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POWhy is my function static variable never different despite being incremented?
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copied!<p>I am writing a callback function in C. It is intended to initialise an I2C sensor, and it called at the conclusion of each (split-phase) configuration step; after the 9th call, the device is almost ready to use.</p> <p>The basic idea of the function is this:</p> <pre><code>void callback(void) { static uint8_t calls = 0; if (++calls == 9) { // Finalise device setup (literally a single line of code) } } </code></pre> <p>My problem is that the above if statement is never being entered, despite the function being called 9 times.</p> <p>The (dis)assembly code for my function seems sane (with the exception of the <code>subi . 0xFF</code> trick for an increment, despite the inclusion of an <code>inc</code> instruction):</p> <pre><code>00000a4c <callback>: a4c: 80 91 9e 02 lds r24, 0x029E a50: 8f 5f subi r24, 0xFF ; 255 a52: 80 93 9e 02 sts 0x029E, r24 a56: 89 30 cpi r24, 0x09 ; 9 a58: 09 f0 breq .+2 ; 0xa5c <callback+0x10> a5a: 08 95 ret a5c: 2e e1 ldi r18, 0x1E ; 30 a5e: 35 e0 ldi r19, 0x05 ; 5 a60: 41 e0 ldi r20, 0x01 ; 1 a62: 60 e0 ldi r22, 0x00 ; 0 a64: 84 e7 ldi r24, 0x74 ; 116 a66: 0c 94 c7 02 jmp 0x58e ; 0x58e <twi_set_register> </code></pre> <p>I am writing the code for an Atmel AVR chip, and thus compiling with avr-gcc. I have no meaningful code debugging capabilities (I don't have access to a JTAG programmer, and the function is asynchronous/split-phase in any case; USART printing is too slow).</p> <p>However, I have access to a logic analyser, and have been able to determine a number of things by placing <code>while (1) ;</code> statements inside the code:</p> <ul> <li><strong>the function is called</strong> - if I place an infinite loop at the start of the function, the microcontroller hangs</li> <li><strong>the function should be called 9 times</strong> - the trigger for the function is an I2C communication, and in the previous step it hangs immediately after the first communication; I can observe 9 complete and valid I2C communications</li> <li><strong>calls is incremented within the function</strong> - if I add <code>if (calls == 0) { while (1) ; }</code> <em>after</em> the increment, it does not hang</li> <li><strong>calls is never non-zero at the start of the function</strong> - if I add <code>if (calls) { while(1) ; }</code> <em>before</em> the increment, it does not hang</li> </ul> <p>I'm completely at a loss for ideas.</p> <p>Does anyone have any suggestions as to what could cause this, or even for new debugging steps I could take?</p>

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