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57 lines
1.8 KiB
Markdown
57 lines
1.8 KiB
Markdown
# Section 1 / Example of calling some common C runtime functions
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This chapter gives an example of calling the most common C I/O functions
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from assembly language.
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There are, by the way, two broad types of functions within the C
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runtime. Some are implemented largely in the C runtime itself. Others
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that exist in the C runtime act as wrappers for functions implemented
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within the OS itself. These are called "system calls".
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For the purposes of calling functions in the C runtime, there is no
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practical difference between these two types. Note however, there are
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ways of calling system calls directly using the `svc` instruction. We
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will cover this way of making system calls as well. See
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[here](../../more/system_calls/README.md).
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## Low level file operations
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This example [program](./file_ops.s) makes use of `open()`, `close()`,
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`read()`, `write()` and `lseek()`. These are implemented in the C
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runtime as wrappers for system calls.
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The program will
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* create a file,
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* write a small amount of text to it,
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* rewind (seek) back to the beginning of the file,
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* read back and print the contents of the file and then
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* close the file.
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A lot of error checking is also implemented (frankly speaking: until we
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got bored writing the example).
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Doing all of these ballooned the example program to about 200 lines. As
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such we won't explain the code line by line but, in compensation, the
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code is liberally commented.
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Here is just a bit:
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```text
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/* off_t lseek(int fd, off_t offset, int whence);
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*/
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seek_zero:
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stp x29, x30, [sp, -16]!
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mov w0, fd // file descriptor
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mov x1, xzr // beginning of file
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mov w2, wzr // SEEK_SET - absolute offset
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bl lseek
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ldp x29, x30, [sp], 16
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ret
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```
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Calling this function rewinds the read / write "head" to position 0.
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