No Memory Alignment with GCC
阿新 • • 發佈：2017-07-18
trac dsm void oid potential () nal class generate
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attributemethod:#include <stdio.h> struct packed { char a; int b; } __attribute__((packed)); struct not_packed { char a; int b; }; int main(void) { printf("Packed: %zu\n", sizeof(struct packed)); printf("Not Packed: %zu\n", sizeof(structnot_packed)); return 0; }Output:
$ make example && ./example cc example.c -o example Packed: 5 Not Packed: 8
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pragma packmethod:#include <stdio.h> #pragma pack(1) struct packed { char a; int b; }; #pragma pack() struct not_packed { char a; intb; }; int main(void) { printf("Packed: %zu\n", sizeof(struct packed)); printf("Not Packed: %zu\n", sizeof(struct not_packed)); return 0; }Output:
$ make example && ./example cc example.c -o example Packed: 5 Not Packed: 8 Add-fpack-structto GCC- -fpack-struct[=n]
- Without a value specified, pack all structure members together without holes. When a value is specified (which must be a small power of two), pack structuremembers according to this value, representing the maximum alignment (that is, objects with default
alignment requirements larger than this will be outputpotentially unaligned at the next fitting location.
Warning: the -fpack-struct switch causes GCC to generate code that is not binary compatible with code generated without thatswitch. Additionally, it makes the code suboptimal. Use it to conform to a non-default application binary interface.
No Memory Alignment with GCC