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POif statement ambiguity
 

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    <p>I'm trying to write my own code to walk the PATH to find an executable as a learning exercise in C programming. (after sucess I might replace it with someone else's code, but for now I want to understand my mistakes).</p> <p>The following section of code is not jumping to the else statement I expect ...</p> <pre><code>#include &lt;stdio.h&gt; #include &lt;sys/types.h&gt; #include &lt;sys/stat.h&gt; #include &lt;fcntl.h&gt; #include &lt;string.h&gt; #include &lt;stdlib.h&gt; #include &lt;unistd.h&gt; #define EXECUTABLE S_IXOTH /* executable by others */ #define MAX_PATH_LEN 1024 void message (const char *msg) { fprintf(stdout, "INFO: %s\n", *msg); } int main (int argc, char *argv[], char *envp[]) { char *editor; struct stat editor_stat; char full_path[MAX_PATH_LEN]; int found_path; memset(full_path,0,MAX_PATH_LEN); strcpy(full_path,"/bin/ed"); found_path=stat(full_path,&amp;editor_stat); if (found_path!=0) { editor=NULL; message("The EDITOR specified is not found in the PATH. Using default editor"); } else { if (editor_stat.st_mode&amp;EXECUTABLE==0) { editor=NULL; message("The EDITOR specified must have world execute permission. using default editoe"); } else { editor=full_path; } } } </code></pre> <p>When I track it with gdb I see it jumps to the 2nd else instead of the first one, and doesn't execute the check for executable ...</p> <pre><code>(gdb) file /tmp/sample2 Reading symbols from /tmp/sample2...done. (gdb) b 28 Breakpoint 1 at 0x400688: file /home/ken/c/shorter_sample.c, line 28. (gdb) r Starting program: /tmp/sample2 Breakpoint 1, main (argc=1, argv=0x7fffffffe1f8, envp=0x7fffffffe208) at /home/ken/c/shorter_sample.c:28 28 if (found_path!=0) { Missing separate debuginfos, use: debuginfo-install glibc-2.13-2.x86_64 (gdb) p found_path $1 = 0 (gdb) s 36 editor=full_path; (gdb) </code></pre> <p>it should jump to line 32 not 36.</p> <p>I've tried searcing here for C ambiguity and I've read the sections from "C" by Kernighan &amp; Ritchie that are referenced in the index under C ambiguity, and stuck as many curly braces as I can in the code but the compiler isn't doing what I intend.</p> <p>FYI, I'm using gcc-4.5.1-4.fc14.x86_64 with kernel 2.6.35.14-106.fc14.x86_64 on Fedora 14.</p>
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    1. USbanjo67xxx
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    1. COWhat is the value of editor_stat.st_mode&EXECUTABLE? Likely, it's just going through the code normally, testing the *if*, and continuing. Your code is compiled to code that doesn't correspond to the source code in a simple way the editor can always understand -- it can't track the flow perfectly, especially if you compile with optimizations of any kind.
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      1. USDavid Schwartz
    2. COThis isn' directly relevant, but your conditional statement is more idiomatically written as an `if` ... `else if` ... `else` chain, with all 3 on the same indentation level. I can't show the formatting in a comment. Let me know if you'd like more explanation and I'll set up a link.
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      1. USKeith Thompson
 

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