l - execl(), execlp(), execle()

The const char *arg and subsequent ellipses can be thought of as arg0, arg1, ..., argn. Together they describe a list of one or more pointers to null-terminated strings that represent the argument list available to the executed program. The first argument, by convention, should point to the filename associated with the file being executed. The list of arguments must be terminated by a null pointer, and, since these are variadic functions, this pointer must be cast (char *) NULL.

By contrast with the 'l' functions, the 'v' functions (below) specify the command-line arguments of the executed program as a vector.

v - execv(), execvp(), execvpe()

The char *const argv[] argument is an array of pointers to null-terminated strings that represent the argument list available to the new program. The first argument, by convention, should point to the filename associated with the file being executed. The array of pointers must be terminated by a null pointer.

e - execle(), execvpe()

The environment of the caller is specified via the argument envp. The envp argument is an array of pointers to null-terminated strings and must be terminated by a null pointer.

All other exec() functions (which do not include 'e' in the suffix) take the environment for the new process image from the external variable environ in the calling process.

p - execlp(), execvp(), execvpe()

These functions duplicate the actions of the shell in searching for an executable file if the specified filename does not contain a slash (/) character. The file is sought in the colon-separated list of directory pathnames specified in the PATH environment variable. If this variable isn't defined, the path list defaults to a list that includes the directories returned by confstr(_CS_PATH) (which typically returns the value "/bin:/usr/bin") and possibly also the current working directory; see NOTES for further details.

If the specified filename includes a slash character, then PATH is ignored, and the file at the specified pathname is executed.

In addition, certain errors are treated specially.

If permission is denied for a file (the attempted execve(2) failed with the error EACCES), these functions will continue searching the rest of the search path. If no other file is found, however, they will return with errno set to EACCES.

If the header of a file isn't recognized (the attempted execve(2) failed with the error ENOEXEC), these functions will execute the shell (/bin/sh) with the path of the file as its first argument. (If this attempt fails, no further searching is done.)

All other exec() functions (which do not include 'p' in the suffix) take as their first argument a (relative or absolute) pathname that identifies the program to be executed.

Architecture-specific details

On sparc and sparc64, execv() is provided as a system call by the kernel (with the prototype shown above) for compatibility with SunOS. This function is not employed by the execv() wrapper function on those architectures.