GNU/Linux man pages
Livre :
Expressions régulières,
Syntaxe et mise en oeuvre :
Unix |
Unix v7 |
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a.out(5) |
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a.out − assembler and link editor output
#include <a.out.h>
A.out is the output file of the assembler as(1) and the link editor ld(1). Both programs make a.out executable if there were no errors and no unresolved external references. Layout information as given in the include file for the PDP11 is:
The file has four sections: a header, the program and data text, relocation information, and a symbol table (in that order). The last two may be empty if the program was loaded with the ’−s’ option of ld or if the symbols and relocation have been removed by strip(1).
In the header the sizes of each section are given in bytes, but are even. The size of the header is not included in any of the other sizes.
When an a.out file is loaded into core for execution, three logical segments are set up: the text segment, the data segment (with uninitialized data, which starts off as all 0, following initialized), and a stack. The text segment begins at 0 in the core image; the header is not loaded. If the magic number in the header is 0407(8), it indicates that the text segment is not to be write-protected and shared, so the data segment is immediately contiguous with the text segment. If the magic number is 0410, the data segment begins at the first 0 mod 8K byte boundary following the text segment, and the text segment is not writable by the program; if other processes are executing the same file, they will share the text segment. If the magic number is 411, the text segment is again pure, write-protected, and shared, and moreover instruction and data space are separated; the text and data segment both begin at location 0. If the magic number is 0405, the text segment is overlaid on an existing (0411 or 0405) text segment and the existing data segment is preserved.
The stack will occupy the highest possible locations in the core image: from 0177776(8) and growing downwards. The stack is automatically extended as required. The data segment is only extended as requested by brk(2).
The start of the text segment in the file is 020(8); the start of the data segment is 020+S t (the size of the text) the start of the relocation information is 020+S t +S d ; the start of the symbol table is 020+2(S t +S d ) if the relocation information is present, 020+S t +S d if not.
The layout of a symbol table entry and the principal flag values that distinguish symbol types are given in the include file. Other flag values may occur if an assembly language program defines machine instructions.
If a symbol’s type is undefined external, and the value field is non-zero, the symbol is interpreted by the loader ld as the name of a common region whose size is indicated by the value of the symbol.
The value of a word in the text or data portions which is not a reference to an undefined external symbol is exactly that value which will appear in core when the file is executed. If a word in the text or data portion involves a reference to an undefined external symbol, as indicated by the relocation information for that word, then the value of the word as stored in the file is an offset from the associated external symbol. When the file is processed by the link editor and the external symbol becomes defined, the value of the symbol will be added into the word in the file.
If relocation information is present, it amounts to one word per word of program text or initialized data. There is no relocation information if the ’relocation info stripped’ flag in the header is on.
Bits 3-1 of a relocation word indicate the segment referred to by the text or data word associated with the relocation word:
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000 |
absolute number |
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002 |
reference to text segment |
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004 |
reference to initialized data |
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006 |
reference to uninitialized data (bss) |
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010 |
reference to undefined external symbol |
Bit 0 of the relocation word indicates, if 1, that the reference is relative to the pc (e.g. ’clr x’); if 0, that the reference is to the actual symbol (e.g., ’clr *$x’).
The remainder of the relocation word (bits 15-4) contains a symbol number in the case of external references, and is unused otherwise. The first symbol is numbered 0, the second 1, etc.
as(1), ld(1), nm(1)
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a.out(5) | |