bfdsumm.texi revision 1.1.1.4
1@c This summary of BFD is shared by the BFD and LD docs. 2@c Copyright (C) 2012-2016 Free Software Foundation, Inc. 3 4When an object file is opened, BFD subroutines automatically determine 5the format of the input object file. They then build a descriptor in 6memory with pointers to routines that will be used to access elements of 7the object file's data structures. 8 9As different information from the object files is required, 10BFD reads from different sections of the file and processes them. 11For example, a very common operation for the linker is processing symbol 12tables. Each BFD back end provides a routine for converting 13between the object file's representation of symbols and an internal 14canonical format. When the linker asks for the symbol table of an object 15file, it calls through a memory pointer to the routine from the 16relevant BFD back end which reads and converts the table into a canonical 17form. The linker then operates upon the canonical form. When the link is 18finished and the linker writes the output file's symbol table, 19another BFD back end routine is called to take the newly 20created symbol table and convert it into the chosen output format. 21 22@menu 23* BFD information loss:: Information Loss 24* Canonical format:: The BFD canonical object-file format 25@end menu 26 27@node BFD information loss 28@subsection Information Loss 29 30@emph{Information can be lost during output.} The output formats 31supported by BFD do not provide identical facilities, and 32information which can be described in one form has nowhere to go in 33another format. One example of this is alignment information in 34@code{b.out}. There is nowhere in an @code{a.out} format file to store 35alignment information on the contained data, so when a file is linked 36from @code{b.out} and an @code{a.out} image is produced, alignment 37information will not propagate to the output file. (The linker will 38still use the alignment information internally, so the link is performed 39correctly). 40 41Another example is COFF section names. COFF files may contain an 42unlimited number of sections, each one with a textual section name. If 43the target of the link is a format which does not have many sections (e.g., 44@code{a.out}) or has sections without names (e.g., the Oasys format), the 45link cannot be done simply. You can circumvent this problem by 46describing the desired input-to-output section mapping with the linker command 47language. 48 49@emph{Information can be lost during canonicalization.} The BFD 50internal canonical form of the external formats is not exhaustive; there 51are structures in input formats for which there is no direct 52representation internally. This means that the BFD back ends 53cannot maintain all possible data richness through the transformation 54between external to internal and back to external formats. 55 56This limitation is only a problem when an application reads one 57format and writes another. Each BFD back end is responsible for 58maintaining as much data as possible, and the internal BFD 59canonical form has structures which are opaque to the BFD core, 60and exported only to the back ends. When a file is read in one format, 61the canonical form is generated for BFD and the application. At the 62same time, the back end saves away any information which may otherwise 63be lost. If the data is then written back in the same format, the back 64end routine will be able to use the canonical form provided by the 65BFD core as well as the information it prepared earlier. Since 66there is a great deal of commonality between back ends, 67there is no information lost when 68linking or copying big endian COFF to little endian COFF, or @code{a.out} to 69@code{b.out}. When a mixture of formats is linked, the information is 70only lost from the files whose format differs from the destination. 71 72@node Canonical format 73@subsection The BFD canonical object-file format 74 75The greatest potential for loss of information occurs when there is the least 76overlap between the information provided by the source format, that 77stored by the canonical format, and that needed by the 78destination format. A brief description of the canonical form may help 79you understand which kinds of data you can count on preserving across 80conversions. 81@cindex BFD canonical format 82@cindex internal object-file format 83 84@table @emph 85@item files 86Information stored on a per-file basis includes target machine 87architecture, particular implementation format type, a demand pageable 88bit, and a write protected bit. Information like Unix magic numbers is 89not stored here---only the magic numbers' meaning, so a @code{ZMAGIC} 90file would have both the demand pageable bit and the write protected 91text bit set. The byte order of the target is stored on a per-file 92basis, so that big- and little-endian object files may be used with one 93another. 94 95@item sections 96Each section in the input file contains the name of the section, the 97section's original address in the object file, size and alignment 98information, various flags, and pointers into other BFD data 99structures. 100 101@item symbols 102Each symbol contains a pointer to the information for the object file 103which originally defined it, its name, its value, and various flag 104bits. When a BFD back end reads in a symbol table, it relocates all 105symbols to make them relative to the base of the section where they were 106defined. Doing this ensures that each symbol points to its containing 107section. Each symbol also has a varying amount of hidden private data 108for the BFD back end. Since the symbol points to the original file, the 109private data format for that symbol is accessible. @code{ld} can 110operate on a collection of symbols of wildly different formats without 111problems. 112 113Normal global and simple local symbols are maintained on output, so an 114output file (no matter its format) will retain symbols pointing to 115functions and to global, static, and common variables. Some symbol 116information is not worth retaining; in @code{a.out}, type information is 117stored in the symbol table as long symbol names. This information would 118be useless to most COFF debuggers; the linker has command line switches 119to allow users to throw it away. 120 121There is one word of type information within the symbol, so if the 122format supports symbol type information within symbols (for example, COFF, 123IEEE, Oasys) and the type is simple enough to fit within one word 124(nearly everything but aggregates), the information will be preserved. 125 126@item relocation level 127Each canonical BFD relocation record contains a pointer to the symbol to 128relocate to, the offset of the data to relocate, the section the data 129is in, and a pointer to a relocation type descriptor. Relocation is 130performed by passing messages through the relocation type 131descriptor and the symbol pointer. Therefore, relocations can be performed 132on output data using a relocation method that is only available in one of the 133input formats. For instance, Oasys provides a byte relocation format. 134A relocation record requesting this relocation type would point 135indirectly to a routine to perform this, so the relocation may be 136performed on a byte being written to a 68k COFF file, even though 68k COFF 137has no such relocation type. 138 139@item line numbers 140Object formats can contain, for debugging purposes, some form of mapping 141between symbols, source line numbers, and addresses in the output file. 142These addresses have to be relocated along with the symbol information. 143Each symbol with an associated list of line number records points to the 144first record of the list. The head of a line number list consists of a 145pointer to the symbol, which allows finding out the address of the 146function whose line number is being described. The rest of the list is 147made up of pairs: offsets into the section and line numbers. Any format 148which can simply derive this information can pass it successfully 149between formats (COFF, IEEE and Oasys). 150@end table 151