atof函数在不包含头文件stdlib.h的情况下也能编译运行，但是转换结果是错误的，为什么

最新推荐文章于 2023-01-03 17:50:32 发布

sesiria

最新推荐文章于 2023-01-03 17:50:32 发布

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分类专栏： C 文章标签： c语言 gcc

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未包含stdlib.h使用atof和atoi不会导致编译错误，但因隐式声明返回类型为int，造成运行错误。GCC开启-Wall警告能检测到此问题。建议始终使用-Wall并考虑使用strtol和strtod替代。

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c - Not including stdlib.h does not produce any compiler error!

For historical reasons -- specifically, compatibility with very old C programs (pre-C89) -- using a function without having declared it first only provokes a warning from GCC, not an error. But the return type of such a function is assumed to be int, not double, which is why the program executes incorrectly.

If you use -Wall on the command line, you get a diagnostic:
$ gcc -Wall test.c
test.c: In function ‘main’:
test.c:5: warning: implicit declaration of function ‘atoi’
test.c:6: warning: implicit declaration of function ‘atof’

You should use -Wall basically always. Other very useful warning options for new code are -Wextra, -Wstrict-prototypes, -Wmissing-prototypes, -pedantic, and -Wwrite-strings, but compared to -Wall they have much higher false positive rates.

Tangentially: never use atoi nor atof, they hide input errors. Use strtol and