Common Functions

By using wrapper calls, we are able to write our programs in C. However, we still need to implement common functions for string management, working with I/O, working with memory.

The simple attempt is to implement these functions (printf() or strcpy() or malloc()) once in a C source code file and then reuse them when needed. This saves us time (we don't have to reimplement) and allows us to constantly improve one implementation constantly; there will only be one implementation that we update to increase its safety, efficiency or performance.

The support/common-functions/ folder stores the implementation of string management functions, in string.c and string.h and of printing functions in printf.c and printf.h. The printf implementation is this one.

There are two programs: main_string.c showcases string management functions, main_printf.c showcases the printf() function.

main_string.c depends on the string.h and string.c files that implement the strlen() and strcpy() functions. We print messages using the write() system call wrapper implemented in syscall.s

Let's build and run the program:

student@os:~/.../lab/support/common-functions$ make main_string
gcc -fno-stack-protector   -c -o main_string.o main_string.c
gcc -fno-stack-protector   -c -o string.o string.c
nasm -f elf64 -o syscall.o syscall.s
gcc -nostdlib -no-pie -Wl,--entry=main -Wl,--build-id=none  main_string.o string.o syscall.o   -o main_string

student@os:~/.../lab/support/common-functions$ ./main_string
Destination string is: warhammer40k

student@os:~/.../lab/support/common-functions$ strace ./main_string
execve("./main_string", ["./main_string"], 0x7ffd544d0a70 /* 63 vars */) = 0
write(1, "Destination string is: ", 23Destination string is: ) = 23
write(1, "warhammer40k\n", 13warhammer40k
)          = 13
exit(0)                                 = ?
+++ exited with 0 +++

When using strace we see that only the write() system call wrapper triggers a system call. There are no system calls triggered by strlen() and strcpy() as can be seen in their implementation.

In addition, main_printf.c depends on the printf.h and printf.c files that implement the printf() function. There is a requirement to implement the _putchar() function; we implement it in the main_printf.c file using the write() syscall call wrapper. The main() function main_printf.c file contains all the string and printing calls. printf() offers a more powerful printing interface, allowing us to print addresses and integers.

Let's build and run the program:

student@os:~/.../lab/support/common-functions$ make main_printf
gcc -fno-stack-protector   -c -o printf.o printf.c
gcc -nostdlib -no-pie -Wl,--entry=main -Wl,--build-id=none  main_printf.o printf.o string.o syscall.o   -o main_printf

student@os:~/.../lab/support/common-functions$ ./main_printf
[before] src is at 00000000004026A0, len is 12, content: "warhammer40k"
[before] dest is at 0000000000603000, len is 0, content: ""
copying src to dest
[after] src is at 00000000004026A0, len is 12, content: "warhammer40k"
[after] dest is at 0000000000603000, len is 12, content: "warhammer40k"

student@os:~/.../lab/support/common-functions$ strace ./main_printf
execve("./main_printf", ["./main_printf"], 0x7ffcaaa1d660 /* 63 vars */) = 0
write(1, "[", 1[)                        = 1
write(1, "b", 1b)                        = 1
write(1, "e", 1e)                        = 1
write(1, "f", 1f)                        = 1
write(1, "o", 1o)                        = 1
write(1, "r", 1r)                        = 1
write(1, "e", 1e)                        = 1
write(1, "]", 1])                        = 1
write(1, " ", 1 )                        = 1
write(1, "s", 1s)                        = 1
write(1, "r", 1r)                        = 1
[...]

We see that we have greater printing flexibility with the printf() function. However, one downside of the current implementation is that it makes a system call for each character. This is inefficient and could be improved by printing a whole string.

Practice

Enter the support/common-functions/ folder and go through the practice items below.

Update string.c and string.h to make available the strcat() function. Call that function in main_string.c and print the result.
Update the main_printf.c file to use the implementation of sprintf() to collect information to be printed inside a buffer. Call the write() function to print the information. The printf() function will no longer be called. This results in a single write system call.

Using previously implemented functions allows us to more efficiently write new programs. These functions provide us with extensive features that we use in our programs.

Quiz 1

Quiz 2

Quiz 3

Common Functions

Practice​

Practice