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  1. #include <stdio.h>
  2. #include <stdint.h>
  3. #include <string.h>
  4.  
  5. #define LONG_DBL_BYTES (80 / 8)
  6.  
  7. static void dump(void *data, size_t length)
  8. {
  9.     for (size_t i = 0; i < length; i++) {
  10.         printf("%.2x ", ((uint8_t *) data)[i]);
  11.     }
  12.     putchar('\n');
  13. }
  14.  
  15. int main(void) {
  16.     long double number = 1.0;
  17.  
  18.     printf("sizeof(long double) = %zu (%zu padding bytes)\n",
  19.         sizeof(long double), sizeof(long double) - LONG_DBL_BYTES);
  20.  
  21.     printf("Unmodified representation of %Lf:\n", number);
  22.     dump(&number, sizeof(number));
  23.  
  24.     // Patch unused (padding) bytes.
  25.     memset(((uint8_t *) &number) + LONG_DBL_BYTES, 0xaa,
  26.         sizeof(long double) - LONG_DBL_BYTES);
  27.  
  28.     printf("After patching unused (padding) bytes of %Lf with 0xaa:\n",
  29.         number);
  30.     dump(&number, sizeof(number));
  31.  
  32.     number += 2.0;
  33.  
  34.     printf("Unused bytes are not touched when doing math on %Lf:\n",
  35.         number);
  36.     dump(&number, sizeof(number));
  37. }
  38.  
Success #stdin #stdout 0s 4400KB
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stdin
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Standard input is empty
stdout
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sizeof(long double) = 16 (6 padding bytes)
Unmodified representation of 1.000000:
00 00 00 00 00 00 00 80 ff 3f 00 00 00 00 00 00 
After patching unused (padding) bytes of 1.000000 with 0xaa:
00 00 00 00 00 00 00 80 ff 3f aa aa aa aa aa aa 
Unused bytes are not touched when doing math on 3.000000:
00 00 00 00 00 00 00 c0 00 40 aa aa aa aa aa aa
https://ideone.com/R2osgB
pr/1206626: 80-bit long double
language:
C99 (gcc 8.3)
created:
5 years ago
visibility:
public

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