Execvp In C
Posted : admin On 05.01.2020This is a video tutorial teaching how to use fork, execvp, and fork and execvp together.
- Nov 02, 2017 String literals in C are const hence the problem with warnings and casting your argv to char. const. is a hack as execvp is now allowed to write to the strings in your argv. Te solution I see is to either allocate a writeble buffer for each item or just use execlp instead which works with const char. args and allows for passing string literals.
- Execve executes the program pointed to by filename. Filename must be either a binary executable, or a script starting with a line of the form: argv is an array of argument strings passed to the new program. By convention, the first of these strings should contain the filename associated with the file being executed.
- However, when we invoke execvp, it replaces the current program with the program passed to it in the arguments. What this means is that although the current text, data, heap and stack segments of the process are replaced, the process id still remains unchanged, but the program gets overwritten completely.
- The functions execlp and execvp will duplicate the actions of the shell in searching for an executable file if the specified file name does not contain a slash (/) character.The search path is the path specified in the environment by the PATH variable.If this variable isn't specified, the default path ``:/bin:/usr/bin' is used.
Name
explain_execvp - explain execvp(3) errors
Synopsis
#include <libexplain/execvp.h>
const char *explain_execvp(const char *pathname, char *const *argv);
const char *explain_errno_execvp(int errnum, const char *pathname, char *const *argv);
void explain_message_execvp(char *message, int message_size, const char *pathname, char *const *argv);
void explain_message_errno_execvp(char *message, int message_size, int errnum, const char *pathname, char *const *argv);
Description
These functions may be used to obtain explanations for errors returned by the execvp(3) system call.
explain_execvp
const char *explain_execvp(const char *pathname, char *const *argv);The explain_execvp function is used to obtain an explanation of an error returned by the execvp(3) system call. The least the message willcontain is the value of strerror(errno), but usually it will do much better, and indicate the underlying cause in more detail.
The errno global variable will be used to obtain the error value to be decoded.
This function is intended to be used in a fashion similar to the following example:
The original argv, exactly as passed to the execvp(3) system call.
explain_errno_execvp
const char *explain_errno_execvp(int errnum, const char *pathname, char *const *argv);The explain_errno_execvp function is used to obtain an explanation of an error returned by the execvp(3) system call. The least the messagewill contain is the value of strerror(errnum), but usually it will do much better, and indicate the underlying cause in more detail.
This function is intended to be used in a fashion similar to the following example:
The error value to be decoded, usually obtained from the errno global variable just before this function is called. This is necessary if you need tocall any code between the system call to be explained and this function, because many libc functions will alter the value of errno.
The original argv, exactly as passed to the execvp(3) system call.
explain_message_execvp
void explain_message_execvp(char *message, int message_size, const char *pathname, char *const *argv);The explain_message_execvp function may be used to obtain an explanation of an error returned by the execvp(3) system call. The least themessage will contain is the value of strerror(errno), but usually it will do much better, and indicate the underlying cause in more detail.
The errno global variable will be used to obtain the error value to be decoded.
This function is intended to be used in a fashion similar to the following example:
The location in which to store the returned message. If a suitable message return buffer is supplied, this function is thread safe.
The original argv, exactly as passed to the execvp(3) system call.
explain_message_errno_execvp
void explain_message_errno_execvp(char *message, int message_size, int errnum, const char *pathname, char *const *argv);The explain_message_errno_execvp function may be used to obtain an explanation of an error returned by the execvp(3) system call. The leastthe message will contain is the value of strerror(errnum), but usually it will do much better, and indicate the underlying cause in more detail.
This function is intended to be used in a fashion similar to the following example:
The location in which to store the returned message. If a suitable message return buffer is supplied, this function is thread safe.
The error value to be decoded, usually obtained from the errno global variable just before this function is called. This is necessary if you need tocall any code between the system call to be explained and this function, because many libc functions will alter the value of errno.
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The original argv, exactly as passed to the execvp(3) system call.
See Also
execvp(3)
man page of the system call.Run Execvp In C
I am super confused. Any help is appreciated.
Anthony Geoghegan2 Answers
this prototype:
Says that execlp ìs a variable argument function. It takes 2 const char *. The rest of the arguments, if any, are the additional arguments to hand over to program we want to run - also char * - all these are C strings (and the last argument must be a NULL pointer)
So, the file argument is the path name of an executable file to be executed. arg is the string we want to appear as argv[0] in the executable. By convention, argv[0] is just the file name of the executable, normally it's set to the same as file.
The .. are now the additional arguments to give to the executable.
Say you run this from a commandline/shell:
That'd be execlp('ls', 'ls', (char *)NULL);Or if you run
That'd be execlp('ls', 'ls', '-l', '/', (char *)NULL);
So on to execlp('/bin/sh', .., 'ls -l /bin/??', ..);
Here you are going to the shell, /bin/sh , and you're giving the shell a command to execute. That command is 'ls -l /bin/??'. You can run that manually from a commandline/shell:
Now, how do you run a shell and tell it to execute a command ? You open up the documentation/man page for your shell and read it.
What you want to run is:
This becomes
Execvp Arguments
Side note:The /bin/?? is doing pattern matching, this pattern matching is done by the shell, and it expands to all files under /bin/ with 2 characters. If you simply did
Probably nothing would happen (unless there's a file actually named /bin/??) as there's no shell that interprets and expands /bin/??
The limitation of execl is that when executing a shell command or any other script that is not in the current working directory, then we have to pass the full path of the command or the script. Example:
Once that is done, restart the program and the Shockwave games will appear.Q5) I want to play this game, but it’s neither in Flashpoint nor the Game Master List!A5) This means that game has not been added to Flashpoint yet. Note that to edit the Google Doc file, you need to request permission first.Even if you have been granted editing privileges, pls check whether the game you want has already been requested before. If there are games you want to be in Flashpoint, you can either wait for it to appear in later versions, or put up a request using the link in Q2. 
The workaround to passing the full path of the executable is to use the function execlp, that searches for the file (1st argument of execlp) in those directories pointed by PATH:
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