1getauxval(3) Library Functions Manual getauxval(3)
2
6 getauxval - retrieve a value from the auxiliary vector
7
9 Standard C library (libc, -lc)
10
12 #include <sys/auxv.h>
13 unsigned long getauxval(unsigned long type);
15
17 The getauxval() function retrieves values from the auxiliary vector, a
18 mechanism that the kernel's ELF binary loader uses to pass certain in‐
19 formation to user space when a program is executed.
20 Each entry in the auxiliary vector consists of a pair of values: a type
22 that identifies what this entry represents, and a value for that type.
23 Given the argument type, getauxval() returns the corresponding value.
24 The value returned for each type is given in the following list. Not
26 all type values are present on all architectures.
27 AT_BASE
29 The base address of the program interpreter (usually, the dy‐
30 namic linker).
31 AT_BASE_PLATFORM
33 A pointer to a string (PowerPC and MIPS only). On PowerPC, this
34 identifies the real platform; may differ from AT_PLATFORM. On
35 MIPS, this identifies the ISA level (since Linux 5.7).
36 AT_CLKTCK
38 The frequency with which times(2) counts. This value can also
39 be obtained via sysconf(_SC_CLK_TCK).
40 AT_DCACHEBSIZE
42 The data cache block size.
43 AT_EGID
45 The effective group ID of the thread.
46 AT_ENTRY
48 The entry address of the executable.
49 AT_EUID
51 The effective user ID of the thread.
52 AT_EXECFD
54 File descriptor of program.
55 AT_EXECFN
57 A pointer to a string containing the pathname used to execute
58 the program.
59 AT_FLAGS
61 Flags (unused).
62 AT_FPUCW
64 Used FPU control word (SuperH architecture only). This gives
65 some information about the FPU initialization performed by the
66 kernel.
67 AT_GID The real group ID of the thread.
69 AT_HWCAP
71 An architecture and ABI dependent bit-mask whose settings indi‐
72 cate detailed processor capabilities. The contents of the bit
73 mask are hardware dependent (for example, see the kernel source
74 file arch/x86/include/asm/cpufeature.h for details relating to
75 the Intel x86 architecture; the value returned is the first
76 32-bit word of the array described there). A human-readable
77 version of the same information is available via /proc/cpuinfo.
78 AT_HWCAP2 (since glibc 2.18)
80 Further machine-dependent hints about processor capabilities.
81 AT_ICACHEBSIZE
83 The instruction cache block size.
84 AT_L1D_CACHEGEOMETRY
86 Geometry of the L1 data cache, encoded with the cache line size
87 in bytes in the bottom 16 bits and the cache associativity in
88 the next 16 bits. The associativity is such that if N is the
89 16-bit value, the cache is N-way set associative.
90 AT_L1D_CACHESIZE
92 The L1 data cache size.
93 AT_L1I_CACHEGEOMETRY
95 Geometry of the L1 instruction cache, encoded as for
96 AT_L1D_CACHEGEOMETRY.
97 AT_L1I_CACHESIZE
99 The L1 instruction cache size.
100 AT_L2_CACHEGEOMETRY
102 Geometry of the L2 cache, encoded as for AT_L1D_CACHEGEOMETRY.
103 AT_L2_CACHESIZE
105 The L2 cache size.
106 AT_L3_CACHEGEOMETRY
108 Geometry of the L3 cache, encoded as for AT_L1D_CACHEGEOMETRY.
109 AT_L3_CACHESIZE
111 The L3 cache size.
112 AT_PAGESZ
114 The system page size (the same value returned by sysconf(_SC_PA‐
115 GESIZE)).
116 AT_PHDR
118 The address of the program headers of the executable.
119 AT_PHENT
121 The size of program header entry.
122 AT_PHNUM
124 The number of program headers.
125 AT_PLATFORM
127 A pointer to a string that identifies the hardware platform that
128 the program is running on. The dynamic linker uses this in the
129 interpretation of rpath values.
130 AT_RANDOM
132 The address of sixteen bytes containing a random value.
133 AT_SECURE
135 Has a nonzero value if this executable should be treated se‐
136 curely. Most commonly, a nonzero value indicates that the
137 process is executing a set-user-ID or set-group-ID binary (so
138 that its real and effective UIDs or GIDs differ from one an‐
139 other), or that it gained capabilities by executing a binary
140 file that has capabilities (see capabilities(7)). Alterna‐
141 tively, a nonzero value may be triggered by a Linux Security
142 Module. When this value is nonzero, the dynamic linker disables
143 the use of certain environment variables (see ld-linux.so(8))
144 and glibc changes other aspects of its behavior. (See also se‐
145 cure_getenv(3).)
146 AT_SYSINFO
148 The entry point to the system call function in the vDSO. Not
149 present/needed on all architectures (e.g., absent on x86-64).
150 AT_SYSINFO_EHDR
152 The address of a page containing the virtual Dynamic Shared Ob‐
153 ject (vDSO) that the kernel creates in order to provide fast im‐
154 plementations of certain system calls.
155 AT_UCACHEBSIZE
157 The unified cache block size.
158 AT_UID The real user ID of the thread.
160
162 On success, getauxval() returns the value corresponding to type. If
163 type is not found, 0 is returned.
164
166 ENOENT (since glibc 2.19)
167 No entry corresponding to type could be found in the auxiliary
168 vector.
169
171 For an explanation of the terms used in this section, see at‐
172 tributes(7).
173 ┌────────────────────────────────────────────┬───────────────┬─────────┐
175 │Interface │ Attribute │ Value │
176 ├────────────────────────────────────────────┼───────────────┼─────────┤
177 │getauxval() │ Thread safety │ MT-Safe │
178 └────────────────────────────────────────────┴───────────────┴─────────┘
179
181 GNU.
182
184 glibc 2.16.
185
187 The primary consumer of the information in the auxiliary vector is the
188 dynamic linker, ld-linux.so(8). The auxiliary vector is a convenient
189 and efficient shortcut that allows the kernel to communicate a certain
190 set of standard information that the dynamic linker usually or always
191 needs. In some cases, the same information could be obtained by system
192 calls, but using the auxiliary vector is cheaper.
193 The auxiliary vector resides just above the argument list and
195 environment in the process address space. The auxiliary vector
196 supplied to a program can be viewed by setting the LD_SHOW_AUXV
197 environment variable when running a program:
198 $ LD_SHOW_AUXV=1 sleep 1
200 The auxiliary vector of any process can (subject to file permissions)
202 be obtained via /proc/pid/auxv; see proc(5) for more information.
203
205 Before the addition of the ENOENT error in glibc 2.19, there was no way
206 to unambiguously distinguish the case where type could not be found
207 from the case where the value corresponding to type was zero.
208
210 execve(2), secure_getenv(3), vdso(7), ld-linux.so(8)
211Linux man-pages 6.05 2023-07-20 getauxval(3)