#include <unistd.h> size_t confstr(int name, char *buf, size_t len);
The name argument represents the system variable to be queried. The implementation shall support the following name values, defined in <unistd.h>. It may support others:
_CS_PATH _CS_POSIX_V7_ILP32_OFF32_CFLAGS _CS_POSIX_V7_ILP32_OFF32_LDFLAGS _CS_POSIX_V7_ILP32_OFF32_LIBS _CS_POSIX_V7_ILP32_OFFBIG_CFLAGS _CS_POSIX_V7_ILP32_OFFBIG_LDFLAGS _CS_POSIX_V7_ILP32_OFFBIG_LIBS _CS_POSIX_V7_LP64_OFF64_CFLAGS _CS_POSIX_V7_LP64_OFF64_LDFLAGS _CS_POSIX_V7_LP64_OFF64_LIBS _CS_POSIX_V7_LPBIG_OFFBIG_CFLAGS _CS_POSIX_V7_LPBIG_OFFBIG_LDFLAGS _CS_POSIX_V7_LPBIG_OFFBIG_LIBS _CS_POSIX_V7_THREADS_CFLAGS _CS_POSIX_V7_THREADS_LDFLAGS _CS_POSIX_V7_WIDTH_RESTRICTED_ENVS _CS_V7_ENV _CS_POSIX_V6_ILP32_OFF32_CFLAGS _CS_POSIX_V6_ILP32_OFF32_LDFLAGS _CS_POSIX_V6_ILP32_OFF32_LIBS _CS_POSIX_V6_ILP32_OFFBIG_CFLAGS _CS_POSIX_V6_ILP32_OFFBIG_LDFLAGS _CS_POSIX_V6_ILP32_OFFBIG_LIBS _CS_POSIX_V6_LP64_OFF64_CFLAGS _CS_POSIX_V6_LP64_OFF64_LDFLAGS _CS_POSIX_V6_LP64_OFF64_LIBS _CS_POSIX_V6_LPBIG_OFFBIG_CFLAGS _CS_POSIX_V6_LPBIG_OFFBIG_LDFLAGS _CS_POSIX_V6_LPBIG_OFFBIG_LIBS _CS_POSIX_V6_WIDTH_RESTRICTED_ENVS _CS_V6_ENV
If len is not 0, and if name has a configuration-defined value, confstr() shall copy that value into the len-byte buffer pointed to by buf. If the string to be returned is longer than len bytes, including the terminating null, then confstr() shall truncate the string to len-1 bytes and null-terminate the result. The application can detect that the string was truncated by comparing the value returned by confstr() with len.
If len is 0 and buf is a null pointer, then confstr() shall still return the integer value as defined below, but shall not return a string. If len is 0 but buf is not a null pointer, the result is unspecified.
After a call to:
confstr(_CS_V7_ENV, buf, sizeof(buf))
the string stored in buf shall contain a <space>-separated list of the variable=value environment variable pairs an implementation requires as part of specifying a conforming environment, as described in the implementations' conformance documentation.
If the implementation supports the POSIX shell option, the string stored in buf after a call to:
confstr(_CS_PATH, buf, sizeof(buf))
can be used as a value of the PATH environment variable that accesses all of the standard utilities of POSIX.1-2008, that are provided in a manner accessible via the exec family of functions, if the return value is less than or equal to sizeof(buf).
If name is invalid, confstr() shall return 0 and set errno to indicate the error.
If name does not have a configuration-defined value, confstr() shall return 0 and leave errno unchanged.
The following sections are informative.
The original need for this function was to provide a way of finding the configuration-defined default value for the environment variable PATH. Since PATH can be modified by the user to include directories that could contain utilities replacing the standard utilities in the Shell and Utilities volume of POSIX.1-2017, applications need a way to determine the system-supplied PATH environment variable value that contains the correct search path for the standard utilities.
An application could use:
confstr(name, (char *)NULL, (size_t)0)
to find out how big a buffer is needed for the string value; use malloc() to allocate a buffer to hold the string; and call confstr() again to get the string. Alternately, it could allocate a fixed, static buffer that is big enough to hold most answers (perhaps 512 or 1024 bytes), but then use malloc() to allocate a larger buffer if it finds that this is too small.
popen("command -p getconf variable", "r");
The confstr() function with a name argument of _CS_PATH returns a string that can be used as a PATH environment variable setting that will reference the standard shell and utilities as described in the Shell and Utilities volume of POSIX.1-2017.
The confstr() function copies the returned string into a buffer supplied by the application instead of returning a pointer to a string. This allows a cleaner function in some implementations (such as those with lightweight threads) and resolves questions about when the application must copy the string returned.
The Base Definitions volume of POSIX.1-2017, <unistd.h>
The Shell and Utilities volume of POSIX.1-2017, c99
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