <code>-Wall -Os -fpack-struct -fshort-enums -funsigned-char -funsigned-bitfields -mmcu=atxmega128a1 -DF_CPU=2000000UL</code></p></li> <li><p>I've tried the same code included directly (rather than via a library) and I've got the same issue.</p></li> </ul> <p>Edits (round 2):</p> <ul> <li>I removed all the optimizations, without my "workaround" the code works as expected. Adding back -Os causes an error. Why is -Os corrupting my code?</li> </ul>

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