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    <p>The reasons for that are purely historical. Note, that in the old days of C language (K&amp;R C) there was no such thing as <em>function prototype</em>. A <code>strchr</code> function in those times would be declared as</p> <pre><code>char *strchr(); </code></pre> <p>and defined in K&amp;R style as</p> <pre><code>char *strchr(s, c) char *s; char c; { /* whatever */ } </code></pre> <p>However, in C language (in K&amp;R C and in the modern one as well) if the function is declared without a prototype (as shown above), the parameters passed in each function call are subjected to so called <em>default argument promotions</em>. Under default argument promotions any integral type smaller than <code>int</code> (or <code>unsigned int</code>) is always converted to <code>int</code> (or <code>unsigned int</code>). I.e. when the parameters are undeclared, whenever you pass a <code>char</code> value as an argument, this value is <em>implicitly</em> converted to <code>int</code>, and actually physically passed as an <code>int</code>. The same is true for <code>short</code>. (BTW, <code>float</code> is converted to <code>double</code> by default argument promotions). If inside the function the parameter is actually declared as a <code>char</code> (as in the K&amp;R style definition above), it is <em>implicitly</em> converted back to <code>char</code> type and used as a <code>char</code> inside the function. This is how it worked in K&amp;R times, and this actually is how it works to this day in modern C when function has no prototype or when variadic parameters are used.</p> <p>Now, cue in the modern C, which has <em>function prototypes</em> and uses modern-style function definition syntax. In order to preserve and reproduce the "traditional" functionality of <code>strchr</code>, as described above, we have no other choice but to declare the parameter of <code>strchr</code> as an <code>int</code> and explicitly convert it to <code>char</code> inside the function. This is exactly what you observe in the code you quoted. This is exactly as the functionality of <code>strchr</code> is described in the standard.</p> <p>Moreover, if you have an already-compiled legacy library, where <code>strchr</code> is defined in K&amp;R style as shown above, and you decided to provide modern prototypes for that library, the proper declaration for <code>strchr</code> would be</p> <pre><code>char *strchr(const char *s, int c); </code></pre> <p>because <code>int</code> is what the above legacy implementation expects to physically receive as <code>c</code>. Declaring it with a <code>char</code> parameter would be incorrect.</p> <p>For this reason, you will never see "traditional" standard library functions expecting parameters of type <code>char</code>, <code>short</code> or <code>float</code>. All these functions will be declared with parameters of type <code>int</code> or <code>double</code> instead.</p> <p>A very same rationale is behind the standard guarantee that char pointers and <code>void *</code> pointers share the same representation and alignment requirements. Relying on this guarantee you can declare <code>malloc</code> as a <code>void *</code>-returning function and then use this declaration with a pre-compiled legacy version of standard library where <code>malloc</code> actually returned <code>char *</code>.</p> <hr> <p>Reference: the C99 rationale, version 5.10</p> <blockquote> <p>7.1.4 Use of library functions<br> /--/<br> All library prototypes are specified in terms of the “widened” types: an argument formerly declared as char is now written as int. This ensures that most library functions can be called with or without a prototype in scope, thus maintaining backwards compatibility with pre-C89 code</p> </blockquote>
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    1. POWhy does strchr take an int for the char to be found?
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    1. POWhy does strchr take an int for the char to be found?
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    1. COOk. many thanks for the detailed reply. Basically, it seems to boil down to legacy issue / historical precedent. I am curious no more...
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      1. USSteve D
    2. COit's not a historical reason -- it has to do with efficiency of implementation of how parameter passing is implemented for function calls, and how the default rules for type promotion work for expressions. It's all still _very_ relevant today. The answer above misleads considerably. The abstract machine model for C makes `int` the most optimal size for a reason, and smaller objects are best handled by promoting them to `int`. Care should always be taken to declare function parameters using their natural promoted types, _not_ any narrower type one might often call them with.
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      1. USGreg A. Woods
    3. CO@Greg A. Woods: That completely misses the point. The prototype-less declarations of standard functions in K&R C and their behavior with regard to smaller types are indisputable historical fact. The reasons it was done that way in K&R (efficiency or something else) are interesting, but irrelevant within the context of the question. The answer to the original question is, again, that when the concept of "prototype" was added to the language, the prototypes for standard functions were *forced* to avoid smaller types in order to preserve the already established legacy behavior. That's all.
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