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POWhy short is stored as 4 bytes in a struct in C?
 

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  1. POWhy short is stored as 4 bytes in a struct in C?
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    <p>I have the following two structs:</p> <p>The problem is the sizeof(Content) returns 160. The struct consists of 11 shorts, 6 ints, 76 chars, 7 floats, 1 double, totally adding to 158 bytes. I have counted three times and there is still a 2 byte difference.</p> <pre><code>typedef struct TIME_T { short year,mon,day; short hour,min,sec; } TIME; typedef struct { int no; char name[20]; char Code[10]; char DASType[10]; short wlen; float VLtd; int samp; int comp; int locationID; short TranMode; char TranIns[12]; short TimerMode; char ClkType[12]; float ClkErr; float lat; float lon; float alt; float azimuth,incident; short weight; short veloc; int oritype; char seismometer[12]; double sens; TIME start_time; int record_samples; } Content; </code></pre> <p>I write a small piece of code to print the position of each variables in the struct, and suddenly I find the <code>float wlen</code> takes 4 bytes. My code is as follows:</p> <pre><code>int main(void) { Content content; printf("Sizeof Content: %d\n", sizeof(content)); printf("Sizeof int content.no: %d\n", (int)&amp;content.name - (int)&amp;content.no); printf("Sizeof char[20] content.name: %d\n", (int)&amp;content.Code - (int)&amp;content.name); printf("Sizeof char[10] content.Code: %d\n", (int)&amp;content.DASType - (int)&amp;content.Code); printf("Sizeof char[10] content.DASType: %d\n", (int)&amp;content.wlen - (int)&amp;content.DASType); printf("Sizeof short content.wlen: %d\n", (int)&amp;content.VLtd - (int)&amp;content.wlen); printf("Sizeof float content.VLtdL %d\n", (int)&amp;content.samp - (int)&amp;content.VLtd); printf("Sizeof int content.samp: %d\n", (int)&amp;content.comp - (int)&amp;content.samp); printf("Sizeof int content.comp: %d\n", (int)&amp;content.locationID - (int)&amp;content.comp); printf("Sizeof int content.locationID: %d\n", (int)&amp;content.TranMode - (int)&amp;content.locationID); printf("Sizeof short content.TranMode: %d\n", (int)&amp;content.TranIns - (int)&amp;content.TranMode); printf("Sizeof char[12] content.TranIns: %d\n", (int)&amp;content.TimerMode - (int)&amp;content.TranIns); printf("Sizeof short content.TimerMode: %d\n", (int)&amp;content.ClkType - (int)&amp;content.TimerMode); printf("Sizeof char[12] content.ClkType: %d\n", (int)&amp;content.ClkErr - (int)&amp;content.ClkType); printf("Sizeof float content.ClkErr: %d\n", (int)&amp;content.lat - (int)&amp;content.ClkErr); printf("Sizeof float content.lat: %d\n", (int)&amp;content.lon - (int)&amp;content.lat); printf("Sizeof floatcontent.lon: %d\n", (int)&amp;content.alt - (int)&amp;content.lon); printf("Sizeof floatcontent.alt: %d\n", (int)&amp;content.azimuth - (int)&amp;content.alt); printf("Sizeof floatcontent.azimuth: %d\n", (int)&amp;content.incident - (int)&amp;content.azimuth); printf("Sizeof floatcontent.incident: %d\n", (int)&amp;content.weight - (int)&amp;content.incident); printf("Sizeof short content.weight: %d\n", (int)&amp;content.veloc - (int)&amp;content.weight); printf("Sizeof short content.veloc: %d\n", (int)&amp;content.oritype - (int)&amp;content.veloc); printf("Sizeof int content.oritype: %d\n", (int)&amp;content.seismometer - (int)&amp;content.oritype); printf("Sizeof char[12] content.seismometer: %d\n", (int)&amp;content.sens - (int)&amp;content.seismometer); printf("Sizeof double content.sens: %d\n", (int)&amp;content.start_time - (int)&amp;content.sens); printf("Sizeof TIME content.start_time: %d\n", (int)&amp;content.record_samples - (int)&amp;content.start_time); printf("Sizeof int content.record_samples: %d\n", sizeof(content.record_samples)); getchar(); return 0; } </code></pre> <p>The output is as follows:</p> <pre><code>Sizeof int content.no: 4 Sizeof char[20] content.name: 20 Sizeof char[10] content.Code: 10 Sizeof char[10] content.DASType: 10 Sizeof short content.wlen: 4 **Sizeof float content.VLtdL 4** Sizeof int content.samp: 4 Sizeof int content.comp: 4 Sizeof int content.locationID: 4 Sizeof short content.TranMode: 2 Sizeof char[12] content.TranIns: 12 Sizeof short content.TimerMode: 2 Sizeof char[12] content.ClkType: 12 Sizeof float content.ClkErr: 4 Sizeof float content.lat: 4 Sizeof floatcontent.lon: 4 Sizeof floatcontent.alt: 4 Sizeof floatcontent.azimuth: 4 Sizeof floatcontent.incident: 4 Sizeof short content.weight: 2 Sizeof short content.veloc: 2 Sizeof int content.oritype: 4 Sizeof char[12] content.seismometer: 12 Sizeof double content.sens: 8 Sizeof TIME content.start_time: 12 Sizeof int content.record_samples: 4 </code></pre> <p>The compiler is MSVC8, no UNICODE defined, no other macro defined. It's x86.</p> <p>I tried to compile the same code in gcc version 3.4.4, the output is the same. <code>Sizeof short content.wlen: 4</code></p> <p>Can anyone explain this to me?</p> <p>Thanks in advance.</p> <p><strong>EDIT:</strong> Thanks for answering! I have got it now.</p>
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    1. COSee here: http://stackoverflow.com/questions/713963/why-does-this-c-code-work for a macro that calculates the offset of a data member. It was a different discussion, but the macro there does the work...
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      1. USCătălin Pitiș
    2. COYou should not print sizes returned by sizeof with %d. The type si size_t, generally a typedef of unsigned int or unsigned long. It's plaform-dependant, so you have to cast it before you can use a type-specific format: "%u", (unsigned int)sizeof(...) or "%lu\n", (unsigned long)sizeof(...). I believe C99 added a size_t format, which makes things much simpler.
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      1. USBastien Léonard
 

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