The result of using strtol() under stdlib.h and stdio.h is different

Question

When trying to parse a number too big to fit a long, strtol() returns 0 instead of LONG_MAX (stdio.h). If I read the POSIX spec correctly, it should be LONG_MAX. There is a different between stdio.h and stdlib.h

#include "stdio.h"
int main(void){
    printf("%ld\n", strtol("99999999999999999999999"));
    return 0;
} # 0

#include "stdio.h"
//#include "stdlib.h"
int main(void){
    char *end[500];
    printf("%ld\n", strtol("99999999999999999999999", end, 10));
    return 0;
} # 9223372036854775807

score 1 · Accepted Answer · answered Feb 06 '19 at 22:20

strtol is declared in header <stdlib.h> as

long strtol( const char *restrict str, char **restrict str_end, int base );
//                       ^^^^^^^^             ^^^^^^^^ since C99

In the first posted snippet, <stdlib.h> is not included and the function is called with one single argument, so that, if compiled with -Wall -Wextra -std=gnu11, gcc produces the following explanatory warnings before outputting 0:

prog.c: In function 'main':
prog.c:5:21: warning: implicit declaration of function 'strtol' [-Wimplicit-function-declaration]
     printf("%ld\n", strtol("99999999999999999999999"));
                     ^~~~~~
prog.c:5:15: warning: format '%ld' expects argument of type 'long int', but argument 2 has type 'int' [-Wformat=]
     printf("%ld\n", strtol("99999999999999999999999"));
             ~~^     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
             %d

Which means that the library function is not called at all and an implicitly declared function with the same name is called, an int with value 0 is returned and printed (with the wrong format specifier, which is itself undefined behavior).

Note that the same code fails to compile with clang, which reports the following:

prog.c:4:21: warning: implicitly declaring library function 'strtol' with type 'long (const char *, char **, int)' [-Wimplicit-function-declaration]
    printf("%ld\n", strtol("99999999999999999999999"));
                    ^
prog.c:4:21: note: include the header &ltstdlib.h&gt or explicitly provide a declaration for 'strtol'
prog.c:4:53: error: too few arguments to function call, expected 3, have 1
    printf("%ld\n", strtol("99999999999999999999999"));
                    ~~~~~~                          ^
1 warning and 1 error generated.

In the second snippet, strtol is called with the right number of arguments, but, as posted (with the #include commented out), has the same missing header problem. To produce the expected output, LONG_MAX, header stdlib.h has to be included.

The result of using strtol() under stdlib.h and stdio.h is different

1 Answers1