Understanding the Linux Kernel, Second Edition

The exec Functions

Unix systems provide a family of functions that replace the execution context of a process with a new context described by an executable file. The names of these functions start with the prefix exec, followed by one or two letters; therefore, a generic function in the family is usually referred to as an exec function.

The exec functions are listed in Table 20-7; they differ in how the parameters are interpreted.

Table 20-7. The exec functions

Function name	PATH search	Command-line arguments	Environment array
execl( )	No	List	No
execlp( )	Yes	List	No
execle( )	No	List	Yes
execv( )	No	Array	No
execvp( )	Yes	Array	No
execve( )	No	Array	Yes

The first parameter of each function denotes the pathname of the file to be executed. The pathname can be absolute or relative to the process’s current directory. Moreover, if the name does not include any / characters, the execlp( ) and execvp( ) functions search for the executable file in all directories specified by the PATH environment variable.

Besides the first parameter, the execl( ), execlp( ), and execle( ) functions include a variable number of additional parameters. Each points to a string describing a command-line argument for the new program; as the "l" character in the function names suggests, the parameters are organized in a list terminated by a NULL value. Usually, the first command-line argument duplicates the executable filename. Conversely, the execv( ), execvp( ), and ...