参考http://blog.163.com/againinput4@yeah/blog/static/122764271200962305432323/
在下面的命令中：

arm-linux-ar cru .libs/libmad.a version.o fixed.o bit.o timer.o stream.o frame.o synth.o decoder.o layer12.o layer3.o huffman.o imdct_l_arm.o

arm-linux-ranlib .libs/libmad.a

关于arm-linux-ar的参数 cru 中，没有 s。

其他有些具体项目的参数有的是，rcs，其中的s的含义，去查了下：

Write an object-file index into the archive, or update an existing one, even if no other change is made to the archive. You may use this modifier flag either with any operation, or alone. Running ar s on an archive is equivalent to running ranlib on it.

就相当于是接着再运行的 arm-linux-ranlib 。

参见
https://www.lifewire.com/ar-linux-command-4093866

NAME

ar - create, modify, and extract from archives
SYNOPSIS

ar [-X32_64] [-]p[mod [relpos] [count]] archive [member...]  
DESCRIPTION

The GNU ar program creates, modifies, and extracts from archives. An archive is a single file holding a collection of other files in a structure that makes it possible to retrieve the original individual files (called members of the archive).

The original files' contents, mode (permissions), timestamp, owner, and group are preserved in the archive, and can be restored on extraction.

GNU ar can maintain archives whose members have names of any length; however, depending on how ar is configured on your system, a limit on member-name length may be imposed for compatibility with archive formats maintained with other tools. If it exists, the limit is often 15 characters (typical of formats related to a.out) or 16 characters (typical of formats related to coff).

ar is considered a binary utility because archives of this sort are most often used aslibraries holding commonly needed subroutines.

ar creates an index to the symbols defined in relocatable object modules in the archive when you specify the modifier s. Once created, this index is updated in the archive whenever ar makes a change to its contents (save for the q update operation). An archive with such an index speeds up linking to the library, and allows routines in the library to call each other without regard to their placement in the archive.

You may use nm -s or nm --print-armap to list this index table. If an archive lacks the table, another form of ar called ranlib can be used to add just the table.

GNU ar is designed to be compatible with two different facilities. You can control its activity using command-line options, like the different varieties of ar on Unix systems; or, if you specify the single command-line option -M, you can control it with a script supplied via standard input, like the MRI ``librarian'' program.


OPTIONS

GNU ar allows you to mix the operation code p and modifier flags mod in any order, within the first command-line argument.

If you wish, you may begin the first command-line argument with a dash.

The p keyletter specifies what operation to execute; it may be any of the following, but you must specify only one of them:

d

Delete modules from the archive. Specify the names of modules to be deleted asmember...; the archive is untouched if you specify no files to delete.

If you specify the v modifier, ar lists each module as it is deleted.

m

Use this operation to move members in an archive.

The ordering of members in an archive can make a difference in how programs are linked using the library, if a symbol is defined in more than one member.

If no modifiers are used with "m", any members you name in the memberarguments are moved to the end of the archive; you can use the a, b, or imodifiers to move them to a specified place instead.

p

Print the specified members of the archive, to the standard output file. If the vmodifier is specified, show the member name before copying its contents to standard output.

If you specify no member arguments, all the files in the archive are printed.

q

Quick append; Historically, add the files member...

to the end of archive, without checking for replacement.

The modifiers a, b, and i do not affect this operation; new members are always placed at the end of the archive.

The modifier v makes ar list each file as it is appended.

Since the point of this operation is speed, the archive's symbol table index is not updated, even if it already existed; you can use ar s or ranlib explicitly to update the symbol table index.

However, too many different systems assume quick append rebuilds the index, so GNU ar implements "q" as a synonym for "r".

r

Insert the files member... into archive (with replacement).

This operation differs from q in that any previously existing members are deleted if their names match those being added.

If one of the files named in member... does not exist, ar displays an error message, and leaves undisturbed any existing members of the archive matching that name.

By default, new members are added at the end of the file; but you may use one of the modifiers a, b, or i to request placement relative to some existing member.

The modifier v used with this operation elicits a line of output for each file inserted, along with one of the letters a or r to indicate whether the file was appended (no old member deleted) or replaced.

t

Display a table listing the contents of archive, or those of the files listed inmember... that are present in the archive. Normally only the member name is shown; if you also want to see the modes (permissions), timestamp, owner, group, and size, you can request that by also specifying the v modifier.

If you do not specify a member, all files in the archive are listed.

If there is more than one file with the same name (say, fie) in an archive (sayb.a), ar t b.a fie lists only the first instance; to see them all, you must ask for a complete listing---in our example, ar t b.a.

x

Extract members (named member) from the archive. You can use the v modifier with this operation, to request that ar list each name as it extracts it.

If you do not specify a member, all files in the archive are extracted.

A number of modifiers (mod) may immediately follow the p keyletter, to specify variations on an operation's behavior:

a

Add new files after an existing member of the archive. If you use the modifier a, the name of an existing archive member must be present as the relpos argument, before the archive specification.

b

Add new files before an existing member of the archive. If you use the modifier b, the name of an existing archive member must be present as the relpos argument, before the archive specification. (same as i).

c

Create the archive. The specified archive is always created if it did not exist, when you request an update. But a warning is issued unless you specify in advance that you expect to create it, by using this modifier.

f

Truncate names in the archive. GNU ar will normally permit file names of any length. This will cause it to create archives which are not compatible with the native ar program on some systems. If this is a concern, the f modifier may be used to truncate file names when putting them in the archive.

i

Insert new files before an existing member of the archive. If you use the modifieri, the name of an existing archive member must be present as the relposargument, before the archive specification. (same as b).

l

This modifier is accepted but not used.

N

Uses the count parameter. This is used if there are multiple entries in the archive with the same name. Extract or delete instance count of the given name from the archive.

o

Preserve the original dates of members when extracting them. If you do not specify this modifier, files extracted from the archive are stamped with the time of extraction.

P

Use the full path name when matching names in the archive. GNU ar can not create an archive with a full path name (such archives are not POSIX complaint), but other archive creators can. This option will cause GNU ar to match file names using a complete path name, which can be convenient when extracting a single file from an archive created by another tool.

s

Write an object-file index into the archive, or update an existing one, even if no other change is made to the archive. You may use this modifier flag either with any operation, or alone. Running ar s on an archive is equivalent to running ranlibon it.

S

Do not generate an archive symbol table. This can speed up building a large library in several steps. The resulting archive can not be used with the linker. In order to build a symbol table, you must omit the S modifier on the last execution of ar, or you must run ranlib on the archive.

u

Normally, ar r... inserts all files listed into the archive. If you would like to insertonly those of the files you list that are newer than existing members of the same names, use this modifier. The u modifier is allowed only for the operation r(replace). In particular, the combination qu is not allowed, since checking the timestamps would lose any speed advantage from the operation q.

v

This modifier requests the verbose version of an operation. Many operations display additional information, such as filenames processed, when the modifier vis appended.

V

This modifier shows the version number of ar.

ar ignores an initial option spelt -X32_64, for compatibility with AIX. The behavior produced by this option is the default for GNU ar. ar does not support any of the other -X options; in particular, it does not support -X32 which is the default for AIX ar. 

Important: Use the man command (% man) to see how a command is used on your particular computer.

来自《LinuxC编程实战》