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POString manipulation & memory allocation - C
 

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    <p>I am in the process of learning C. I have a method that takes 3 strings and combines them to do some operation. Following was my first implementation using a GCC compiler. </p> <pre><code>void foo(const char *p1, const char *p2, const char *p3) { size_t length = strlen(p1) + strlen(p2) + strlen(p3); char combined[length + 1]; memset(combined, 0, length + 1); strcat(combined, p1); strcat(combined, p2); strcat(combined, p3); printf("Result : %s", combined); } int main() { foo("hello ", "world ", "how"); return 0; } </code></pre> <p>This works well. But when I compiled this using, <code>cc -Wall -pedantic -g foo.c -o foo</code>, I started getting warnings like <code>ISO C90 forbids variable length array ‘combined’</code>. MSVC was not compiling this code. Changed the code like</p> <pre><code>void foo(const char *p1, const char *p2, const char *p3) { size_t length = strlen(p1) + strlen(p2) + strlen(p3); char *combined = (char *) malloc(length + 1); memset(combined, 0, length + 1); strcat(combined, p1); strcat(combined, p2); strcat(combined, p3); printf("Result : %s", combined); free(combined); } </code></pre> <p><strong>Questions</strong></p> <ol> <li>Is this the correct implementation? </li> <li>If variable length arrays are not part of standard, why GCC implemented it? If the code is expected to compile only on GCC, using variable arrays will be a better alternative than using malloc? </li> <li>I think the thumb rule is, if memory required is knows at compile time, use arrays else use malloc to allocate required memory. Is this correct?</li> <li>My code is expected to compile on GCC and MSVC. I will be developing on GCC usually. So what are the compiler flags that ensures maximum portability? Currently I am using <code>-Wall -pedantic</code>. Should I use <code>-ansi</code> too? What would be the equivalent flags available in MSVC?</li> <li>What are the other common things to consider when writing portable C code?</li> </ol>
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    1. COSee http://stackoverflow.com/questions/3093049/different-behavior-of-compilers-with-array-allocation
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      1. USPeterK
    2. CO(1)Yes it is, as you have mentioned in point 3 (2)Because gcc also supports other standards besides ISO C90. (5)http://www.google.com/search?q=writing+portable+C+code It's always better to dynamically allocate VLAs because you can check the return value of (m/c)alloc to see if the allocation was successful or not
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    3. CO@itisravi - You should answer, instead of answering in a comment. Due to comment length restrictions, your recommendation on VLAs vs. dynamic allocation is a bit sparse. I agree with it, and I know what you're getting at, but it's unclear because comments are too short for answers. That's why we have answers. ;)
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