how do I know what system calls are invoked by a specific library call

[forrestgump]

In Linux, a library call is a "normal" function call, while a system call goes through the kernel.
Library functions do not go to kernel mode for the call instruction, but often do so within the actual library function. In fact, a single library function may make zero, one, or several system calls and do context switches to kernel mode.

My question is how do I know what system calls are invoked by a specific library to fulfill the library function?

For example, fgets() is a shared library function. What system calls are invoked by fgets() to really do its job? What is the implementation detail for the library?

Your response will be appreciated.

[Lakshmipathi]

how do I know what system calls are invoked by a specific library call

strace. Run the program with prefix strace.

strace ./a.out

It show you the system calls used by that binary.

[forrestgump]

strace. Run the program with prefix strace.


It show you the system calls used by that binary.

I know if I use "ltrace " and "strace" as a prefix, I will get all the library calls and system calls used by the executables. Since I will have a list of library calls and a list of system calls used, how do I know which system calls are invoked by a specific library call?

Thanks.

[Lakshmipathi]

I know if I use "ltrace " and "strace" as a prefix, I will get all the library calls and system calls used by the executables. Since I will have a list of library calls and a list of system calls used, how do I know which system calls are invoked by a specific library call?

Thanks.

I don't know about group by glibc call options - One option would be create simple programs with specific glibc function call and use strace to debug.
But ,I think if you go through the strace checking system calls along with its arguments and return types. you can know which glibc call invoked what kind of sys.call.

[forrestgump]

I don't know about group by glibc call options - One option would be create simple programs with specific glibc function call and use strace to debug.
But ,I think if you go through the strace checking system calls along with its arguments and return types. you can know which glibc call invoked what kind of sys.call.

You mean I need to go through the source code of strace to check system calls along with its arguments and return types? or is there a command to use?

Thanks,

[Bemk]

If you want to know for a specific library function, can't you just make 3 or so tiny programs that run one library call and then exit.
Then you run those trace things over them and compare the results. The differences will probably be of most interest.