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71+ < header data-kunai-mdinfo ="{"meta": {"header": ["cstdio"], "namespace": ["std"], "id-type": ["function"]}, "sources": [{"id": "e8da0f4d5689dd5aef360271f856d5c63dbbc1cb", "source": "#include <cstdio>\n\nint main() {\n std::FILE *file = std::fopen(\"output.txt\", \"w\");\n if (!file) {\n std::perror(\"\u30d5\u30a1\u30a4\u30eb\u3092\u958b\u3051\u307e\u305b\u3093\u3067\u3057\u305f\");\n return 1;\n }\n\n const char data[] = \"Hello, World!\\n\";\n /*\n \u53b3\u5bc6\u306b\u306f\u3001sizeof(char)\u306f1\u30d0\u30a4\u30c8\u3067\u3042\u308b\u3053\u3068\u304c\u4fdd\u8a3c\u3055\u308c\u3066\u3044\u308b\u305f\u3081\u3001\n sizeof(data) - 1\u306f\u3001\u6587\u5b57\u5217\u306e\u9577\u3055\uff08\u30cc\u30eb\u7d42\u7aef\u6587\u5b57\u3092\u9664\u304f\uff09\u3068\u7b49\u3057\u304f\u306a\u308b\u3002\n */\n std::size_t count = std::fwrite(data, sizeof(char), sizeof(data) - 1, file);\n std::printf(\"\u66f8\u304d\u8fbc\u3093\u3060\u30c7\u30fc\u30bf\u306e\u9577\u3055: %zu\\n\", count);\n\n std::fclose(file);\n return 0;\n}\n"}], "page_id": ["reference", "cstdio", "fwrite"]} ">
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190+ 最終更新日時(UTC):
191+ < span itemprop ="datePublished " content ="2025-09-05T12:29:52 ">
192+ 2025年09月05日 12時29分52秒
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196+ < span itemprop ="name "> rotarymars</ span >
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198+ が更新
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204+ < span class ="fa fa-fw fa-clock-o fa-flip-horizontal "> </ span > 履歴
205+ </ a >
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207+ < span class ="fa fa-fw fa-pencil "> </ span > 編集
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214+
215+ < div class ="identifier-type "> function</ div > < div class ="header "> <cstdio></ div > < h1 itemprop ="name "> < span class ="namespace " title ="namespace std "> std::</ span > < span class ="token "> fwrite</ span > </ h1 >
216+ < div itemprop ="articleBody "> < p > < div class ="codehilite "> < pre > < span > </ span > < code > < span class ="k "> namespace</ span > < span class ="n "> std</ span > < span class ="p "> {</ span >
217+ < span class ="kt "> size_t</ span > < span class ="n "> fwrite</ span > < span class ="p "> (</ span > < span class ="k "> const</ span > < span class ="kt "> void</ span > < span class ="o "> *</ span > < span class ="n "> buffer</ span > < span class ="p "> ,</ span > < span class ="kt "> size_t</ span > < span class ="n "> size</ span > < span class ="p "> ,</ span > < span class ="kt "> size_t</ span > < span class ="n "> count</ span > < span class ="p "> ,</ span > < span class ="kt "> FILE</ span > < span class ="o "> *</ span > < span class ="n "> stream</ span > < span class ="p "> );</ span >
218+ < span class ="p "> }</ span >
219+ </ code > </ pre > </ div >
220+ </ p >
221+ < h2 > 概要</ h2 >
222+ < p > ストリームに< code > count</ code > 個の< code > size</ code > バイトの要素を書き込む。</ p >
223+ < p > ストリーム内の位置は、書き込まれたバイトだけ進む。</ p >
224+ < p > 書き込み時にエラーが発生した場合、ストリーム内の位置は不定である。</ p >
225+ < p > また、書き込みには内部的に< code > < a href ="fputc.html "> fputc</ a > </ code > を使用する。</ p >
226+ < h2 > 要件</ h2 >
227+ < ul >
228+ < li > < code > buffer</ code > は有効なポインタであること。</ li >
229+ < li > < code > stream</ code > は有効なファイルストリームであること。</ li >
230+ < li > < code > buffer</ code > が指すメモリ領域と< code > stream</ code > が指すファイルストリームの内部バッファが重複していないこと。</ li >
231+ </ ul >
232+ < h2 > < a class ="cpprefjp-defined-word " data-desc ="関数呼び出し式の評価結果となるオブジェクト・値 "> 戻り値</ a > </ h2 >
233+ < p > 正常に書き込むことのできた要素の数を返す。</ p >
234+ < h2 > 効果</ h2 >
235+ < p > < code > buffer</ code > が指すメモリから< code > count</ code > 個の< code > size</ code > バイトの要素を< code > stream</ code > に書き込む。</ p >
236+ < p > 書き込みエラーが生じなければ、書き込むデータの大きさは< code > count * size</ code > バイトである。</ p >
237+ < h2 > 例</ h2 >
