For newbies, problems that may occur when some variadic functions are in use

During the learning process operation testing the Linux exec function, a very simple piece of test code was written, as follows: 650) this.width=650; "Src=" http://s4.51cto.com/wyfs02/M02/88/5C/ Wkiol1fzyaybbmjbaaatd1678rc582.png "title=" screenshot from 2016-10-04 14:34:33.png "alt=" Wkiol1fzyaybbmjbaaatd1678rc582.png "/>

When compiling with GCC version 4.6.3 (Ubuntu/linaro 4.6.3-1ubuntu5), ask the question:
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Later, compared to a reference book, and did not find the error (may not be the same as the GCC version), and later looked for additional information, found that the problem: for some parameters are variable-length array function, you must add a NULL value at the end of the parameter to indicate the end of the passed parameter. For example, the EXECL format ( A form of exec, which has a total of six expressions).

For functions with variable parameter arrays, some problems may arise. Take the form of execl as an example.

This function: execl (const char * path, const char * arg0, .../* (char*) 0 */);

When compiling with GCC, the following two types of problems may occur:
1 warning:not enough variable arguments to fit a sentinel (insufficient number of parameters);

workaround : The parameters can be filled. For the EXECL function above, a minimum of two parameters is required. (The path executable file name, which is the full path; arg, as a convention, indicates the destination file's paths as in path);

2 Warning:missing Sentinel in function call;

workaround : In the formal parameter list, add a null or take (char*) 0 can also, to the compiler (complier) Some hints: "Tell the compiler, do not look down, no parameters need to pass".

such as Execl ("/bin/date", "/bin/date", (char*) 0) with execl ("/bin/date", "/bin/date", NULL);

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In addition, for 0 and (char*) 0, in 64-bit operating systems, the integer 0 is 32 bits, and pointer 0 is 64 bits, but the compiler (complier) does not know, so the problem occurs.

This article is from the "slow Forward in pain" blog, please be sure to keep this source http://jjyynice.blog.51cto.com/10193818/1858709

For newbies, problems that may occur when some variadic functions are in use