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Difference between system() and execl() Call
In programming, system−level calls are used to interact with the operating system and perform various tasks. Two commonly used system−level calls are system() and execl(). While both these calls allow executing external programs, they differ in their functionality and usage.
What is system() Call?
The system() call is a higher−level function that allows the execution of shell commands or scripts. When system() is invoked with a command as its argument, it starts a new shell process, which then interprets and executes the specified command. The system() call provides a simple way to interact with the command line and execute external programs. It can be useful for tasks such as running a system command, executing a shell script, or invoking another program.
Command Execution: system() allows you to execute an entire command or shell script as a single string. The command is passed to the operating system's command interpreter, which executes it as if it were typed directly into the shell.
Shell Invocation: system() invokes the default shell of the operating system, which is typically a command−line interpreter like Bash or Command Prompt. The shell processes the command and performs shell−specific operations such as shell expansions, environment variable substitution, and input/output redirection.
Simplicity and Convenience: system() provides a simple and convenient way to execute external commands from within a C program without the need for complex argument handling or low−level process management.
Limited Control: system() does not offer fine−grained control over the execution of the command. It starts a new shell process and waits for its completion before returning control to the calling program. This can limit flexibility and performance in certain scenarios.
What is execl() Call?
The execl() call is a lower−level function that belongs to the exec family of functions. It replaces the current process image with a new one. The execl() call requires specifying the exact path to the executable file, along with the command−line arguments. It directly loads and runs the specified program, replacing the current process with the new program's image. This means that once execl() is called, the original program's execution stops, and the new program takes its place.
Process Replacement: execl() replaces the currently running process image with a new executable file. It loads the specified program into memory, discarding the existing code, data, and stack of the calling process.
Direct Execution: execl() directly executes a specific program or command without invoking a separate shell. It does not interpret shell−specific syntax or perform shell expansions.
Fine−Grained Control: execl() provides fine−grained control over the execution of the new program. It allows you to specify the path to the executable and provide a list of arguments as separate string parameters.
No Return: execl() does not return to the calling program unless an error occurs during execution. Instead, it transfers control to the new program, which becomes the new running process.
Lower Overhead: Compared to system(), execl() typically has lower overhead because it bypasses the need for a shell process. It directly replaces the current process image, resulting in faster execution.
Argument Handling: execl() requires you to specify the program path and individual arguments as separate string parameters. This may require additional effort for handling and formatting arguments compared to system().
Difference between system() and execl() Call
The following table highlights the major differences between system() and excel() calls:
Feature |
system() Call |
execl() Call |
|---|---|---|
Definition |
A library function that allows executing shell commands or running external programs from within a C or C++ program. |
A system call that replaces the current process with a new process, loading a new program into the current process image. |
Execution |
Executes the specified command by invoking the operating system's command interpreter (shell). |
Replaces the current process image with a new program image specified by the provided path. |
Command |
Accepts a string containing the command to be executed, which can include shell commands, executable files, or scripts. |
Requires the full path to the executable file and the command−line arguments as separate parameters. |
Return Value |
Returns an integer representing the termination status of the executed command or −1 if the command cannot be executed. |
Does not return if successful; the new program starts executing in place of the calling program. Returns −1 if an error occurs. |
Control Flow |
system() call blocks the execution of the program until the executed command completes. |
execl() call does not return control to the calling program; the new program takes over execution. |
Flexibility |
Offers flexibility in executing shell commands and external programs without explicitly managing process creation or termination. |
Provides more control over the execution environment, allowing the program to explicitly replace itself with a new program. |
Portability |
Available across different platforms and operating systems, as it relies on the standard C library. |
The execl() function call is specific to UNIX−like systems. Similar functions exist on other platforms, but with different names and slightly different behavior. |
Use Cases |
Suitable for executing simple shell commands or invoking external programs without the need for fine−grained control or interaction. |
Used when explicit control is needed to replace the current process with a new program, such as for process management or specialized execution requirements. |
Conclusion
system() allows you to execute commands in the operating system's shell, while execl() replaces the current process image with a new program. system() provides simplicity and convenience, while execl() offers finer control over program execution and potentially lower overhead. The choice between them depends on the specific requirements of the task at hand.
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