238+ < p > < div class ="yata " id ="e8da0f4d5689dd5aef360271f856d5c63dbbc1cb "> < div class ="codehilite "> < pre > < span > </ span > < code > < span class ="cp "> #include</ span > < span class ="cpf "> < a href ="../cstdio.html "> <cstdio></ a > </ span > < span class ="cp "> </ span >
239+
240+ < span class ="kt "> int</ span > < span class ="nf "> main</ span > < span class ="p "> ()</ span > < span class ="p "> {</ span >
241+ < span class ="n "> std</ span > < span class ="o "> ::</ span > < span class ="kt "> FILE</ span > < span class ="o "> *</ span > < span class ="n "> file</ span > < span class ="o "> =</ span > < span class ="n "> < a href ="fopen.html "> std::fopen</ a > </ span > < span class ="p "> (</ span > < span class ="s "> "output.txt"</ span > < span class ="p "> ,</ span > < span class ="s "> "w"</ span > < span class ="p "> );</ span >
242+ < span class ="k "> if</ span > < span class ="p "> (</ span > < span class ="o "> !</ span > < span class ="n "> file</ span > < span class ="p "> )</ span > < span class ="p "> {</ span >
243+ < span class ="n "> < span href ="https://cpprefjp.github.io/reference/cstdio/perror.md.nolink "> std::perror</ span > </ span > < span class ="p "> (</ span > < span class ="s "> "ファイルを開けませんでした"</ span > < span class ="p "> );</ span >
244+ < span class ="k "> return</ span > < span class ="mi "> 1</ span > < span class ="p "> ;</ span >
245+ < span class ="p "> }</ span >
246+
247+ < span class ="k "> const</ span > < span class ="kt "> char</ span > < span class ="n "> data</ span > < span class ="p "> []</ span > < span class ="o "> =</ span > < span class ="s "> "Hello, World!</ span > < span class ="se "> \n</ span > < span class ="s "> "</ span > < span class ="p "> ;</ span >
248+ < span class ="cm "> /*</ span >
249+ < span class ="cm "> 厳密には、sizeof(char)は1バイトであることが保証されているため、</ span >
250+ < span class ="cm "> sizeof(data) - 1は、文字列の長さ（ヌル終端文字を除く）と等しくなる。</ span >
251+ < span class ="cm "> */</ span >
252+ < span class ="n "> < a href ="../cstddef/size_t.html "> std::size_t</ a > </ span > < span class ="n "> count</ span > < span class ="o "> =</ span > < span class ="n "> < span style ="color:#ff0000 "> std::fwrite</ span > </ span > < span class ="p "> (</ span > < span class ="n "> data</ span > < span class ="p "> ,</ span > < span class ="k "> sizeof</ span > < span class ="p "> (</ span > < span class ="kt "> char</ span > < span class ="p "> ),</ span > < span class ="k "> sizeof</ span > < span class ="p "> (</ span > < span class ="n "> data</ span > < span class ="p "> )</ span > < span class ="o "> -</ span > < span class ="mi "> 1</ span > < span class ="p "> ,</ span > < span class ="n "> file</ span > < span class ="p "> );</ span >
253+ < span class ="n "> < a href ="printf.html "> std::printf</ a > </ span > < span class ="p "> (</ span > < span class ="s "> "書き込んだデータの長さ: %zu</ span > < span class ="se "> \n</ span > < span class ="s "> "</ span > < span class ="p "> ,</ span > < span class ="n "> count</ span > < span class ="p "> );</ span >
254+
255+ < span class ="n "> < a href ="fclose.html "> std::fclose</ a > </ span > < span class ="p "> (</ span > < span class ="n "> file</ span > < span class ="p "> );</ span >
256+ < span class ="k "> return</ span > < span class ="mi "> 0</ span > < span class ="p "> ;</ span >
257+ < span class ="p "> }</ span >
258+ </ code > </ pre > </ div >
259+ </ div > </ p >
260+ < h3 > 出力例</ h3 >
261+ < p > < pre > < code > 書き込んだデータの長さ: 13
262+ </ code > </ pre > </ p >
263+ < h3 > ファイル出力(output.txt)</ h3 >
264+ < p > < pre > < code > Hello, World!
265+ </ code > </ pre > </ p >
266+ < h2 > 処理系</ h2 >
267+ < ul >
268+ < li > < a href ="../../implementation.html#clang "> Clang</ a > : ??</ li >
269+ < li > < a href ="../../implementation.html#gcc "> GCC</ a > : ??</ li >
270+ < li > < a href ="../../implementation.html#visual_cpp "> Visual C++</ a > : ??</ li >
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