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174+ < span itemprop ="name "> # atoi,atol,atoll</ span >
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190+ 最終更新日時(UTC):
191+ < span itemprop ="datePublished " content ="2025-07-05T10:01:28 ">
192+ 2025年07月05日 10時01分28秒
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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 >
206+ < a class ="edit " target ="_blank " href ="https://github.com/cpprefjp/site/edit/master/reference/cstdlib/atoi_atol_atoll.md ">
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 "> <cstdlib></ div > < h2 > atoi,atol,atoll</ h2 >
216+ < p > < div class ="codehilite "> < pre > < span > </ span > < code > < span class ="kt "> int</ span > < span class ="nf "> atoi</ span > < span class ="p "> (</ span > < span class ="k "> const</ span > < span class ="kt "> char</ span > < span class ="o "> *</ span > < span class ="n "> str</ span > < span class ="p "> );</ span >
217+ < span class ="kt "> long</ span > < span class ="nf "> atol</ span > < span class ="p "> (</ span > < span class ="k "> const</ span > < span class ="kt "> char</ span > < span class ="o "> *</ span > < span class ="n "> str</ span > < span class ="p "> );</ span >
218+ < span class ="kt "> long</ span > < span class ="kt "> long</ span > < span class ="nf "> atoll</ span > < span class ="p "> (</ span > < span class ="k "> const</ span > < span class ="kt "> char</ span > < span class ="o "> *</ span > < span class ="n "> str</ span > < span class ="p "> );</ span >
219+ </ code > </ pre > </ div >
220+ </ p >
221+ < h2 > 概要</ h2 >
222+ < p > < code > str</ code > を< code > int long long long</ code > に変換する。</ p >
223+ < p > 基数は常に10である。</ p >
224+ < h2 > 変換方法</ h2 >
225+ < p > 空白文字から始まる場合、最初の非空白文字から変換する。</ p >
226+ < p > + ,-は変換後の符号に適用される。</ p >
227+ < p > 変換不可能ならば0を返す.。</ p >
228+ < h2 > 備考</ h2 >
229+ < p > 変換後の数値が< 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 > の範囲外の場合、< a class ="cpprefjp-defined-word " data-desc ="未定義の動作。処理系は予期せぬ動作をする可能性がある。要するに動作保証対象外。undefined behavior (UB)。 " href ="../../implementation-compliance.html#dfn-undefined-behavior "> 動作は未定義</ a > である。</ p >
230+ < p > 名前は、ASCII to integerから命名。</ p >
231+ < h2 > 例</ h2 >
232+ < p > < div class ="yata " id ="9ab591e74c8ad634e355e9a471b3d11d31181f2a "> < div class ="codehilite "> < pre > < span > </ span > < code > < span class ="cp "> #include</ span > < span class ="cpf "> <stdio.h></ span > < span class ="cp "> </ span >
233+ < span class ="cp "> #include</ span > < span class ="cpf "> <stdlib.h></ span > < span class ="cp "> </ span >
234+
235+ < span class ="kt "> int</ span > < span class ="nf "> main</ span > < span class ="p "> (</ span > < span class ="kt "> void</ span > < span class ="p "> )</ span >
236+ < span class ="p "> {</ span >
237+ < span class ="n "> printf</ span > < span class ="p "> (</ span > < span class ="s "> "%i</ span > < span class ="se "> \n</ span > < span class ="s "> "</ span > < span class ="p "> ,</ span > < span class ="n "> atoi</ span > < span class ="p "> (</ span > < span class ="s "> " -123junk"</ span > < span class ="p "> ));</ span >
238+ < span class ="n "> printf</ span > < span class ="p "> (</ span > < span class ="s "> "%i</ span > < span class ="se "> \n</ span > < span class ="s "> "</ span > < span class ="p "> ,</ span > < span class ="n "> atoi</ span > < span class ="p "> (</ span > < span class ="s "> " +321dust"</ span > < span class ="p "> ));</ span >
239+ < span class ="n "> printf</ span > < span class ="p "> (</ span > < span class ="s "> "%i</ span > < span class ="se "> \n</ span > < span class ="s "> "</ span > < span class ="p "> ,</ span > < span class ="n "> atoi</ span > < span class ="p "> (</ span > < span class ="s "> "0"</ span > < span class ="p "> ));</ span >
240+ < span class ="n "> printf</ span > < span class ="p "> (</ span > < span class ="s "> "%i</ span > < span class ="se "> \n</ span > < span class ="s "> "</ span > < span class ="p "> ,</ span > < span class ="n "> atoi</ span > < span class ="p "> (</ span > < span class ="s "> "0042"</ span > < span class ="p "> ));</ span > < span class ="c1 "> // 先頭の0を含む10進数として変換</ span >
241+ < span class ="n "> printf</ span > < span class ="p "> (</ span > < span class ="s "> "%i</ span > < span class ="se "> \n</ span > < span class ="s "> "</ span > < span class ="p "> ,</ span > < span class ="n "> atoi</ span > < span class ="p "> (</ span > < span class ="s "> "0x2A"</ span > < span class ="p "> ));</ span > < span class ="c1 "> // 0のみを変換、"x2A"は破棄される</ span >
242+ < span class ="n "> printf</ span > < span class ="p "> (</ span > < span class ="s "> "%i</ span > < span class ="se "> \n</ span > < span class ="s "> "</ span > < span class ="p "> ,</ span > < span class ="n "> atoi</ span > < span class ="p "> (</ span > < span class ="s "> "junk"</ span > < span class ="p "> ));</ span > < span class ="c1 "> // 変換不可</ span >
243+ < span class ="n "> printf</ span > < span class ="p "> (</ span > < span class ="s "> "%i</ span > < span class ="se "> \n</ span > < span class ="s "> "</ span > < span class ="p "> ,</ span > < span class ="n "> atoi</ span > < span class ="p "> (</ span > < span class ="s "> "2147483648"</ span > < span class ="p "> ));</ span > < span class ="c1 "> //intの範囲外</ span >
244+ < span class ="p "> }</ span >
245+ </ code > </ pre > </ div >
246+ </ div > </ p >
247+ < h2 > 出力例</ h2 >
248+ < p > < pre > < code > -123
249+ 321
250+ 0
251+ 42
252+ 0
253+ 0
254+ -2147483648
255+ </ code > </ pre > </ p >
256+ < h2 > 参照</ h2 >
257+ < ul >
258+ < li > < a href ="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3220.pdf " target ="_blank "> 7.22.1.2 The atoi, atol, and atoll functions</ a > </ li >
259+ < li > < a href ="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2310.pdf " target ="_blank "> 7.22.1.2 The atoi, atol, and atoll functions (p: 249)</ a > </ li >
260+ < li > < a href ="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1548.pdf " target ="_blank "> 7.22.1.2 The atoi, atol, and atoll functions (p: 341)</ a > </ li >
261+ < li > < a href ="https://www.dii.uchile.cl/~daespino/files/Iso_C_1999_definition.pdf " target ="_blank "> 7.20.1.2 The atoi, atol, and atoll functions (p: 307)</ a > </ li >
262+ < li > < a href ="https://www.dii.uchile.cl/~daespino/files/Iso_C_1999_definition.pdf " target ="_blank "> 4.10.1.2 The atoi function</ a > </ li >
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